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{{Short description|Order of amphibians}}
{{About|the group of amphibians}}
{{pp-semi-indef|small=yes}}
{{Use British English|date=October 2024}}
{{Use mdy dates|date=October 2024}}
{{Automatic taxobox
| name = Frogs
| fossil_range = [[Early Jurassic]] – [[Holocene|Present]], {{fossil range|200|0}}
| image = Anoures.jpg
| image_caption =
|
| authority = [[André Marie Constant Duméril|Duméril]], 1806 (as Anoures)
| range_map = Distribution.anura.1.png
| range_map_caption = Native distribution of frogs (in green)
| subdivision_ranks = Subgroups
| subdivision = See text
}}
A '''frog''' is any member of a diverse and largely [[semiaquatic]] group of short-bodied, [[tail]]less [[amphibian]] [[vertebrate]]s composing the [[order (biology)|order]] '''Anura'''<ref name="AOTW">{{cite web |url=https://amphibiansoftheworld.amnh.org/Amphibia/Anura |title=Anura |last=Frost |first=Darrel R. |date=2021 |orig-date=1998 |website=Amphibian Species of the World |publisher=American Museum of Natural History |access-date=November 29, 2022 |quote=}}</ref> (coming from the [[Ancient Greek]] {{lang|grc|ἀνούρα}}, literally 'without tail'). Frog species with rough [[skin]] texture due to [[wart]]-like [[parotoid gland]]s tend to be called [[toad]]s, but the distinction between frogs and toads is informal and purely cosmetic, not from [[taxonomy (biology)|taxonomy]] or evolutionary history.
Frogs are widely distributed, ranging from the [[tropics]] to [[subarctic]] regions, but the greatest concentration of [[species diversity]] is in [[tropical rainforest]] and associated [[wetland]]s. They account for around 88% of extant amphibian species, and are one of the five most diverse [[vertebrate]] orders. The oldest fossil "proto-frog" ''[[Triadobatrachus]]'' is known from the [[Early Triassic]] of [[Madagascar]] (250{{nbsp}}[[Myr|million years ago]]), but [[molecular clock|molecular clock dating]] suggests their [[divergent evolution|divergence]] from other amphibians may extend further back to the [[Permian]], 265{{nbsp}}million years ago.
Adult frogs have a stout body, protruding [[eye]]s, anteriorly-attached [[tongue]], limbs folded underneath, and no [[tail]] (the "tail" of [[tailed frog]]s is an extension of the male [[cloaca]]). Frogs have [[gland]]ular skin, with [[secretion]]s ranging from distasteful to toxic. Their skin varies in colour from well-[[camouflage]]d dappled brown, grey and green, to vivid patterns of bright red or yellow and black to show toxicity and [[aposematism|ward off predators]]. Adult frogs live in both [[fresh water]] and on dry [[land]]; some species are adapted for [[burrowing|living underground]] or [[arboreal|in trees]]. As their skin is [[semi-permeable]], making them susceptible to [[dehydration]], they either live in moist [[ecological niche|niche]]s or have special [[adaptation]]s to deal with drier habitats. Frogs produce a wide range of [[animal communication|vocalisations]], particularly in their [[breeding season]], and exhibit many different kinds of complex [[ethology|behaviors]] to attract mates, to fend off predators and to generally survive.
Being [[oviparous]] [[anamniote]]s, frogs typically [[spawning|spawn]] their [[egg]]s in [[bodies of water]]. The eggs then hatch into fully aquatic [[larva]]e called [[tadpole]]s, which have tails and internal [[gill]]s. A few species lay eggs on land or bypass the tadpole stage altogether. Tadpoles have highly specialised rasping mouth parts suitable for [[herbivorous]], [[omnivorous]] or [[planktivorous]] diets. The [[biological life cycle|life cycle]] is completed when they [[metamorphosis|metamorphose]] into semiaquatic [[adult]]s capable of [[terrestrial locomotion]] and hybrid [[respiration (physiology)|respiration]] using both [[lung]]s aided by [[buccal pumping]] and [[cutaneous respiration|gas exchange across the skin]], and the larval tail regresses into an internal [[urostyle]]. Adult frogs generally have a [[carnivorous]] diet consisting of small [[invertebrates]], especially [[insect]]s, but omnivorous species exist and a few feed on plant matter. Frogs generally seize and ingest [[food]] by protruding their adhesive tongue and then swallow the item whole, often using their [[eyeball]]s and [[extraocular muscle]]s to help pushing down the throat, and their [[digestive system]] is extremely efficient at converting what they eat into body mass. Being [[trophic level|low-level]] [[consumer (food chain)|consumer]]s, both tadpoles and adult frogs are an important food source for other [[predator]]s and a vital part of the [[food web]] dynamics of many of the world's [[ecosystem]]s.
Frogs (especially [[frog legs|their muscular hindlimbs]]) are eaten by [[human]]s as food in many [[cuisine]]s, and also have many [[cultural]] roles in literature, symbolism and religion. They are [[bioindicator|environmental bellwethers]], with declines in frog populations considered early warning signs of [[environmental degradation]]. Global frog populations and diversities have declined significantly since the 1950s. More than one third of species are considered to be [[threatened species|threatened]] with [[extinct]]ion, and over 120 are believed to have become extinct since the 1980s. Frog malformations are on the rise as an emerging [[fungus|fungal]] disease, [[chytridiomycosis]], has spread around the world. [[Conservation biology|Conservation biologists]] are working to solve these problems.
==Etymology and taxonomy==
The use of the common names ''frog'' and ''[[toad]]'' has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family [[Bufonidae]] are considered "true toads". The use of the term ''frog'' in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins; the term ''toad'' generally refers to species that are terrestrial with dry, warty skins.<ref name=Salientia/><ref>{{cite book |title=Frogs |last=Badger |first=D. |author2=Netherton, J. |year=1995 |publisher=Airlife Publishing |isbn=978-1-85310-740-5 |page=19 }}</ref> There are numerous exceptions to this rule. The [[European fire-bellied toad]] (''Bombina bombina'') has a slightly warty skin and prefers a watery habitat<ref name=Bb>{{cite web |url=http://amphibiaweb.org/cgi/amphib_query?where-genus=Bombina&where-species=bombina |title=''Bombina bombina'' |author=Kuzmin, Sergius L. |date=September 29, 1999 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=June 15, 2012}}</ref> whereas the [[Panamanian golden frog]] (''Atelopus zeteki'') is in the toad family Bufonidae and has a smooth skin.<ref>{{cite iucn |url=https://www.iucnredlist.org/species/54563/54341110 |title=''Atelopus zeteki'' |author1=Lips, K |author2=Solís, F. |author3=Ibáñez, R. |author4=Jaramillo, C. |author5=Fuenmayor, Q. |year=2010 |access-date=August 2, 2012}}</ref>
=== Etymology ===
The origin of the order name ''Anura''—and its original spelling ''Anoures''—is the [[Ancient Greek]] [[alpha privative]] prefix {{wikt-lang|grc|ἀν-}} ({{grc-transl|ἀν-}} from {{wikt-lang|grc|ἀ-}} before a vowel) 'without',<ref>{{cite web|url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=a)1 |title=ἀ |last1=Liddell |first1=Henry George |last2=Scott |first2=Robert |work=A Greek-English Lexicon |publisher=Perseus Digital Library |date=1940 }}</ref> and {{wikt-lang|grc|οὐρά}} ({{grc-transl|οὐρά}}) 'animal tail'.<ref>{{cite web |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=ou)ra/ |title=οὐρά |last1=Liddell |first1=Henry George |last2=Scott |first2=Robert |work=A Greek-English Lexicon |publisher=Perseus Digital Library |date=1940 }}</ref> meaning "tailless". It refers to the tailless character of these amphibians.<ref name="BaillyBook">{{Cite book |last=Bailly |first=Anatole |date=1981 |title=Abrégé du dictionnaire grec français |___location=Paris |publisher=Hachette |isbn=978-2010035289 |oclc=461974285 }}</ref><ref name="BaillyWeb">{{Cite web |url=http://www.tabularium.be/bailly/ |title=Greek-french dictionary online |last=Bailly |first=Anatole |website=www.tabularium.be |access-date=December 9, 2018}}</ref><ref>{{cite OED|anuran, n. and adj.}}</ref>
The origins of the word ''frog'' are uncertain and debated.<ref name=":0">{{cite OED|frog, n.1 and adj.}}</ref> The word is first attested in [[Old English]] as {{lang|ang|frogga}}, but the usual Old English word for the frog was {{lang|ang|frosc}} (with variants such as {{lang|ang|frox}} and {{lang|ang|forsc}}), and it is agreed that the word ''frog'' is somehow related to this. Old English {{lang|ang|frosc}} remained in dialectal use in English as ''frosh'' and ''frosk'' into the nineteenth century,<ref>{{cite OED|frosh {{!}} frosk, n.1.}}</ref> and is paralleled widely in other [[Germanic languages]], with examples in the modern languages including [[German language|German]] {{lang|de|Frosch}}, [[Norwegian language|Norwegian]] {{lang|no|frosk}}, [[Icelandic language|Icelandic]] {{lang|is|froskur}}, and [[Dutch language|Dutch]] ''{{Not a typo|(kik)vors}}''.<ref name=":0" /> These words allow reconstruction of a [[Proto-Germanic language|Common Germanic]] ancestor {{lang|gem-x-proto|*froskaz}}.<ref>Jerzy Wełna, '[https://repozytorium.amu.edu.pl/bitstream/10593/18454/1/31welna.pdf Metathetic and Non-Metathetic Form Selection in Middle English]', ''Studia Anglica Posnaniensia'', 30 (2002), 501–18 (p. 504).</ref> The third edition of the ''[[Oxford English Dictionary]]'' finds that the etymology of {{lang|gem-x-proto|*froskaz}} is uncertain, but agrees with arguments that it could plausibly derive from a [[Proto-Indo-European language|Proto-Indo-European]] base along the lines of {{lang|ine-x-proto|*preu}}, meaning 'jump'.<ref name=":0" />
How Old English {{lang|ang|frosc}} gave rise to {{lang|ang|frogga}} is, however, uncertain, as the development does not involve a regular [[Sound change|sound-change]]. Instead, it seems that there was a trend in Old English to coin nicknames for animals ending in -''g'', with examples—themselves all of uncertain etymology—including ''dog'', ''hog'', ''pig, stag'', and ''{{Not a typo|(ear)wig}}''. ''Frog'' appears to have been adapted from {{lang|ang|frosc}} as part of this trend.<ref name=":0" />
Meanwhile, the word ''toad'', first attested as Old English {{lang|ang|tādige}}, is unique to English and is likewise of uncertain etymology.<ref>{{cite OED|toad, n.}}</ref> It is the basis for the word ''tadpole'', first attested as [[Middle English]] {{lang|enm|taddepol}}, apparently meaning 'toad-head'.<ref>{{cite OED|tadpole, n.1.}}</ref>
=== Taxonomy ===
About 88% of [[amphibian]] species are [[Biological classification|classified]] in the [[Order (biology)|order]] Anura.<ref name="Pou92">{{cite book |last1=Pough |first1=F. H. |last2=Andrews |first2=R. M. |last3=Cadle |first3=J. E. |last4=Crump |first4=M. L. |last5=Savitsky |first5=A. H. |last6=Wells |first6=K. D. |title=Herpetology |edition=third |year=2003 |publisher=Benjamin Cummings |isbn=978-0-13-100849-6 }}</ref> These include over 7,700 species<ref name="AOTW"/> in 59 [[Family (biology)|families]], of which the [[Hylidae]] (1062 spp.), [[Strabomantidae]] (807 spp.), [[Microhylidae]] (758 spp.), and [[True toad|Bufonidae]] (657 spp.) are the [[Species richness|richest in species]].<ref name=amdb>{{cite web |url=http://research.amnh.org/vz/herpetology/amphibia/Amphibia/Anura |title=Anura Fischer von Waldheim, 1813 |last=Frost |first=Darrel R. |year=2023 |work=Amphibian Species of the World: an Online Reference. Version 6.2 |publisher=American Museum of Natural History |access-date=November 12, 2024}}</ref>
[[File:Bombina bombina 1 (Marek Szczepanek) tight crop.jpg|thumb|left|alt=Dark-coloured toad facing left|[[European fire-bellied toad]] (''Bombina bombina'')]]
The Anura include all modern frogs and any [[fossil]] species that fit within the anuran definition. The characteristics of anuran adults include: 9 or fewer presacral vertebrae, the presence of a urostyle formed of fused vertebrae, no tail, a long and forward-sloping ilium, shorter fore limbs than hind limbs, [[Radius (bone)|radius]] and [[ulna]] fused, [[tibia]] and [[fibula]] fused, elongated [[ankle bone]]s, absence of a prefrontal bone, presence of a [[hyoid bone|hyoid plate]], a [[Mandible|lower jaw]] without teeth (with the exception of ''[[Gastrotheca guentheri]]'') consisting of three pairs of bones (angulosplenial, dentary, and mentomeckelian, with the last pair being absent in [[Pipoidea]]),<ref>Duellman, William E. {{Google books|CzxVvKmrtIgC|Biology of Amphibians|page=319}}</ref> an unsupported tongue, [[lymph]] spaces underneath the skin, and a muscle, the protractor lentis, attached to the [[Lens (anatomy)|lens of the eye]].<ref name=ToLAnura>{{cite web |url=http://www.tolweb.org/Anura/16963 |title=Anura |author=Cannatella, David |author-link=David C. Cannatella |date=January 11, 2008 |publisher=Tree of Life web project |access-date=August 8, 2012}}</ref> The anuran larva or tadpole has a single central respiratory [[Spiracle (vertebrates)|spiracle]] and mouthparts consisting of [[keratin]]ous beaks and [[Denticle (tooth feature)|denticles]].<ref name=ToLAnura/>
[[File:Atelopus zeteki1.jpg|thumb|alt=Panamanian golden frog|[[Panamanian golden frog]] (''Atelopus zeteki'')]]
Frogs and toads are broadly classified into three suborders: [[Archaeobatrachia]], which includes four families of primitive frogs; [[Mesobatrachia]], which includes five families of more evolutionary intermediate frogs; and [[Neobatrachia]], by far the largest group, which contains the remaining families of modern frogs, including most common species throughout the world. The suborder Neobatrachia is further divided into the two superfamilies [[Hyloidea]] and [[Ranoidea]].<ref>{{cite journal |last=Ford |first=L.S. |author2=Cannatella, D. C. |year=1993 |title=The major clades of frogs |journal=Herpetological Monographs |volume=7 |pages=94–117 |doi=10.2307/1466954 |jstor=1466954}}</ref> This classification is based on such [[Morphology (biology)|morphological]] features as the number of vertebrae, the structure of the [[pectoral girdle]], and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated.<ref>{{cite journal |last=Faivovich |first=J. |author2=Haddad, C. F. B. |author3=Garcia, P. C. A. |author4=Frost, D. R. |author5=Campbell, J. A. |author6=Wheeler, W. C. |title=Systematic review of the frog family Hylidae, with special reference to Hylinae: Phylogenetic analysis and revision |journal=Bulletin of the American Museum of Natural History |volume=294 |pages=1–240 |doi=10.1206/0003-0090(2005)294[0001:SROTFF]2.0.CO;2 |year=2005 |citeseerx=10.1.1.470.2967 |s2cid=83925199}}</ref>
Some species of anurans [[hybrid (biology)|hybridise]] readily. For instance, the [[edible frog]] (''Pelophylax esculentus'') is a hybrid between the [[pool frog]] (''P. lessonae'') and the [[marsh frog]] (''P. ridibundus'').<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Pelophylax&where-species=esculentus |title=''Pelophylax esculentus'' |author=Kuzmin, S. L. |date=November 10, 1999 |access-date=October 12, 2012}}</ref> The fire-bellied toads ''Bombina bombina'' and ''[[Yellow-bellied toad|B. variegata]]'' are similar in forming hybrids. These are less fertile than their parents, giving rise to a [[hybrid zone]] where the hybrids are prevalent.<ref>{{cite web |url=http://edoc.ub.uni-muenchen.de/1521/1/Koehler_Sonja.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://edoc.ub.uni-muenchen.de/1521/1/Koehler_Sonja.pdf |archive-date=October 9, 2022 |url-status=live |title=Mechanisms for partial reproductive isolation in a ''Bombina'' hybrid zone in Romania |last=Köhler |first=Sonja |year=2003 |work=Dissertation for thesis |access-date=June 5, 2012}}</ref>
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==Evolution==
The origins and evolutionary relationships between the three main groups of amphibians are hotly debated. A [[molecular phylogeny]] based on [[Ribosomal DNA|rDNA]] analysis dating from 2005 suggests that [[salamanders]] and [[caecilian]]s are more closely related to each other than they are to frogs and the [[Genetic divergence|divergence]] of the three groups took place in the [[Paleozoic]] or early [[Mesozoic]] before the break-up of the supercontinent [[Pangaea]] and soon after their divergence from the [[Sarcopterygii|lobe-finned fishes]]. This would help account for the relative scarcity of amphibian fossils from the period before the groups split.<ref>{{cite journal |author1=San Mauro, Diego |author2=Vences, Miguel |author3=Alcobendas, Marina |author4=Zardoya, Rafael |author5=Meyer, Axel |year=2005 |title=Initial Diversification of living amphibians predated the breakup of Pangaea |journal=The American Naturalist |volume=165 |issue=5 |pages=590–599 |jstor=429523 |doi=10.1086/429523 |pmid=15795855 |bibcode=2005ANat..165..590S |s2cid=17021360 |url=http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-33053 }}{{subscription required}}</ref> Another molecular phylogenetic analysis conducted about the same time concluded that [[lissamphibian]]s first appeared about 330 million years ago and that the [[Temnospondyli|temnospondyl-origin]] hypothesis is more credible than other theories. The [[neobatrachia]]ns seemed to have originated in Africa/India, the salamanders in East Asia and the caecilians in tropical Pangaea.<ref>{{cite journal |author1=Zhang, Peng |author2=Zhou, Hui |author3=Chen, Yue-Qin |author4=Liu, Yi-Fei |author5=Qu, Liang-Hu |year=2005 |title=Mitogenomic perspectives on the origin and phylogeny of living amphibians |journal=Systematic Biology |volume=54 |issue=3 |pages=391–400 |doi=10.1080/10635150590945278 |pmid=16012106 |doi-access=free }}</ref> Other researchers, while agreeing with the main thrust of this study, questioned the choice of calibration points used to synchronise the data. They proposed that the date of lissamphibian diversification should be placed in the [[Permian]], rather less than 300 million years ago, a date in better agreement with the palaeontological data.<ref>{{cite journal |author1=Marjanović, David |author2=Laurin, Michel |year=2007 |title=Fossils, molecules, divergence times, and the origin of lissamphibians |journal=Systematic Biology |volume=56 |issue=3 |pages=369–388 |doi=10.1080/10635150701397635 |pmid=17520502|doi-access=free }}</ref> A further study in 2011 using both extinct and living taxa sampled for morphological, as well as molecular data, came to the conclusion that Lissamphibia is [[Monophyly|monophyletic]] and that it should be nested within [[Lepospondyli]] rather than within [[Temnospondyli]]. The study postulated that Lissamphibia originated no earlier than the late [[Carboniferous]], some 290 to 305 million years ago. The split between Anura and [[Caudata]] was estimated as taking place 292 million years ago, rather later than most molecular studies suggest, with the caecilians splitting off 239 million years ago.<ref>{{cite journal |author=Pyron, R. Alexander |year=2011 |title=Divergence time estimation using fossils as terminal taxa and the origins of Lissamphibia |journal=Systematic Biology |volume=60 |issue=4 |pages=466–481 |doi=10.1093/sysbio/syr047 |pmid=21540408 |url=https://zenodo.org/record/889970 |doi-access=free }}</ref>
[[File:Fossilised frog.jpg|thumb|A fossilised frog from the [[Czech Republic]], possibly ''[[Palaeobatrachus gigas]]'']]
In 2008, ''[[Gerobatrachus hottoni]]'', a [[Temnospondyli|temnospondyl]] with many frog- and salamander-like characteristics, was discovered in [[Texas]]. It dated back 290 million years and was hailed as a [[Transitional fossil|missing link]], a [[Stem group|stem]] batrachian close to the [[common ancestor]] of frogs and salamanders, consistent with the widely accepted hypothesis that frogs and salamanders are more closely related to each other (forming a [[clade]] called Batrachia) than they are to caecilians.<ref>{{cite news |title="Frog-amander" fossil may be amphibian missing link |author=Casselman, Anne |url=http://news.nationalgeographic.com/news/2008/05/080521-frog-fossil.html |archive-url=https://web.archive.org/web/20080525210713/http://news.nationalgeographic.com/news/2008/05/080521-frog-fossil.html |url-status=dead |archive-date=May 25, 2008 |newspaper=National Geographic News |date=May 21, 2008 |access-date=July 5, 2012}}</ref><ref>{{cite journal |author1=Anderson, Jason S. |author2=Reisz, Robert R. |author3=Scott, Diane |author4=Fröbisch, Nadia B. |author5=Sumida, Stuart S. |year=2008 |title=A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders |journal=Nature |volume=453 |pages=515–518 |doi=10.1038/nature06865 |issue=7194 |pmid=18497824 |bibcode=2008Natur.453..515A |s2cid=205212809 }}</ref> However, others have suggested that ''Gerobatrachus hottoni'' was only a [[Dissorophoidea|dissorophoid]] temnospondyl unrelated to extant amphibians.<ref name="M&L09">{{cite journal |last1=Marjanović |first1=D. |last2=Laurin |first2=M. |year=2009 |title=The origin(s) of modern amphibians: a commentary |journal=Evolutionary Biology |volume=36 |issue=3 |pages=336–338 |doi=10.1007/s11692-009-9065-8 |bibcode=2009EvBio..36..336M |s2cid=12023942 |url=https://hal.archives-ouvertes.fr/hal-00549002/file/MARJANOVIC_David.pdf }}</ref>
[[Salientia]] (Latin ''salire'' (''salio''), "to jump") is the name of the total group that includes modern frogs in the order Anura as well as their close fossil relatives, the "proto-frogs" or "stem-frogs". The common features possessed by these proto-frogs include 14 [[Vertebral column|presacral vertebrae]] (modern frogs have eight or 9), a long and forward-sloping [[Ilium (bone)|ilium]] in the [[pelvis]], the presence of a [[Parietal bone|frontoparietal bone]], and a [[Mandible|lower jaw]] without teeth. The earliest known amphibians that were more closely related to frogs than to salamanders are ''[[Triadobatrachus massinoti]]'', from the early [[Triassic]] period of [[Madagascar]] (about 250 million years ago), and ''[[Czatkobatrachus polonicus]]'', from the Early Triassic of [[Poland]] (about the same age as ''Triadobatrachus'').<ref name=TOLweb>{{cite web |last=Cannatella |first=David |title=''Triadobatrachus massinoti'' |url=http://www.tolweb.org/Triadobatrachus_massinoti/16962 |work=Tree of Life |year=1995 |access-date=June 26, 2008 }}</ref> The skull of ''Triadobatrachus'' is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body with more [[vertebrae]]. The tail has separate vertebrae unlike the fused urostyle or coccyx in modern frogs. The tibia and fibula bones are also separate, making it probable that ''Triadobatrachus'' was not an efficient leaper.<ref name=TOLweb/> A 2019 study has noted the presence of Salientia from the [[Chinle Formation]], and suggested that anurans might have first appeared during the [[Late Triassic]].<ref>{{cite journal |author1=Michelle R. Stocker |author2=Sterling J. Nesbitt |author3=Ben T. Kligman |author4=Daniel J. Paluh |author5=Adam D. Marsh |author6=David C. Blackburn |author7=William G. Parker |year=2019 |title=The earliest equatorial record of frogs from the Late Triassic of Arizona |journal=Biology Letters |volume=15 |issue=2 |article-number=20180922 |doi=10.1098/rsbl.2018.0922 |pmid=30958136 |pmc=6405462 |hdl=10919/87931 }}</ref>
On the basis of fossil evidence, the earliest known "true frogs" that fall into the anuran lineage proper all lived in the early [[Jurassic]] period.<ref name=Salientia>{{cite web |url=http://tolweb.org/Salientia/ |title=Salientia |author=Cannatella, David C.|year=1997 |work=Tree of Life Web Project |access-date=August 7, 2012}}</ref><ref name="Heatwole00">{{cite book |title=Amphibian Biology: Paleontology: The Evolutionary History of Amphibians |editor1-last=Heatwole |editor1-first=H. |editor2-last=Carroll |editor2-first=R. L. |year=2000 |volume=4 |publisher=Surrey Beatty & Sons |isbn=978-0-949324-87-0 |chapter=14. Mesozoic Amphibians |last1=Roček |first1=Z. |pages=1295–1331 |chapter-url=http://rocek.gli.cas.cz/Reprints/AmphBiol3.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://rocek.gli.cas.cz/Reprints/AmphBiol3.pdf |archive-date=October 9, 2022 |url-status=live}}</ref> One such early frog species, ''[[Prosalirus bitis]]'', was discovered in [[1995 in paleontology|1995]] in the [[Kayenta Formation]] of [[Arizona]] and dates back to the [[Early Jurassic]] epoch (199.6 to 175 million years ago), making ''Prosalirus'' somewhat more recent than ''Triadobatrachus''.<ref name="jurassicdistribution">{{cite book |title=Dinosaur distribution (Early Jurassic, North America): The Dinosauria |editor1-last=Weishampel |editor1-first=D. B. |editor2-last=Dodson |editor2-first=P. |editor3-last=Osmólska |editor3-first=H. |year=2004 |publisher=University of California Press |isbn=978-0-520-24209-8 |pages=530–532 |edition=2nd }}</ref> Like the latter, ''Prosalirus'' did not have greatly enlarged legs, but had the typical three-pronged [[pelvic]] structure of modern frogs. Unlike ''Triadobatrachus'', ''Prosalirus'' had already lost nearly all of its tail<ref>{{cite journal |journal=Nature |volume=377 |pages=49–52 |year=1995 |doi=10.1038/377049a0 |title=An Early Jurassic jumping frog |author1=Shubin, N. H. |author2=Jenkins, F. A. Jr |issue=6544 |bibcode=1995Natur.377...49S |s2cid=4308225 }}</ref> and was well adapted for jumping.<ref name="foster-anura">{{cite book |last1=Foster |first1=J. |year=2007 |chapter=Anura (Frogs) |title=Jurassic west: the dinosaurs of the Morrison Formation and their world |publisher=Indiana University Press |pages=135–136 |isbn=978-0-253-34870-8 }}</ref> Another Early Jurassic frog is ''[[Vieraella herbsti]]'', which is known only from [[dorsum (biology)|dorsal]] and [[ventral]] impressions of a single animal and was estimated to be {{convert|33|mm|in|frac=8|abbr=on}} from snout to vent. ''[[Notobatrachus degiustoi]]'' from the [[middle Jurassic]] is slightly younger, about 155–170 million years old. The main evolutionary changes in this species involved the shortening of the body and the loss of the tail. Tadpoles of ''N. degiustoi'' constitute the oldest tadpoles found as of 2024, dating back to 168–161 million years ago. These tadpoles also showed adaptations for [[filter-feeding]], implying residence in temporary pools by filter-feeding larvae was already commonplace.<ref>{{Cite journal |last1=Chuliver |first1=Mariana |last2=Agnolín |first2=Federico L. |last3=Scanferla |first3=Agustín |last4=Aranciaga Rolando |first4=Mauro |last5=Ezcurra |first5=Martín D. |last6=Novas |first6=Fernando E. |last7=Xu |first7=Xing |date=2024-10-30 |title=The oldest tadpole reveals evolutionary stability of the anuran life cycle |url=https://www.nature.com/articles/s41586-024-08055-y |journal=Nature |volume=636 |issue=8041 |language=en |pages=138–142 |doi=10.1038/s41586-024-08055-y |pmid=39478214 |bibcode=2024Natur.636..138C |issn=1476-4687|url-access=subscription }}</ref> The evolution of modern Anura likely was complete by the Jurassic period. Since then, evolutionary changes in [[Ploidy|chromosome numbers]] have taken place about 20 times faster in mammals than in frogs, which means [[speciation]] is occurring more rapidly in mammals.<ref>{{cite journal |author1=Wilson, A. C. |author2=Sarich, V. M. |author3=Maxson, L. R. |year=1974 |title=The importance of gene rearrangement in evolution: evidence from studies on rates of chromosomal, protein, and anatomical evolution |journal=Proceedings of the National Academy of Sciences |volume=71 |issue=8 |pages=3028–3030 |doi=10.1073/pnas.71.8.3028 |pmid=4528784 |bibcode=1974PNAS...71.3028W |pmc=388613 |doi-access=free }}</ref>
According to genetic studies, the families [[Hyloidea]], [[Microhylidae]], and the clade [[Natatanura]] (comprising about 88% of living frogs) diversified simultaneously some 66 million years ago, soon after the [[Cretaceous–Paleogene extinction event]] associated with the [[Chicxulub impactor]]. All origins of arboreality (e.g. in Hyloidea and Natatanura) follow from that time and the resurgence of forest that occurred afterwards.<ref>{{cite news |title=Frog evolution linked to dinosaur asteroid strike |url=https://www.bbc.com/news/science-environment-40482039 |newspaper=BBC News |date=July 3, 2017 |access-date=July 3, 2017}}</ref><ref>{{cite journal |periodical=Proceedings of the National Academy of Sciences of the United States of America |title=Phylogenomics reveals rapid, simultaneous diversification of three major clades of Gondwanan frogs at the Cretaceous–Paleogene boundary |author1=Feng, Yan-Jie |author2=Blackburn, David C. |author3=Liang, Dan |author4=Hillis, David M. |author5=Wake, David B. |author6=Cannatella, David C. |author7=Zhang, Peng |year=2017 |doi=10.1073/pnas.1704632114 |pmc=5530686 |pmid=28673970 |volume=114 |issue=29 |pages=E5864–E5870 |bibcode=2017PNAS..114E5864F |doi-access=free}}</ref>
Frog fossils have been found on all of the Earth's continents.<ref>{{cite journal |last1=Evans |first1=S. E. |last2=Jones |first2=M. E. H. |last3=Krause |first3=D. W. |title=A giant frog with South American affinities from the Late Cretaceous of Madagascar |journal=Proceedings of the National Academy of Sciences |volume=105 |pages=2951–2956 |year=2008 |doi=10.1073/pnas.0707599105 |pmid=18287076 |issue=8 |pmc=2268566 |bibcode=2008PNAS..105.2951E |doi-access=free }}</ref><ref>{{cite journal |last1=Mörs |first1=Thomas |last2=Reguero |first2=Marcelo |last3=Vasilyan |first3=Davit |title=First fossil frog from Antarctica: implications for Eocene high latitude climate conditions and Gondwanan cosmopolitanism of Australobatrachia |journal=Scientific Reports |date=2020 |volume=10 |issue=1 |page=5051 |doi=10.1038/s41598-020-61973-5 |pmid=32327670 |pmc=7181706 |bibcode=2020NatSR..10.5051M |doi-access=free}}</ref> In 2020, it was announced that 40 million year old [[Calyptocephalellidae|helmeted frog]] fossils had been discovered by a team of vertebrate palaeontologists in [[Seymour Island]] on the [[Antarctic Peninsula]], indicating that this region was once home to frogs related to those now living in South American [[Valdivian temperate rain forest|''Nothofagus'' forest]].<ref>{{Cite news |last=Joel |first=Lucas |date=April 23, 2020 |title=Fossil Shows Cold-Blooded Frogs Lived on Warm Antarctica |language=en-US |work=The New York Times|url=https://www.nytimes.com/2020/04/23/science/frog-antarctica-fossil.html |archive-url=https://web.archive.org/web/20200423151008/https://www.nytimes.com/2020/04/23/science/frog-antarctica-fossil.html |archive-date=April 23, 2020 |url-access=subscription |url-status=live |access-date=May 13, 2020 |issn=0362-4331}}</ref>
===Phylogeny===
A [[cladogram]] showing the relationships of the different [[Family (biology)|families]] of frogs in the clade Anura can be seen in the table below. This diagram, in the form of a [[Phylogenetic tree|tree]], shows how each frog family is related to other families, with each node representing a point of common ancestry. It is based on Frost ''et al.'' (2006),<ref>{{Cite journal | last1 = Frost | first1 = D. R. | last2 = Grant | first2 = T. | last3 = Faivovich | first3 = J. N. | last4 = Bain | first4 = R. H. | last5 = Haas | first5 = A. | last6 = Haddad | first6 = C. L. F. B. | last7 = De Sá | first7 = R. O. | last8 = Channing | first8 = A. | last9 = Wilkinson | first9 = M. | last10 = Donnellan | first10 = S. C. | last11 = Raxworthy | first11 = C. J. | last12 = Campbell | first12 = J. A. | last13 = Blotto | first13 = B. L. | last14 = Moler | first14 = P. | last15 = Drewes | first15 = R. C. | last16 = Nussbaum | first16 = R. A. | last17 = Lynch | first17 = J. D. | last18 = Green | first18 = D. M. | last19 = Wheeler | first19 = W. C. | title = The Amphibian Tree of Life | journal = Bulletin of the American Museum of Natural History | volume = 297 | pages = 1–291| year = 2006 | doi = 10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2 | hdl = 2246/5781| s2cid = 86140137 | doi-access = free }}</ref> Heinicke ''et al.'' (2009)<ref>{{cite journal |author1=Heinicke M. P. |author2=Duellman, W. E. |author3=Trueb, L. |author-link3=Linda Trueb |author4=Means, D. B. |author5=MacCulloch, R. D. |author6=Hedges, S. B. | year = 2009 | title = A new frog family (Anura: Terrarana) from South America and an expanded direct-developing clade revealed by molecular phylogeny | url = http://www.mapress.com/zootaxa/2009/f/z02211p035f.pdf | journal= Zootaxa | volume = 2211 | pages = 1–35 |doi=10.11646/zootaxa.2211.1.1 }}</ref> and Pyron and Wiens (2011).<ref>{{cite journal |author1=R. Alexander Pyron |author2=John J. Wiens | year = 2011 | title= A large-scale phylogeny of Amphibia including over 2800 species, and a revised classification of extant frogs, salamanders, and caecilians | journal = Molecular Phylogenetics and Evolution | volume = 61 | issue = 2 | pages = 543–583 | doi=10.1016/j.ympev.2011.06.012| pmid = 21723399 | doi-access = free |bibcode=2011MolPE..61..543A }}</ref>
{{clade| style=font-size:80%;line-height:100%
|label1='''Anura'''
|1={{clade
|label1=
|1={{clade
|1=[[Leiopelmatidae]]
|2=[[Ascaphidae]]
}}
|label2=[[Bombianura]]
|2={{clade
|label1=[[Costata]]
|1={{clade
|1=[[Bombinatoridae]]
|label2=
|2={{clade
|1=[[Alytidae]]
|2=[[Discoglossidae]]
}}
}}
|label2=[[Pipanura]]
|2={{clade
|label1=[[Xenoanura]]
|1={{clade
|1=[[Pipidae]]
|2=[[Rhinophrynidae]]
}}
|label2=[[Acosmanura]]
|2={{clade
|label1=[[Anomocoela]]
|1={{clade
|1=[[Scaphiopodidae]]
|2={{clade
|1=[[Pelodytidae]]
|label2=[[Pelobatoidea]]
|2={{clade
|1=[[Pelobatidae]]
|2=[[Megophryidae]]
}}
}}
}}
|label2=[[Neobatrachia]]
|2=NEOBATRACHIA
}}
}}
}}
}}
<!----- The main frog cladogram has three subtrees: NEOBATRACHIA, HYLOIDEA, RANO------------------->
|targetA=NEOBATRACHIA
|subcladeA={{clade
|1=[[Heleophrynidae]]
|label2=[[Phthanobatrachia]]
|2={{clade
|1={{clade <!--dummy clade for reduced horizontal spacing-->
|label1=[[Sooglossoidea]]
|1={{clade
|1=[[Sooglossidae]]
|2=[[Nasikabatrachidae]]
}}
}}
|label2=
|2={{clade
|label1=[[Notogaeanura]]
|1={{clade
|label1=[[Australobatrachia]]
|1={{clade
|1=[[Calyptocephalellidae]]
|label2=
|2={{clade
|1=[[Myobatrachidae]]
|2=[[Limnodynastidae]]
}}
}}
|label2=[[Nobleoanura]]
|2=HYLOIDEA
}}
|label2=[[Ranoidea|Ranoides]]
|2=RANOIDEA
}}
}}
}} <!-- end NEOBATRACHIA subtree-->
|targetB=HYLOIDEA
|subcladeB={{clade
|1={{clade <!--dummy clade for reduced horizontal spacing-->
|label1=[[Brachycephaloidea]]
|sublabel1=([[Terrarana]])
|1={{clade
|1=[[Ceuthomantidae]]
|label2=
|2={{clade
|1=[[Brachycephalidae]]
|label2=
|2={{clade
|1=[[Eleutherodactylidae]]
|2=[[Craugastoridae]]
}}
}}
}}
}}
|2={{clade
|1=[[Hemiphractidae]]
|label2=[[Athesphatanura]]
|2={{clade
|1=[[Hylidae]]
|label2=[[Leptodactyliformes]]
|2={{clade
|label1=
|1={{clade
|1={{clade <!--dummy clade for reduced horizontal spacing-->
|label1=[[Agastorophrynia]]
|1={{clade
|1=[[Bufonidae]]
|2={{clade
|1=[[Aromobatidae]]
|2=[[Dendrobatidae]]
}}
}}
}}
|2={{clade
|1={{clade <!--dummy clade for reduced horizontal spacing-->
|label1=[[Diphyabatracea]]
|1={{clade
|1=[[Leptodactylidae]]
|2={{clade
|1=[[Allophrynidae]]
|2=[[Centrolenidae]]
}}
}}
}}
|2={{clade
|1=[[Ceratophryidae]]
|2={{clade
|1=[[Odontophrynidae]]
|2={{clade
|1={{clade
|1=[[Cycloramphidae]]
|2={{clade
|1=[[Alsodidae]]
|2=[[Hylodidae]]
}}
}}
|2={{clade
|1=[[Telmatobiidae]]
|2={{clade
|1=[[Batrachylidae]]
|2=[[Rhinodermatidae]]
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
}} <!-- end HYLOIDEA subtree-->
|targetC=RANOIDEA
|subcladeC={{clade
|label1=[[Allodapanura]]
|1={{clade
|1=[[Microhylidae]]
|label2=Afrobatrachia
|2={{clade
|label1=[[Xenosyneunitanura]]
|1={{clade
|1=[[Brevicipitidae]]
|2=[[Hemisotidae]]
}}
|label2=[[Laurentobatrachia]]
|2={{clade
|1=[[Hyperoliidae]]
|2=[[Arthroleptidae]]
}}
}}
}}
|label2=[[Natatanura]]
|2={{clade
|1=[[Ptychadenidae]]
|label2=[[Victoranura]]
|2={{clade
|1=[[Micrixalidae]]
|2={{clade
|1=[[Phrynobatrachidae]]
|2={{clade
|1=[[Conrauidae]]
|2={{clade
|1={{clade <!--dummy clade for reduced horizontal spacing-->
|label1=[[Pyxicephaloidea]]
|1={{clade
|1=[[Petropedetidae]]
|2=[[Pyxicephalidae]]
}}
}}
|2={{clade
|1={{clade
|1=[[Nyctibatrachidae]]
|2=[[Ceratobatrachidae]]
}}
|label2=[[Saukrobatrachia]]
|2={{clade
|1={{clade
|1=[[Ranixalidae]]
|2=[[Dicroglossidae]]
}}
|label2=[[Aglaioanura]]
|2={{clade
|1={{clade
|1=[[Rhacophoridae]]
|2=[[Mantellidae]]
}}
|2=[[Ranidae]]
}}
}}
}}
}}
}}
}}
}}
}}
}} <!-- end RANOIDEA subtree-->
}}
== Morphology and physiology ==
[[File:Frog limbs.jpg|thumb|right|A [[American bullfrog|bullfrog]] skeleton, showing elongated limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.]]
Frogs have no tail, except as larvae. Most frogs have long hind legs, elongated ankle bones, webbed toes, no claws, large eyes, and either smooth or warty skin. They have short vertebral columns, with no more than 10 free vertebrae and fused tailbones (urostyle or coccyx).<ref name=Flam>{{cite web |author=Flam, F. |year=1995 |title=Finding earliest true frog will help paleontologists understand how frog evolved its jumping ability |url=http://www.highbeam.com/doc/1G1-17454183.html |archive-url=https://web.archive.org/web/20130513111921/http://www.highbeam.com/doc/1G1-17454183.html |url-status=dead |archive-date=May 13, 2013 |publisher=Knight Ridder/Tribune News Service |access-date=June 10, 2012}}</ref> Frogs range in size from a [[snout–vent length]] of {{cvt|7.7|mm}} (the ''[[Paedophryne amauensis]]'' of [[Papua New Guinea]])<ref>{{cite news |title=Tiny frog claimed as world's smallest vertebrate |url=https://www.theguardian.com/environment/2012/jan/12/world-smallest-frog |newspaper=The Guardian |date=January 12, 2012 |access-date=September 28, 2012}}</ref> to about {{cvt|35|cm}} (the [[goliath frog]] (''Conraua goliath'') of central Africa, which is about {{cvt|3.3|kg}}).<ref name=Colgan1982>{{cite book | last=Colgan | first=P.V | year=1982 |title=The Guinness Book of Animal Facts and Feats | url=https://archive.org/details/guinnessbookofan00wood/ | edition=3 | publisher=Guinness Superlatives | pages=118–119 | isbn=978-0851122359 }}</ref> Some extinct prehistoric species were even larger.<ref>{{cite journal |author1=Otero, R.A. |author2=P. Jimenez-Huidobro |author3=S. Soto-Acuña |author4=R.E.Yury-Yáñez |year=2014 |title=Evidence of a giant helmeted frog (Australobatrachia, Calyptocephalellidae) from Eocene levels of the Magallanes Basin, southernmost Chile |journal=Journal of South American Earth Sciences |volume=55 |pages=133–140 |doi=10.1016/j.jsames.2014.06.010 |bibcode=2014JSAES..55..133O }}</ref>
===Feet and legs===
A frog's foot and leg structure is related to its habitat. Across species, these structures vary based on whether the species lives primarily on the ground, in water, in trees, or in burrows. Adult anurans have four fingers on the hands and five toes on the feet,<ref>[https://pubmed.ncbi.nlm.nih.gov/28597591/ Morphological Variation in Anuran Limbs: Constraints and Novelties]</ref> but the smallest species often have hands and feet where some of the digits are vestigial.<ref>[https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0213314 Morphological and ecological convergence at the lower size limit for vertebrates highlighted by five new miniaturised microhylid frog species from three different Madagascan genera]</ref> Frogs must be able to move quickly through their environment to catch prey and escape predators, and numerous adaptations help them to do so. Most frogs are either proficient jumpers or descend from ancestors that were, with much of the [[musculoskeletal]] [[morphology (biology)|morphology]] modified for this purpose. The tibia, fibula, and [[tarsus (skeleton)|tarsals]] have been fused into a single strong [[bone]], as have the radius and ulna in the fore limbs (which must absorb the impact on landing). The [[metatarsals]] have become elongated to add to the leg length, allowing frogs to push against the ground for a longer period on take-off. The [[ilium (bone)|ilium]] has elongated and formed a mobile joint with the [[sacrum]] which, in specialist jumpers such as ranids and hylids, functions as an additional limb joint to further power the leaps. The tail vertebrae have fused into a urostyle which is retracted inside the pelvis. This enables frogs to transfer force from the legs to the body during a leap.<ref name=Flam/>
[[File:Rana temporaria 04 by-dpc.jpg|thumb|alt=Webbed foot|Webbed hind foot of [[common frog]] <br /> (''Rana temporaria'')]]
[[File:Litoria tyleri.jpg|thumb|[[Tyler's tree frog]] (''Litoria tyleri'') has large toe pads and webbed feet.]]
The muscular system has been similarly modified. The hind limbs of ancestral frogs presumably contained pairs of muscles which would act in opposition (one muscle to flex the knee, a different muscle to extend it), as is seen in most other limbed animals. However, in modern frogs, almost all muscles have been modified to contribute to the action of jumping, with only a few small muscles remaining to bring the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the main leg muscles accounting for over 17% of the total mass of frogs.<ref name=Frogjump/>
Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water.<ref name=Exploratorium>{{cite web |url=http://www.exploratorium.edu/frogs/mainstory/ |title=The amazing adaptable frog |author=Tesler, P. |year=1999 |publisher=Exploratorium:: The museum of science, art and human perception |access-date=June 4, 2012}}</ref> The completely aquatic [[African dwarf frog]] (''Hymenochirus'' sp.) has fully webbed toes, whereas those of [[White's tree frog]] (''Litoria caerulea''), an arboreal species, are only a quarter or half webbed.<ref>{{cite web |author=Vincent, L. |year=2001 |publisher=James Cook University |title=''Litoria caerulea'' |url=http://www.jcu.edu.au/school/tbiol/zoology/herp/Litoriacaerulea.PDF |archive-url=https://web.archive.org/web/20040422212337/http://www.jcu.edu.au/school/tbiol/zoology/herp/Litoriacaerulea.PDF |archive-date=April 22, 2004 |access-date=August 3, 2012 }}</ref> Exceptions include [[flying frog]]s in the [[Hylidae]] and [[Rhacophoridae]], which also have fully webbed toes used in gliding.
[[Tree frog|Arboreal frogs]] have pads located on the ends of their toes to help grip vertical surfaces. These are not suction pads, the surface consisting instead of columnar cells with flat tops with small gaps between them lubricated by mucous glands. When the frog applies pressure, the cells adhere to irregularities on the surface and the grip is maintained through [[adhesion]]. This allows the frog to climb on smooth surfaces, but the system does not function efficiently when the pads are excessively wet.<ref>{{cite journal |last=Emerson |first=S. B. |author2=Diehl, D. |year=1980| title=Toe pad morphology and mechanisms of sticking in frogs |journal=Biological Journal of the Linnean Society |volume=13 |issue=3 |pages=199–216 |doi=10.1111/j.1095-8312.1980.tb00082.x}}</ref>
In many arboreal frogs, a small "intercalary structure" on each toe increases the surface area touching the [[Substrate (biology)|substrate]]. Furthermore, many arboreal frogs have hip joints that allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes. This allows the frogs to "parachute" or make a controlled glide from one position in the canopy to another.<ref>{{cite journal |last=Harvey |first=M. B. |author2= Pemberton, A. J. |author3=Smith, E. N. |year=2002 |title=New and poorly known parachuting frogs (Rhacophoridae : ''Rhacophorus'') from Sumatra and Java |journal=Herpetological Monographs |volume=16 |pages=46–92 |doi=10.1655/0733-1347(2002)016[0046:NAPKPF]2.0.CO;2 |s2cid=86616385 }}</ref>
Ground-dwelling frogs generally lack the adaptations of aquatic and arboreal frogs. Most have smaller toe pads, if any, and little webbing. Some burrowing frogs such as [[Couch's Spadefoot Toad|Couch's spadefoot]] (''Scaphiopus couchii'') have a flap-like toe extension on the hind feet, a [[keratin]]ised [[tubercle]] often referred to as a spade, that helps them to burrow.<ref>{{cite web |url=http://www.desertmuseum.org/books/nhsd_spadefoot.php |title=Couch's spadefoot (''Scaphiopus couchi'') |publisher=Arizona-Sonora Desert Museum |access-date=August 3, 2012}}</ref>
Sometimes during the tadpole stage, one of the developing rear legs is eaten by a predator such as a [[dragonfly nymph]]. In some cases, the full leg still grows, but in others it does not, although the frog may still live out its normal lifespan with only three limbs. Occasionally, a parasitic [[flatworm]] (''[[Ribeiroia ondatrae]]'') digs into the rear of a tadpole, causing a rearrangement of the limb bud cells and the frog develops one or more extra legs.<ref>{{cite news |url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8116000/8116692.stm |title=Legless frogs mystery solved |work=BBC News |date=June 25, 2009 |first=M. |last=Walker}}</ref>
[[File:Ranapipiensmoulting.jpg|thumb|left|[[Northern leopard frog]] (''Rana pipiens'') moulting and eating its skin]]
===Skin===
A frog's skin is protective, has a respiratory function, can absorb water, and helps control body temperature. It has many glands, particularly on the head and back, which often exude distasteful and toxic substances ([[Skin|granular glands]]). The secretion is often sticky and helps keep the skin moist, protects against the entry of moulds and bacteria, and makes the animal slippery and more able to escape from predators.<ref>{{cite book |title=A Natural History of Amphibians |last=Stebbins |first=Robert C. |author-link1=Robert C. Stebbins |author2=Cohen, Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=10–14 }}</ref> The skin is shed every few weeks. It usually splits down the middle of the back and across the belly, and the frog pulls its arms and legs free. The sloughed skin is then worked towards the head where it is quickly eaten.<ref>{{cite journal |jstor=2456779 |pages=530–540 |last1=Frost |first1=S. W. |title=Notes on feeding and molting in frogs |volume=66 |issue=707 |journal=The American Naturalist |year=1932 |doi=10.1086/280458 |bibcode=1932ANat...66..530F |s2cid=84796411 }}</ref>
Being cold-blooded, frogs have to adopt suitable behaviour patterns to regulate their temperature. To warm up, they can move into the sun or onto a warm surface; if they overheat, they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This posture is also used to prevent water loss and involves the frog squatting close to the substrate with its hands and feet tucked under its chin and body.<ref name=BandN>{{cite book |title=Frogs |last=Badger |first=D. |author2=Netherton, J. |year=1995 |publisher=Airlife Publishing |isbn=978-1-85310-740-5 |page=27 }}</ref> The colour of a frog's skin is used for thermoregulation. In cool damp conditions, the colour will be darker than on a hot dry day. The [[Grey Foam-nest Treefrog|grey foam-nest tree frog]] (''Chiromantis xerampelina'') is even able to turn white to minimise the chance of overheating.<ref>{{cite book |title=Amphibians and Their Ways |last=Smyth |first=H. R. |year=1962 |publisher=Macmillan |isbn=978-0-02-612190-3 }}</ref>
Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area, but the permeability of a frog's skin can also result in water loss. Glands located all over the body exude mucus which helps keep the skin moist and reduces evaporation. Some glands on the hands and chest of males are specialised to produce sticky secretions to aid in [[amplexus]]. Similar glands in tree frogs produce a glue-like substance on the adhesive discs of the feet. Some arboreal frogs reduce water loss by having a waterproof layer of skin, and several South American species coat their skin with a waxy secretion. Other frogs have adopted behaviours to conserve water, including becoming [[nocturnal]] and resting in a water-conserving position. Some frogs may also rest in large groups with each frog pressed against its neighbours. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss.<ref name=BandN/> [[Woodhouse's toad]] (''Bufo woodhousii''), if given access to water after confinement in a dry ___location, sits in the shallows to rehydrate.<ref>{{cite book |title=The Frog Book: North American Frogs and Toads |last=Dickerson |first=M. C. |year=1969 |publisher=Dover Publications |isbn=978-0-486-21973-8 |url=https://archive.org/details/frogbooknorthame00dick_0 }}</ref> The male [[hairy frog]] (''Trichobatrachus robustus'') has [[dermal papillae]] projecting from its lower back and thighs, giving it a bristly appearance. These contain blood vessels and are thought to increase the area of the skin available for respiration.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Trichobatrachus |title=''Trichobatrachus robustus'' |author=Blackburn, D. C. |date=November 14, 2002 |publisher=AmphibiaWeb |access-date=August 18, 2012}}</ref>
[[File:Hip-pocket Frog - Assa darlingtoni.jpg|thumb|alt=Frog barely recognisable against brown decaying leaf litter.|[[Pouched frog]] (''Assa darlingtoni'') camouflaged against leaf litter]]
[[File:WoodFrog DarienLakesStatePark 2020-06-16 (02).jpg|thumb|[[Wood frog]] (''Lithobates sylvaticus'') uses [[disruptive coloration]].]]
Some species have [[Osteoderm|bony plates]] embedded in the skin, a trait that appears to have evolved independently several times.<ref>{{cite journal |author1=Ruibal, Rodolfo |author2=Shoemaker, Vaughan |year=1985 |title=Osteoderms in Anurans |journal=Journal of Herpetology |volume=18 |issue=3 |pages=313–328 |jstor=1564085 |doi=10.2307/1564085}}</ref> In certain other species, the skin at the top of the head is compacted and the connective tissue of the dermis is co-ossified with the bones of the skull ([[exostosis]]).<ref>{{cite book |title=Herpetology: An Introductory Biology of Amphibians and Reptiles |last1=Vitt |first1=Laurie J. |last2=Caldwell |first2=Janalee P. |year=2013 |publisher=Academic Press |isbn=9780123869203 |page=50 |url={{Google books|Gay9N_ry79kC|page=50|plainurl=yes}} }}</ref><ref>{{cite journal |author1=Jared, C. |author2=Antoniazzi, M. M. |author3=Navas, C. A. |author4=Katchburian, E. |author5=Freymüller, E. |author6=Tambourgi, D. V. |author7=Rodrigues, M. T. |year=2005 |title=Head co-ossification, phragmosis and defence in the casque-headed tree frog ''Corythomantis greeningi'' |journal=Journal of Zoology |volume=265 |issue=1 |pages=1–8 |doi=10.1017/S0952836904005953 |s2cid=59449901 }}</ref>
[[Camouflage]] is a common defensive mechanism in frogs. Features such as warts and [[skin fold]]s are usually on ground-dwelling frogs, for whom smooth skin would not provide such effective camouflage. Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.<ref name=Observer>{{cite book |title=The Observer's Book of British Wild Animals |last=Burton |first=Maurice |year=1972 |publisher=Frederick Warne & Co |isbn=978-0-7232-1503-5 |pages=204–209 }}</ref> Some are even able to control their skin texture.<ref>{{Cite journal|last1=Guayasamin|first1=Juan M.|last2=Krynak|first2=Tim|last3=Krynak|first3=Katherine|last4=Culebras|first4=Jaime|last5=Hutter|first5=Carl R.|date=2015|title=Phenotypic plasticity raises questions for taxonomically important traits: a remarkable new Andean rainfrog ( Pristimantis ) with the ability to change skin texture: Phenotypic plasticity in Andean rainfrog|journal=Zoological Journal of the Linnean Society|language=en|volume=173|issue=4|pages=913–928|doi=10.1111/zoj.12222|doi-access=free}}</ref> The [[Pacific tree frog]] (''Pseudacris regilla'') has green and brown morphs, plain or spotted, and changes colour depending on the time of year and general background colour.<ref>{{cite journal |author1=Wente, W. H. |author2=Phillips, J. B. |year=2003 |title=Fixed green and brown color morphs and a novel color-changing morph of the Pacific tree frog ''Hyla regilla'' |journal=The American Naturalist |volume=162 |issue=4 |pages=461–473 | jstor=10.1086/378253 |doi=10.1086/378253 |pmid=14582008 |bibcode=2003ANat..162..461W |s2cid=25692966 }}</ref> The [[Wood frog]] (''Lithobates sylvaticus'') uses [[disruptive coloration]] including black eye markings similar to voids between leaves, bands of the dorsal skin (dorsolateral dermal plica) similar to a leaf [[midrib]] as well as stains, spots and leg stripes similar to fallen leaf features.
===Respiration and circulation===
Like other amphibians, [[oxygen]] can pass through their highly permeable skins. This unique feature allows them to remain in places without access to the air, respiring through their skins. Ribs are generally absent, so the lungs are filled by [[buccal pumping]] and a frog deprived of its lungs can maintain its body functions without them.<ref name=Observer/> The fully aquatic [[Bornean flat-headed frog]] (''Barbourula kalimantanensis'') is the first frog known to lack lungs entirely.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Barbourula&where-species=kalimantanensis |title=''Barbourula kalimantanensis'' |author=Boisvert, Adam |date=October 23, 2007 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 9, 2012}}</ref>
Frogs have three-chambered [[heart]]s, a feature they share with [[lizard]]s. Oxygenated blood from the lungs and de-oxygenated blood from the [[Respiration (physiology)|respiring]] tissues enter the heart through separate [[atrium (anatomy)|atria]]. When these chambers contract, the two blood streams pass into a common [[Ventricle (heart)|ventricle]] before being pumped via a spiral valve to the appropriate vessel, the [[aorta]] for oxygenated blood and [[pulmonary artery]] for deoxygenated blood.<ref name=Kimball>{{cite web |url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalHearts.html |title=Animal Circulatory Systems: Three Chambers: The Frog and Lizard |author=Kimball, John |year=2010 |work=Kimball's Biology Pages |access-date=July 9, 2012 |archive-url= https://web.archive.org/web/20160511032359/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalHearts.html |archive-date= May 11, 2016}}</ref>
Some species of frog have adaptations that allow them to survive in oxygen deficient water. The [[Titicaca water frog]] (''Telmatobius culeus'') is one such species and has wrinkly skin that increases its surface area to enhance gas exchange. It normally makes no use of its rudimentary lungs but will sometimes raise and lower its body rhythmically while on the lake bed to increase the flow of water around it.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Telmatobius&where-species=culeus |title=''Telmatobius culeus'' |author=Lee, Deborah |date=April 23, 2010 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 9, 2012}}</ref>
[[File:Frog anatomy tags.PNG|thumb|right|alt=Dissected frog|Anatomical model of a dissected frog: 1 Right atrium, 2 Lungs, 3 Aorta, 4 Egg mass, 5 Colon, 6 Left atrium, 7 Ventricle, 8 Stomach, 9 Liver, 10 Gallbladder, 11 Small intestine, 12 Cloaca]]
===Digestion and excretion===
Frogs have maxillary teeth along their upper jaw which are used to hold food before it is swallowed. These teeth are very weak, and cannot be used to chew or catch and harm agile prey. Instead, the frog uses its sticky, cleft tongue to catch insects and other small moving prey. The tongue normally lies coiled in the mouth, free at the back and attached to the mandible at the front. It can be shot out and retracted at great speed.<ref name=Exploratorium/> In amphibians there are salivary glands on the tongue, which in frogs produce what is called a two-phase viscoelastic fluid. When exposed to pressure, like when the tongue is wrapping around a prey, it becomes runny and covers the prey's body. As the pressure drops, it returns to a thick and elastic state, which gives the tongue an extra grip.<ref>[https://stao.ca/the-secret-to-the-stickiness-of-frog-spit/ The Secret to the Stickiness of Frog Spit | STAO]</ref> Some frogs have no tongue and just stuff food into their mouths with their hands.<ref name=Exploratorium/> The African bullfrog (''[[Pyxicephalus]]''), which preys on relatively large animals such as mice and other frogs, has cone shaped bony projections called odontoid processes at the front of the lower jaw which function like teeth.<ref name="Pou92"/> The eyes assist in the swallowing of food as they can be retracted through holes in the skull and help push food down the throat.<ref name=Exploratorium/><ref>{{cite journal |last1=Levine |first1=R. P. |last2=Monroy |first2=J. A. |last3=Brainerd|first3=E. L. |title=Contribution of eye retraction to swallowing performance in the northern leopard frog, ''Rana pipiens'' |doi=10.1242/jeb.00885|pmid=15010487 |date=March 15, 2004|pages=1361–1368 |issue=Pt 8|volume=207 |journal=Journal of Experimental Biology |doi-access=free |bibcode=2004JExpB.207.1361L }}</ref>
The food then moves through the oesophagus into the stomach where digestive enzymes are added and it is churned up. It then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Pancreatic juice from the pancreas, and bile, produced by the liver and stored in the gallbladder, are secreted into the small intestine, where the fluids digest the food and the nutrients are absorbed. The food residue passes into the large intestine where excess water is removed and the wastes are passed out through the [[cloaca]].<ref>{{cite web |url=http://www.tutorvista.com/biology/frog-digestive-system-diagram |title=Frog Digestive System |year=2010 |publisher=TutorVista.com |access-date=August 4, 2012|archive-url=https://web.archive.org/web/20100603080405/http://www.tutorvista.com/biology/frog-digestive-system-diagram|archive-date=June 3, 2010|url-status=dead}}</ref>
Although adapted to terrestrial life, frogs resemble freshwater fish in their inability to conserve body water effectively. When they are on land, much water is lost by evaporation from the skin. The excretory system is similar to that of mammals and there are two [[kidney]]s that remove nitrogenous products from the blood. Frogs produce large quantities of dilute urine in order to flush out toxic products from the kidney tubules.<ref name=Doritexcretion>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last=Dorit |first=R. L. |author2=Walker, W. F.|author3=Barnes, R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |page=[https://archive.org/details/zoology0000dori/page/849 849] }}</ref> The nitrogen is excreted as [[ammonia]] by tadpoles and aquatic frogs but mainly as [[urea]], a less toxic product, by most terrestrial adults. A few species of tree frog with little access to water excrete the even less toxic [[uric acid]].<ref name=Doritexcretion/> The urine passes along paired [[ureter]]s to the [[urinary bladder]] from which it is vented periodically into the cloaca. All bodily wastes exit the body through the cloaca which terminates in a cloacal vent.<ref name=TutorVista>{{cite web |url=http://www.tutorvista.com/content/biology/biology-iii/animal-morphology/respiratory-excretory-nervous-reproductive-system-frog.php |title=Frog's internal systems |year=2010 |publisher=TutorVista.com |access-date=June 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20080121101351/http://www.tutorvista.com/content/biology/biology-iii/animal-morphology/respiratory-excretory-nervous-reproductive-system-frog.php |archive-date=January 21, 2008}}</ref>
===Reproductive system===
{{See also|Sexual selection in amphibians}}
In the male frog, the two [[Testis|testes]] are attached to the kidneys and [[semen]] passes into the kidneys through fine tubes called [[efferent ducts]]. It then travels on through the ureters, which are consequently known as urinogenital ducts. There is no penis, and sperm is ejected from the cloaca directly onto the eggs as the female lays them. The ovaries of the female frog are beside the kidneys and the eggs pass down a pair of oviducts and through the cloaca to the exterior.<ref name=TutorVista/>
When frogs mate, the male climbs on the back of the female and wraps his fore limbs round her body, either behind the front legs or just in front of the hind legs. This position is called [[amplexus]] and may be held for several days.<ref>{{cite book | last1=Duellman |first1=William E. |last2=Trueb |first2=Linda |author-link2=Linda Trueb | date = 1986 | title = Biology of Amphibians | ___location = New York | publisher = McGraw-Hill Publishing Company }}</ref> The male frog has certain hormone-dependent [[secondary sexual characteristic]]s. These include the development of special pads on his thumbs in the breeding season, to give him a firm hold.<ref>{{cite web |url=http://www2.southeastern.edu/Academics/Faculty/dsever/SeverandStaub.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www2.southeastern.edu/Academics/Faculty/dsever/SeverandStaub.pdf |archive-date=October 9, 2022 |url-status=live |title=Hormones, sex accessory structures, and secondary sexual characteristics in amphibians |author1=Sever, David M. |author2=Staub, Nancy L. |pages=83–98 |work=Hormones and Reproduction of Vertebrates – Vol 2: Amphibians |access-date=August 4, 2012}}</ref> The grip of the male frog during amplexus stimulates the female to release eggs, usually wrapped in jelly, as spawn. In many species the male is smaller and slimmer than the female. Males have vocal cords and make a range of croaks, particularly in the breeding season, and in some species they also have [[vocal sac]]s to amplify the sound.<ref name=TutorVista/>
===Nervous system===
Frogs have a highly developed nervous system that consists of a brain, spinal cord and nerves. Many parts of frog brains correspond with those of humans. It consists of two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a cerebellum and a medulla oblongata. Muscular coordination and posture are controlled by the [[cerebellum]], and the [[medulla oblongata]] regulates respiration, digestion and other automatic functions. The relative size of the [[cerebrum]] in frogs is much smaller than it is in humans. Frogs have ten pairs of [[cranial nerves]] which pass information from the outside directly to the brain, and ten pairs of [[spinal nerves]] which pass information from the extremities to the brain through the spinal cord.<ref name=TutorVista/> By contrast, all [[amniotes]] (mammals, birds and reptiles) have twelve pairs of cranial nerves.<ref>{{cite web |url=http://tolweb.org/Amniota |title=Amniota |author1=Laurin, Michel |author2=Gauthier, Jacques A. |year=2012 |publisher=Tree of Life Web Project |access-date=August 4, 2012}}</ref>
[[File:Groene kikker achter Bekaert-draad-detail oog.jpg|thumb|left|Close-up of frog's head showing eye, nostril, mouth, and tympanum]]
===Sight===
The eyes of most frogs are located on either side of the head near the top and project outwards as hemispherical bulges. They provide [[binocular vision]] over a field of 100° to the front and a total visual field of almost 360°.<ref name="Howard">{{cite book |last1=Howard |first1=Ian P. |url={{Google books|I8vqITdETe0C|page=651|plainurl=yes}} |title=Binocular Vision and Stereopsis |last2=Rogers |first2=Brian J. |publisher=Oxford University Press |year=1995 |isbn=978-0195084764 |page=651}}</ref> They may be the only part of an otherwise submerged frog to protrude from the water. Each eye has closable upper and lower lids and a [[nictitating membrane]] which provides further protection, especially when the frog is swimming.<ref name=Badger>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing Ltd |isbn=978-1-85310-740-5 |pages=31–35 }}</ref> Members of the aquatic family [[Pipidae]] have the eyes located at the top of the head, a position better suited for detecting prey in the water above.<ref name=Howard/> The [[iris (anatomy)|iris]]es come in a range of colours and the pupils in a range of shapes. The [[common toad]] (''Bufo bufo'') has golden irises and horizontal slit-like pupils, the [[Agalychnis callidryas|red-eyed tree frog]] (''Agalychnis callidryas'') has vertical slit pupils, the [[poison dart frog]] has dark irises, the [[fire-bellied toad]] (''Bombina spp.'') has triangular pupils and the [[Dyscophus antongilii|tomato frog]] (''Dyscophus spp.'') has circular ones. The irises of the [[southern toad]] (''Anaxyrus terrestris'') are patterned so as to blend in with the surrounding camouflaged skin.<ref name=Badger/>
The distant vision of a frog is better than its near vision. Calling frogs will quickly become silent when they see an intruder or even a moving shadow but the closer an object is, the less well it is seen.<ref name=Badger/> When a frog shoots out its tongue to catch an insect it is reacting to a small moving object that it cannot see well and must line it up precisely beforehand because it shuts its eyes as the tongue is extended.<ref name=Exploratorium/> Although it was formerly debated,<ref>{{cite journal|title=Vision in frogs|last=Muntz|first=W. R. A.|author2=Scientific American Offprints|journal=Scientific American|publisher=W. H. Freeman|year=1964|volume=210|issue=3|pages=110–9|doi=10.1038/scientificamerican0364-110|pmid=14133069|bibcode=1964SciAm.210c.110M|asin=B0006RENBO|oclc=15304238}}</ref> more recent research has shown that frogs can see in colour, even in very low light.<ref>{{Cite journal|last1=Kelber|first1=Almut|last2=Yovanovich|first2=Carola|last3=Olsson|first3=Peter|date=April 5, 2017|title=Thresholds and noise limitations of colour vision in dim light|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|language=en|volume=372|issue=1717|pages=20160065|doi=10.1098/rstb.2016.0065|issn=0962-8436|pmc=5312015|pmid=28193810}}</ref>
{{Clear}}
===Hearing===
[[File:Synchrotron microtomography of Atelopus franciscus head - pone.0022080.s003.ogv|thumb|Surface rendering of the head of the frog ''[[Atelopus franciscus]]'', with ear parts highlighted]]
Frogs can hear both in the air and below water. They do not have [[Pinna (anatomy)|external ears]]; the eardrums ([[Tympanum (anatomy)|tympanic membranes]]) are directly exposed or may be covered by a layer of skin and are visible as a circular area just behind the eye. The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls. In some species such as the bullfrog, the size of the tympanum indicates the sex of the frog; males have tympani that are larger than their eyes while in females, the eyes and tympani are much the same size.<ref name=Neth38>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing |isbn=978-1-85310-740-5 |page=38 }}</ref> A noise causes the tympanum to vibrate and the sound is transmitted to the middle and inner ear. The middle ear contains semicircular canals which help control balance and orientation. In the inner ear, the auditory hair cells are arranged in two areas of the cochlea, the basilar papilla and the amphibian papilla. The former detects high frequencies and the latter low frequencies.<ref name=Cohen>{{cite book |title=A Natural History of Amphibians |last1=Stebbins |first1=Robert C.|author-link1=Robert C. Stebbins| last2=Cohen|first2= Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=67–69 }}</ref> Because the cochlea is short, frogs use [[electrical tuning]] to extend their range of audible frequencies and help discriminate different sounds.<ref>{{cite journal |author1=Armstrong, Cecilia E. |author2=Roberts, William M. |year=1998 |title=Electrical properties of frog saccular hair cells: distortion by enzymatic dissociation |journal=Journal of Neuroscience |volume=18 |issue=8 |pages= 2962–2973 |pmid=9526013 |doi=10.1523/JNEUROSCI.18-08-02962.1998 |pmc=6792591 |doi-access=free }}</ref> This arrangement enables detection of the territorial and breeding calls of their [[Conspecificity|conspecifics]]. In some species that inhabit arid regions, the sound of thunder or heavy rain may arouse them from a dormant state.<ref name=Cohen/> A frog may be startled by an unexpected noise but it will not usually take any action until it has located the source of the sound by sight.<ref name=Neth38/>
===Call===
{{See also|Sexual selection in frogs}}
[[File:Dendropsophus microcephalus - calling male (Cope, 1886).jpg|thumb|A male ''[[Dendropsophus microcephalus]]'' displaying its vocal sac during its call]]
[[File:Atelopus franciscus male territorial call - pone.0022080.s002.oga|thumb|Advertisement call of male ''[[Atelopus franciscus]]'']]
[[File:Frogs croak calling chorus at night.ogg|thumb|frogs croak]]
The call or croak of a frog is unique to its species. Frogs create this sound by passing air through the [[larynx]] in the throat. In most calling frogs, the sound is amplified by one or more vocal sacs, membranes of skin under the throat or on the corner of the mouth, that distend during the amplification of the call. Some frog calls are so loud that they can be heard up to a mile (1.6{{nbsp}}km) away.<ref>{{cite web |url=http://www.dnr.state.oh.us/Home/species_a_to_z/SpeciesGuideIndex/bullfrog/tabid/6576/Default.aspx |title=Bullfrog |publisher=Ohio Department of Natural Resources |access-date=June 19, 2012 |archive-date=August 18, 2012 |archive-url=https://web.archive.org/web/20120818105255/http://www.dnr.state.oh.us/Home/species_a_to_z/SpeciesGuideIndex/bullfrog/tabid/6576/Default.aspx |url-status=dead }}</ref> Additionally, some species have been found to use man-made structures such as drain pipes for artificial amplification of their call.<ref>{{Cite journal|last1=Tan|first1=W.-H.|last2=Tsai|first2=C.-G.|last3=Lin|first3=C.|last4=Lin|first4=Y. K.|date=June 5, 2014|title=Urban canyon effect: storm drains enhance call characteristics of the Mientien tree frog|journal=Journal of Zoology|language=en|volume=294|issue=2|pages=77–84|doi=10.1111/jzo.12154|issn=0952-8369|url=http://ntur.lib.ntu.edu.tw//handle/246246/260607|url-access=subscription}}</ref> The [[Ascaphus truei|coastal tailed frog]] (''Ascaphus truei'') lives in mountain streams in North America and does not vocalise.<ref>{{cite web |url=http://www.californiaherps.com/frogs/pages/a.truei.html |title=''Ascaphus truei'': Coastal Tailed Frog |author=Nafis, Gary |year=2012 |publisher=California Herps |access-date=June 19, 2012}}</ref>
The main function of calling is for male frogs to attract mates. Males may call individually or there may be a chorus of sound where numerous males have converged on breeding sites. In many frog species, such as the [[Common Tree Frog|common tree frog]] (''Polypedates leucomystax''), females reply to males' calls, which acts to reinforce reproductive activity in a breeding colony.<ref>{{cite journal| last=Roy| first=Debjani| year=1997| title=Communication signals and sexual selection in amphibians| journal=Current Science| volume=72| pages=923–927| url=http://www.ias.ac.in/jarch/currsci/72/00000944.pdf| url-status=dead| archive-url=https://web.archive.org/web/20120923071112/http://www.ias.ac.in/jarch/currsci/72/00000944.pdf| archive-date=September 23, 2012}}</ref> Female frogs prefer males that produce sounds of greater intensity and lower frequency, attributes that stand out in a crowd. The rationale for this is thought to be that by demonstrating his prowess, the male shows his fitness to produce superior offspring.<ref>{{cite journal |author=Gerhardt, H. C. |year=1994 |title=The evolution of vocalization in frogs and toads |journal=Annual Review of Ecology and Systematics |volume=25 |issue=1 |pages=293–324 |doi=10.1146/annurev.es.25.110194.001453 |bibcode=1994AnRES..25..293G }}</ref>
A different call is emitted by a male frog or unreceptive female when mounted by another male. This is a distinct chirruping sound and is accompanied by a vibration of the body.<ref name=Badger3>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing Ltd |isbn=978-1-85310-740-5 |pages=39–44 }}</ref> Tree frogs and some non-aquatic species have a rain call that they make on the basis of humidity cues prior to a shower.<ref name=Badger3/> Many species also have a territorial call that is used to drive away other males. All of these calls are emitted with the mouth of the frog closed.<ref name=Badger3/> A distress call, emitted by some frogs when they are in danger, is produced with the mouth open resulting in a higher-pitched call. It is typically used when the frog has been grabbed by a predator and may serve to distract or disorient the attacker so that it releases the frog.<ref name=Badger3/>
[[file:Banded_Bull_Frog_Call.ogg|left|thumb|Distinctive low "jug-o-rum" sound of [[banded bullfrog]]]]
Many species of frog have deep calls. The croak of the [[American bullfrog]] (''Rana catesbiana'') is sometimes written as "jug o' rum".<ref>{{cite book|last=Hilton|first=Bill Jr.|chapter=9. 'Jug-o-Rum': Call of the Amorous Bullfrog |title=The Piedmont Naturalist, Volume 1 |publisher=Hilton Pond Center for Piedmont Natural History |___location=York, SC |year=1986 |isbn=978-0-9832151-0-3}}</ref> The [[Pacific Tree Frog|Pacific tree frog]] (''Pseudacris regilla'') produces the [[onomatopoeia|onomatopoeic]] "ribbit" often heard in films.<ref>{{cite web |url=http://www.beachwatchers.wsu.edu/island/essays/TreeFrogs.htm |title=The RRRRRRRRiveting Life of Tree Frogs |last=Nash |first=Pat |date=February 2005 |access-date=August 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120309105032/http://www.beachwatchers.wsu.edu/island/essays/TreeFrogs.htm |archive-date=March 9, 2012 }}</ref> Other renderings of frog calls into speech include "brekekekex koax koax", the call of the marsh frog (''[[Pelophylax ridibundus]]'') in ''The Frogs'', an Ancient Greek comic drama by [[Aristophanes]].<ref>{{cite web |url=http://records.viu.ca/~johnstoi/aristophanes/frogs.htm |title=The Frogs |author=Aristophanes |access-date=June 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120513143510/http://records.viu.ca/~johnstoi/aristophanes/frogs.htm |archive-date=May 13, 2012 }}</ref> The calls of the [[Concave-eared torrent frog]] (''Amolops tormotus'') are unusual in many aspects. The males are notable for their varieties of calls where upward and downward frequency modulations take place. When they communicate, they produce calls that fall in the [[ultrasound]] frequency range. The last aspect that makes this species of frog's calls unusual is that nonlinear acoustic phenomena are important components in their acoustic signals.<ref name="ultrasonic">{{cite journal|author1=Suthers, R.A.|author2=Narins, P.M.|author3=Lin, W|author4=Schnitzler, H|author5=Denzinger, A|author6=Xu, C|author7=Feng, A.S.|year=2006|title= Voices of the dead: complex nonlinear vocal signals from the larynx of an ultrasonic frog|journal=Journal of Experimental Biology|volume=209|issue=24|pages=4984–4993| doi= 10.1242/jeb.02594|pmid=17142687|doi-access=free|bibcode=2006JExpB.209.4984S }}</ref>
===Torpor===
During extreme conditions, some frogs enter a state of [[torpor]] and remain inactive for months. In colder regions, many species of frog [[Hibernation|hibernate]] in winter. Those that live on land such as the [[American toad]] (''Bufo americanus'') dig a burrow and make a [[Hibernaculum (zoology)|hibernaculum]] in which to lie [[Dormancy|dormant]]. Others, less proficient at digging, find a crevice or bury themselves in dead leaves. Aquatic species such as the [[American bullfrog]] (''Rana catesbeiana'') normally sink to the bottom of the pond where they lie, semi-immersed in mud but still able to access the oxygen dissolved in the water. Their metabolism slows down and they live on their energy reserves. Some frogs such as the [[wood frog]], [[moor frog]], or [[spring peeper]] can even survive being frozen. Ice crystals form under the skin and in the body cavity but the essential organs are protected from freezing by a high concentration of glucose. An apparently lifeless, frozen frog can resume respiration and its heartbeat can restart when conditions warm up.<ref>{{cite magazine |url=http://www.scientificamerican.com/article.cfm?id=how-do-frogs-survive-wint |title=How do frogs survive winter? Why don't they freeze to death? |author=Emmer, Rick |date=November 24, 1997 |magazine=Scientific American |access-date=June 15, 2012}}</ref>
At the other extreme, the [[striped burrowing frog]] (''Cyclorana alboguttata'') regularly [[Aestivation|aestivates]] during the hot, dry season in Australia, surviving in a dormant state without access to food and water for nine or ten months of the year. It burrows underground and curls up inside a protective [[Cocoon (silk)|cocoon]] formed by its shed skin. Researchers at the [[University of Queensland]] have found that during aestivation, the [[metabolism]] of the frog is altered and the operational efficiency of the [[mitochondria]] is increased. This means that the limited amount of energy available to the comatose frog is used in a more efficient manner. This survival mechanism is only useful to animals that remain completely unconscious for an extended period of time and whose energy requirements are low because they are cold-blooded and have no need to generate heat.<ref>{{cite journal |author1=Kayes, Sara M. |author2=Cramp, Rebecca L. |author3=Franklin, Craig E. |year=2009 |title=Metabolic depression during aestivation in ''Cyclorana alboguttata'' |journal=Comparative Biochemistry and Physiology – Part A: Molecular & Integrative Physiology |volume=154 |issue=4 |pages=557–563 |doi=10.1016/j.cbpa.2009.09.001 |pmid=19737622 }}</ref> Other research showed that, to provide these energy requirements, muscles atrophy, but hind limb muscles are preferentially unaffected.<ref>{{cite journal |author1=Hudson, N. J. |author2=Lehnert, S. A. |author3=Ingham, A. B. |author4=Symonds, B. |author5=Franklin, C. E. |author6=Harper, G. S. |year=2005 |title=Lessons from an estivating frog: sparing muscle protein despite starvation and disuse |journal=[[American Journal of Physiology|AJP: Regulatory, Integrative and Comparative Physiology]] |volume=290 |issue=3 |pages = R836–R843 | doi = 10.1152/ajpregu.00380.2005 | pmid = 16239372 |s2cid=8395980 }}</ref> Frogs have been found to have upper critical temperatures of around 41 degrees Celsius.<ref>{{Cite book|title=Environmental Physiology of Animals|last=Wilmer|first=Pat|publisher=Wiley|year=2009|isbn=9781405107242|pages=[https://archive.org/details/environmentalphy00will/page/188 188]|url=https://archive.org/details/environmentalphy00will/page/188}}</ref>
==Locomotion==
Different species of frog use a number of methods of moving around including [[jumping]], [[running]], [[walking]], [[aquatic locomotion|swimming]], [[burrow]]ing, [[climbing]] and [[Flying frog|gliding]].
[[File:Colostethus flotator jumping.jpg|thumb|left|[[Rainforest rocket frog]] jumping]]
=== Jumping ===
Frogs are generally recognised as exceptional jumpers and, relative to their size, the best jumpers of all vertebrates.<ref>{{cite web |url=http://scienceray.com/biology/zoology/top-10-best-jumper-animals/ |title=Top 10 best jumper animals |publisher=Scienceray |access-date=June 11, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120810065127/http://scienceray.com/biology/zoology/top-10-best-jumper-animals/ |archive-date=August 10, 2012 }}</ref> The [[striped rocket frog]], ''Litoria nasuta'', can leap over {{convert|2|m|ft|frac=2|abbr=off|spell=in}}, a distance that is more than fifty times its body length of {{convert|55|mm|in|frac=4|abbr=on}}.<ref>{{cite journal |author1=James, R. S. |author2=Wilson, R. S. |year=2008 |title=Explosive jumping: extreme morphological and physiological specializations of Australian rocket frogs (''Litoria nasuta'') |journal=Physiological and Biochemical Zoology |volume=81 |issue=2 |pages=176–185 |pmid=18190283 |doi=10.1086/525290 |s2cid=12643425 |url=http://espace.library.uq.edu.au/view/UQ:175725/UQ175725_OA.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://espace.library.uq.edu.au/view/UQ:175725/UQ175725_OA.pdf |archive-date=October 9, 2022 |url-status=live }}</ref> There are tremendous differences between species in jumping capability. Within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases. The [[Euphlyctis cyanophlyctis|Indian skipper frog]] (''Euphlyctis cyanophlyctis'') has the ability to leap out of the water from a position floating on the surface.<ref>{{cite journal |author1=Nauwelaerts, S. |author2=Schollier, J. |author3=Aerts, P. |year=2004 |title=A functional analysis of how frogs jump out of water |journal=Biological Journal of the Linnean Society |volume=83 |issue=3 |pages=413–420 |doi=10.1111/j.1095-8312.2004.00403.x |doi-access=free }}</ref> The tiny [[northern cricket frog]] (''Acris crepitans'') can "skitter" across the surface of a pond with a series of short rapid jumps.<ref name=Badger2>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing |isbn=978-1-85310-740-5 |page=51 }}</ref>
Slow-motion photography shows that the muscles have passive flexibility. They are first stretched while the frog is still in the crouched position, then they are contracted before being stretched again to launch the frog into the air. The fore legs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump.<ref name=Frogjump>{{cite magazine |url=http://www.komverse.com/2010/05/15/how-frogs-jump-nationalgeographic/ |title=How frogs jump |author=Minott, Kevin |date=May 15, 2010 |magazine=National Geographic |access-date=June 10, 2012 |archive-url=https://web.archive.org/web/20131104045512/http://www.komverse.com/2010/05/15/how-frogs-jump-nationalgeographic/ |archive-date=November 4, 2013 |url-status=dead }}</ref> In some extremely capable jumpers, such as the [[Cuban tree frog]] (''Osteopilus septentrionalis'') and the [[northern leopard frog]] (''Lithobates pipiens''), the peak power exerted during a jump can exceed that which the muscle is theoretically capable of producing. When the muscles contract, the energy is first transferred into the stretched tendon which is wrapped around the ankle bone. Then the muscles stretch again at the same time as the tendon releases its energy like a [[catapult]] to produce a powerful acceleration beyond the limits of muscle-powered acceleration.<ref>{{cite journal |author1=Astley, H. C. |author2=Roberts, T. J. |year=2011 |title=Evidence for a vertebrate catapult: elastic energy storage in the plantaris tendon during frog jumping |journal=Biology Letters |volume=8 |issue=3 |pages=386–389 |doi=10.1098/rsbl.2011.0982 |pmid=22090204 |pmc=3367733 }}</ref> A similar mechanism has been documented in [[locust]]s and [[grasshopper]]s.<ref>{{cite journal |last1=Scott |first1=J. |title=The locust jump: an integrated laboratory investigation |doi=10.1152/advan.00037.2004 |year=2005 |pages=21–26 |volume=29 |journal=Advances in Physiology Education |issue=1 |pmid=15718379|s2cid=13423666 }}</ref>
Early hatching of froglets can have negative effects on frog jumping performance and overall locomotion.<ref name=":1">{{Cite journal|last1=Buckley|first1=C. R.|last2=Michael|first2=S. F.|last3=Irschick|first3=D. J.|date=2005|title=Early Hatching Decreases Jumping Performance in a Direct-Developing Frog, Eleutherodactylus coqui|journal=Functional Ecology|volume=19|issue=1|pages=67–72|doi=10.1111/j.0269-8463.2005.00931.x|jstor=3599272|issn=0269-8463|doi-access=free|bibcode=2005FuEco..19...67B }}</ref> The hindlimbs are unable to completely form, which results in them being shorter and much weaker relative to a normal hatching froglet.<ref name=":1" /> Early hatching froglets may tend to depend on other forms of locomotion more often, such as swimming and walking.<ref name=":1" />
=== Walking and running ===
[[File:Phrynosequence bodyFoR2.gif|thumb|right|''[[Phrynomantis bifasciatus]]'' walking on a level surface]]
Frogs in the families Bufonidae, [[Rhinophrynidae]], and [[Microhylidae]] have short back legs and tend to walk rather than jump.<ref name=Britannica>{{cite encyclopedia |last1=Zug |first1=George R. |last2=Duellman |first2=William E. |encyclopedia=Encyclopædia Britannica Online |title=Anura |url=https://www.britannica.com/EBchecked/topic/29023/Anura |access-date=April 26, 2015 |date=May 14, 2014 }}</ref> When they try to move rapidly, they speed up the rate of movement of their limbs or resort to an ungainly hopping gait. The [[Great Plains narrow-mouthed toad]] (''Gastrophryne olivacea'') has been described as having a gait that is "a combination of running and short hops that are usually only an inch or two in length".<ref>{{cite journal |author=Fitch, H. S. |year=1956 |title=An ecological study of the collared lizard (''Crotaphytus collaris'') |journal=University of Kansas Publications |volume=8 |pages=213–274 }}</ref> In an experiment, [[Bufo fowleri|Fowler's toad]] (''Anaxyrus'' ''fowleri'') was placed on a treadmill which was turned at varying speeds. By measuring the toad's uptake of oxygen it was found that hopping was an inefficient use of resources during sustained locomotion but was a useful strategy during short bursts of high-intensity activity.<ref>{{cite journal |author1=Walton, M. |author2=Anderson, B. D. |year=1988 |title=The aerobic cost of saltatory locomotion in the fowler's toad (''Bufo woodhousei fowleri'') |journal=Journal of Experimental Biology |volume=136 |pages=273–288 |url=http://jeb.biologists.org/content/136/1/273 |pmid= 3404074 |issue=1|doi=10.1242/jeb.136.1.273 |doi-access=free |bibcode=1988JExpB.136..273W }} {{free access}}</ref>
The [[Kassina maculata|red-legged running frog]] (''Kassina maculata'') has short, slim hind limbs unsuited to jumping. It can move fast by using a running gait in which the two hind legs are used alternately. Slow-motion photography shows, unlike a horse that can trot or gallop, the frog's gait remained similar at slow, medium, and fast speeds.<ref>{{cite journal |author1=Ahn, A. N. |author2=Furrow, E. |author3=Biewener, A. A. |year=2004 |title=Walking and running in the red-legged running frog, ''Kassina maculata'' |journal=Journal of Experimental Biology |volume=207 |pages=399–410 |doi=10.1242/jeb.00761 |pmid= 14691087 |issue= Pt 3 |doi-access=free |bibcode=2004JExpB.207..399A }}</ref> This species can also climb trees and shrubs, and does so at night to catch insects.<ref>{{cite iucn |url=https://www.iucnredlist.org/species/56231/11445543 |title=''Kassina maculata'' |author1=Pickersgill, M. |author2=Schiøtz, A. |author3=Howell, K. |author4=Minter, L. |year=2004 |access-date=June 11, 2012}}</ref> The Indian skipper frog (''Euphlyctis cyanophlyctis'') has broad feet and can run across the surface of the water for several metres (yards).<ref name=Badger2/>
=== Swimming ===
[[File:Frog žába.gif|thumb|alt=Common toad swimming|[[Common toad]] (''Bufo bufo'') swimming]]
Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water. Members of the family [[Pipidae]] are wholly aquatic and show the most marked specialisation. They have inflexible vertebral columns, flattened, streamlined bodies, [[lateral line]] systems, and powerful hind limbs with large webbed feet.<ref name=Pipidae>{{cite web |url=http://amphibiaweb.org/lists/Pipidae.shtml |title=Pipidae |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=June 14, 2012}}</ref> Tadpoles mostly have large tail fins which provide thrust when the tail is moved from side to side.<ref name="Duellman"/>
=== Burrowing ===
Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes, and hind feet adapted for excavation. An extreme example of this is the [[purple frog]] (''Nasikabatrachus sahyadrensis'') from southern India which feeds on [[termite]]s and spends almost its whole life underground. It emerges briefly during the [[monsoon]] to mate and breed in temporary pools. It has a tiny head with a pointed snout and a plump, rounded body. Because of this [[fossorial]] existence, it was [[Species description|first described]] in 2003, being new to the scientific community at that time, although previously known to local people.<ref>{{cite journal |author1=Radhakrishnan, C. |author2=Gopi, K. C. |year=2007 |title=Extension of range of distribution of ''Nasikabatrachus sahyadrensis'' Biju & Bossuyt ( Amphibia : Anura : Nasikabatrachidae ) along Western Ghats, with some insights into its bionomics |journal=Current Science |volume=92 |issue=2 |pages=213–216 |issn=0011-3891 |url=http://www.iisc.ernet.in/currsci/jan252007/213.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.iisc.ernet.in/currsci/jan252007/213.pdf |archive-date=October 9, 2022 |url-status=live }}</ref>
[[File:Nasikabatrachus sahyadrensis.jpg|thumb|left|alt=Purple frog|[[Purple frog]] (''Nasikabatrachus sahyadrensis'')]]
The spadefoot toads of North America are also adapted to underground life. The [[Plains spadefoot toad]] (''Spea bombifrons'') is typical and has a flap of keratinised bone attached to one of the [[Metatarsus|metatarsals]] of the hind feet which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side to sink into the loose soil. It has a shallow burrow in the summer from which it emerges at night to forage. In winter, it digs much deeper and has been recorded at a depth of {{convert|4.5|m|ftin|abbr=on}}.<ref name=Farrar>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Spea&where-species=bombifrons |title=''Spea bombifrons'' |author1=Farrar, Eugenia |author2=Hey, Jane |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=June 16, 2012}}</ref> The tunnel is filled with soil and the toad hibernates in a small chamber at the end. During this time, urea accumulates in its tissues and water is drawn in from the surrounding damp soil by [[osmosis]] to supply the toad's needs.<ref name=Farrar/> Spadefoot toads are explosive breeders, all emerging from their burrows at the same time and converging on temporary pools, attracted to one of these by the calling of the first male to find a suitable breeding ___location.<ref>{{cite web |url=http://amphibiaweb.org/lists/Scaphiopodidae.shtml |title=Scaphiopodidae |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=June 16, 2012}}</ref>
The burrowing frogs of Australia have a different lifestyle. The [[western spotted frog]] (''Heleioporus albopunctatus'') digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage. Mating takes place and eggs are laid in a foam nest inside the burrow. The eggs partially develop there but do not hatch until they are submerged following heavy rainfall. The tadpoles then swim out into the open water and rapidly complete their development.<ref>{{cite iucn |url=https://www.iucnredlist.org/species/9763/13014492 |title=''Heleioporus albopunctatus'' |author1=Roberts, Dale |author2=Hero, Jean-Marc |year=2011 |access-date=June 16, 2012}}</ref> Madagascan burrowing frogs are less fossorial and mostly bury themselves in leaf litter. One of these, the [[Scaphiophryne marmorata|green burrowing frog]] (''Scaphiophryne marmorata''), has a flattened head with a short snout and well-developed metatarsal tubercles on its hind feet to help with excavation. It also has greatly enlarged terminal discs on its fore feet that help it to clamber around in bushes.<ref>{{cite web |url=http://www.amphibian.co.uk/scaphiophryne.html |title=Madagascan Burrowing Frogs: Genus: ''Scaphiophryne'' (Boulenger, 1882) |author=Staniszewski, Marc |date=September 30, 1998 |access-date=June 16, 2012}}</ref> It breeds in temporary pools that form after rains.<ref>{{cite journal |author1=Venesci, M |author2=Raxworthy, C. J. |author3=Nussbaum, R. A. |author4=Glaw, F. |year=2003 |title=A revision of the ''Scaphiophryne marmorata'' complex of marbled toads from Madagascar, including the description of a new species |journal=Herpetological Journal |volume=13 |pages=69–79 |url=http://www.mvences.de/p/p1/Vences_A51.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.mvences.de/p/p1/Vences_A51.pdf |archive-date=October 9, 2022 |url-status=live }}</ref>
=== Climbing ===
[[File:Phyllomedusa burmeisteri01.jpg|thumb|alt=Frog climbing|[[Phyllomedusa burmeisteri|Burmeister's leaf frog]]]]
[[File:Glass frogs.jpg|thumb|Group of [[glass frog]]s]]
Tree frogs live high in the [[canopy (biology)|canopy]], where they scramble around on the branches, twigs, and leaves, sometimes never coming down to earth. The "true" tree frogs belong to the family Hylidae, but members of other frog families have independently adopted an arboreal habit, a case of [[convergent evolution]]. These include the [[glass frogs]] (Centrolenidae), the [[Hyperoliidae|bush frogs]] (Hyperoliidae), some of the narrow-mouthed frogs (Microhylidae), and the [[Rhacophoridae|shrub frogs]] (Rhacophoridae).<ref name=Britannica/> Most tree frogs are under {{convert|10|cm|in|0|abbr=on}} in length, with long legs and long toes with adhesive pads on the tips. The surface of the toe pads is formed from a closely packed layer of flat-topped, hexagonal [[epidermis (skin)|epidermal]] cells separated by grooves into which glands secrete [[mucus]]. These toe pads, moistened by the mucus, provide the grip on any wet or dry surface, including glass. The forces involved include [[boundary friction]] of the toe pad epidermis on the surface and also [[surface tension]] and [[viscosity]].<ref>{{cite journal |author1=Federle, W. |author2=Barnes, W. J. P. |author3=Baumgartner, W. |author4=Drechsler, P. |author5=Smith, J. M. |year=2006 |title=Wet but not slippery: boundary friction in tree frog adhesive toe pads |journal=Journal of the Royal Society Interface |volume=3 |issue=10 |pages=689–697 |doi=10.1098/rsif.2006.0135 |pmc=1664653 |pmid=16971337}}</ref> Tree frogs are very acrobatic and can catch insects while hanging by one toe from a twig or clutching onto the blade of a windswept reed.<ref>{{cite book |title=Living Amphibians of the World |last=Cochran |first=Doris Mabel |year=1961 |publisher=Doubleday |isbn=978-0-241-90338-4 |page=112 |url={{Google books|HOk6AAAAMAAJ|page=112|plainurl=yes}} }}</ref> Some members of the subfamily [[Phyllomedusinae]] have [[Opposable thumb#Opposition and apposition|opposable toes]] on their feet. The [[Phyllomedusa ayeaye|reticulated leaf frog]] (''Phyllomedusa ayeaye'') has a single opposed [[Digit (anatomy)|digit]] on each fore foot and two opposed digits on its hind feet. This allows it to grasp the stems of bushes as it clambers around in its riverside habitat.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Phyllomedusa&where-species=ayeaye |title=''Phyllomedusa ayeaye'' |publisher=AmphibiaWeb |access-date=June 14, 2012}}</ref>
=== Gliding ===
During the evolutionary history of frogs, several different groups have independently taken to the air.<ref>{{cite journal |author1=Emerson, Sharon B. |author2=Koehl, M. A. R. |year=1990 |title=The interaction of behavioral and morphological change in the evolution of a novel locomotor type: "flying frogs |journal=Evolution |volume=44 |issue=8 |pages=1931–1946 |jstor=2409604 |doi=10.2307/2409604 |pmid=28564439 }}</ref> Some frogs in the tropical rainforest are specially adapted for gliding from tree to tree or parachuting to the forest floor. Typical of them is [[Rhacophorus nigropalmatus|Wallace's flying frog]] (''Rhacophorus nigropalmatus'') from Malaysia and Borneo. It has large feet with the fingertips expanded into flat adhesive discs and the digits fully webbed. Flaps of skin occur on the lateral margins of the limbs and across the tail region. With the digits splayed, the limbs outstretched, and these flaps spread, it can glide considerable distances, but is unable to undertake powered flight.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Rhacophorus&where-species=nigropalmatus |title=''Rhacophorus nigropalmatus'' |date=November 29, 2001 |author1=Shah, Sunny |author2=Tiwari, Rachna |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=June 11, 2012}}</ref> It can alter its direction of travel and navigate distances of up to {{convert|15|m|ft|round=5|abbr=on}} between trees.<ref>{{cite web |url=http://animals.nationalgeographic.com/animals/amphibians/wallaces-flying-frog/ |archive-url=https://web.archive.org/web/20100118144053/http://animals.nationalgeographic.com/animals/amphibians/wallaces-flying-frog |url-status=dead |archive-date=January 18, 2010 |title=Wallace's Flying Frog ''Rhacophorus nigropalmatus'' |publisher=National Geographic: Animals |access-date=June 5, 2012|date=September 10, 2010 }}</ref>
==Life history==
[[File:Greenfrog life stages.svg|thumb|left|Life cycle of the [[Rana clamitans|green frog]] <br /> (''Rana clamitans'')]]
===Reproduction===
Two main types of reproduction occur in frogs, prolonged breeding and explosive breeding. In the former, adopted by the majority of species, adult frogs at certain times of year assemble at a pond, lake or stream to breed. Many frogs return to the bodies of water in which they developed as larvae. This often results in annual migrations involving thousands of individuals. In explosive breeders, mature adult frogs arrive at breeding sites in response to certain trigger factors such as rainfall occurring in an arid area. In these frogs, mating and spawning take place promptly and the speed of larval growth is rapid in order to make use of the ephemeral pools before they dry up.<ref name=Stebbins>{{cite book |title=A Natural History of Amphibians |last1=Stebbins |first1=Robert C. |author-link1=Robert C. Stebbins|last2=Cohen|first2= Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=154–162 }}</ref>
Among prolonged breeders, males usually arrive at the breeding site first and remain there for some time whereas females tend to arrive later and depart soon after they have spawned. This means that males outnumber females at the water's edge and defend territories from which they expel other males. They advertise their presence by calling, often alternating their croaks with neighbouring frogs. Larger, stronger males tend to have deeper calls and maintain higher quality territories. Females select their mates at least partly on the basis of the depth of their voice.<ref>{{cite journal |author1=Davies, N. B. |author2=Halliday, T. R. |year=1978 |title=Deep croaks and fighting assessment in toads ''Bufo bufo'' |journal=Nature |volume=274 |issue=5672 |pages=683–685 |doi=10.1038/274683a0|bibcode=1978Natur.274..683D |s2cid=4222519 }}</ref> In some species there are satellite males who have no territory and do not call. They may intercept females that are approaching a calling male or take over a vacated territory. Calling is an energy-sapping activity. Sometimes the two roles are reversed and a calling male gives up its territory and becomes a satellite.<ref name=Stebbins/>
[[File:Bufo bufo couple during migration(2005).jpg|thumb|Male and female [[common toad]]s (''Bufo bufo'') in [[amplexus]]]]
In explosive breeders, the first male that finds a suitable breeding ___location, such as a temporary pool, calls loudly and other frogs of both sexes converge on the pool. Explosive breeders tend to call in unison creating a chorus that can be heard from far away. The spadefoot toads (''[[Scaphiopus]] spp.'') of North America fall into this category. Mate selection and courtship is not as important as speed in reproduction. In some years, suitable conditions may not occur and the frogs may go for two or more years without breeding.<ref name=Stebbins/> Some female [[New Mexico Spadefoot Toad|New Mexico spadefoot toads]] (''Spea multiplicata'') only spawn half of the available eggs at a time, perhaps retaining some in case a better reproductive opportunity arises later.<ref>{{cite journal |author=Long, David R. |year=1989 |title=Energetics and reproduction in female ''Scaphiopus multiplicatus'' from Western Texas |journal=Journal of Herpetology |volume=23 |issue=2 |pages=176–179 |jstor=1564026 |doi=10.2307/1564026 }}</ref>
At the breeding site, the male mounts the female and grips her tightly round the body. Typically, [[amplexus]] takes place in the water, the female releases her eggs and the male covers them with sperm; fertilisation is [[external fertilization|external]]. In many species such as the [[Great Plains toad]] (''Bufo cognatus''), the male restrains the eggs with his back feet, holding them in place for about three minutes.<ref name=Stebbins/> Members of the West African genus ''[[Nimbaphrynoides]]'' are unique among frogs in that they are [[Viviparity|viviparous]]; ''[[Limnonectes larvaepartus]]'', ''[[Eleutherodactylus jasperi]]'' and members of the [[Tanzania]]n genus ''[[Nectophrynoides]]'' are the only frogs known to be [[Ovoviviparity|ovoviviparous]]. In these species, fertilisation is [[Internal fertilization|internal]] and females give birth to fully developed juvenile frogs, except ''L. larvaepartus'', which give birth to tadpoles.<ref name="Iskandar2014">{{Cite journal | doi = 10.1371/journal.pone.0115884| pmid = 25551466| pmc = 4281041| title = A Novel Reproductive Mode in Frogs: A New Species of Fanged Frog with Internal Fertilization and Birth of Tadpoles| journal = PLOS ONE| volume = 9| issue = 12| pages = e115884| year = 2014| last1 = Iskandar | first1 = D. T. | last2 = Evans | first2 = B. J. | last3 = McGuire | first3 = J. A. | bibcode = 2014PLoSO...9k5884I| doi-access = free}}</ref><ref>{{cite book |title=Amphibians of East Africa |last=Channing |first=Alan |author2=Howell, Kim M. |year=2006 |publisher=Comstock Publishing |isbn=978-0-8014-4374-9 |pages=104–117 }}</ref><ref>{{cite journal |author1=Sandberger, L. |author2=Hillers, A. |author3=Doumbia, J. |author4=Loua, N-S. |author5=Brede C. |author6=Rödel, M-O. |year=2010 |title=Rediscovery of the Liberian Nimba toad, ''Nimbaphrynoides liberiensis'' (Xavier, 1978) (Amphibia: Anura: Bufonidae), and reassessment of its taxonomic status |journal=Zootaxa |volume=4355 |pages=56–68 | issn=1175-5334 |url=http://www.nimbadarwin.org/resources/nimbatoad.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nimbadarwin.org/resources/nimbatoad.pdf |archive-date=October 9, 2022 |url-status=live |doi=10.11646/zootaxa.2355.1.3 }}</ref>
===Life cycle===
====Eggs / frogspawn {{anchor|Frogspawn}}{{anchor|Frog spawn}}====
[[File:Frogspawn closeup.jpg|left|thumb|Frogspawn]]
Frogs may lay their eggs as clumps, surface films, strings, or individually. Around half of species deposit eggs in water, others lay eggs in vegetation, on the ground or in excavations.<ref>{{cite book|author=Mattison, Chris|year=2011|title=Frogs and Toads of the Worlds|publisher=Princeton University Press|page=91|isbn=978-0-691-14968-4}}</ref><ref>{{cite encyclopedia |url=https://www.britannica.com/animal/Anura/Natural-history#ref275977 |title=Anura: Egg laying and hatching|author1=Duellman, W. E. |author2=Zug, G. R. |encyclopedia=Encyclopædia Britannica |access-date=April 9, 2022}}</ref><ref>{{cite book |last1=Duellman|first1=William E.|last2=Trueb|first2=Linda|author-link2=Linda Trueb|last3=Arak|first3=Anthony|year=2002 |contribution=Frogs and toads |title=The Firefly Encyclopedia of Reptiles and Amphibians |editor=Halliday, Tim |editor2=Adler, Kraig |publisher=Firefly Books |page=67 |isbn=978-1-55297-613-5 }}</ref> The tiny [[Yellow-Striped Pygmy Eleuth|yellow-striped pygmy eleuth]] (''Eleutherodactylus limbatus'') lays eggs singly, burying them in moist soil.<ref>{{cite journal |author1=Estrada, Alberto R. |author2=Hedges, S. Blair |year=1996 |title=At the lower size limit in tetrapods: a new diminutive frog from Cuba (Leptodactylidae: Eleutherodactylus) |journal=Copeia |volume=1996 |issue=4 |pages=852–859 |jstor=1447647 |doi=10.2307/1447647 }}</ref> The [[Smoky Jungle Frog|smoky jungle frog]] (''Leptodactylus pentadactylus'') makes a nest of foam in a hollow. The eggs hatch when the nest is flooded, or the tadpoles may complete their development in the foam if flooding does not occur.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Leptodactylus&where-species=pentadactylus |title=''Leptodactylus pentadactylus'' |author1=Whittaker, Kellie |author2=Chantasirivisal, Peera | date=December 2, 2005 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 19, 2012}}</ref> The red-eyed treefrog (''Agalychnis callidryas'') deposits its eggs on a leaf above a pool and when they hatch, the larvae fall into the water below.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Agalychnis&where-species=callidryas |title=''Agalychnis callidryas'' |date=June 27, 2007 |author=Whittaker, Kellie |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 19, 2012}}</ref>
In certain species, such as the [[wood frog]] (''Rana sylvatica''), [[Symbiosis|symbiotic]] unicellular green algae are present in the gelatinous material. It is thought that these may benefit the developing larvae by providing them with extra oxygen through [[photosynthesis]].<ref>{{cite journal |author=Gilbert, Perry W. |year=1942 |title=Observations on the eggs of ''Ambystoma maculatum'' with especial reference to the green algae found within the egg envelopes |journal=Ecology |volume=23 |issue=2 |pages=215–227 |jstor=1931088 |doi=10.2307/1931088|bibcode=1942Ecol...23..215G }}</ref> The interior of globular egg clusters of the [[Wood Frog|wood frog]] has also been found to be up to 6 °C (11 °F) warmer than the surrounding water and this speeds up the development of the larvae.<ref>{{cite journal |author1=Waldman, Bruce |author2=Ryan, Michael J. |year=1983 |title=Thermal advantages of communal egg mass deposition in wood frogs (''Rana sylvatica'') |journal=Journal of Herpetology |volume=17 |issue=1 |pages=70–72 |jstor=1563783 |doi=10.2307/1563783 }}</ref> The larvae developing in the eggs can detect vibrations caused by nearby predatory wasps or snakes, and will hatch early to avoid being eaten.<ref>{{cite journal| last=Warkentin| first=K.M.| year=1995| title=Adaptive plasticity in hatching age: a response to predation risk trade-offs| journal=Proceedings of the National Academy of Sciences| volume=92| pages=3507–3510| doi=10.1073/pnas.92.8.3507| pmid=11607529| issue=8| pmc=42196| bibcode=1995PNAS...92.3507W| doi-access=free}}</ref> In general, the length of the egg stage depends on the species and the environmental conditions. Aquatic eggs normally hatch within one week when the capsule splits as a result of [[enzyme]]s released by the developing larvae.<ref name=Stebbins2>{{cite book |title=A Natural History of Amphibians |last=Stebbins |first=Robert C. |author-link1=Robert C. Stebbins|author2=Cohen, Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=179–194 }}</ref>
'''Direct development''', where eggs hatch into juveniles like small adults, is also known in many frogs, for example, ''[[Ischnocnema henselii]],''<ref>{{Cite journal|last1=Goldberg|first1=Javier|last2=Taucce|first2=Pedro P. G.|last3=Quinzio|first3=Silvia Inés|last4=Haddad|first4=Célio F. B.|last5=Vera Candioti|first5=Florencia|date=January 1, 2020|title=Increasing our knowledge on direct-developing frogs: The ontogeny of Ischnocnema henselii (Anura: Brachycephalidae)|url=http://www.sciencedirect.com/science/article/pii/S0044523119301196|journal=Zoologischer Anzeiger|language=en|volume=284|pages=78–87|doi=10.1016/j.jcz.2019.11.001|bibcode=2020ZooAn.284...78G |s2cid=209576535|issn=0044-5231|url-access=subscription|hdl=11449/199791|hdl-access=free}}</ref> ''[[Common coquí|Eleutherodactylus coqui]]'',<ref>{{Cite journal|last=Elinson|first=Richard P.|date=2001|title=Direct development: an alternative way to make a frog|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/1526-968X%28200102%2929%3A2%3C91%3A%3AAID-GENE1009%3E3.0.CO%3B2-6|journal=Genesis|language=en|volume=29|issue=2|pages=91–95|doi=10.1002/1526-968X(200102)29:2<91::AID-GENE1009>3.0.CO;2-6|pmid=11170349|s2cid=32550621 |issn=1526-968X|url-access=subscription}}</ref> and ''[[Raorchestes ochlandrae]]'' and ''[[Raorchestes chalazodes]].''<ref>{{Cite journal|last=Seshadri|first=K.S.|date=2015|title=Rhacophorid Frogs Breeding in Bamboo: Discovery of a Novel Reproductive Mode from Western Ghats|url=https://www.researchgate.net/publication/287209620|journal=FrogLog|volume=23(4)|issue=116|pages=46–49}}</ref>
====Tadpoles====
[[File:Frog spawn time-lapse.gif|thumb|Frogspawn development]]
The larvae that emerge from the eggs are known as tadpoles (or occasionally polliwogs). Tadpoles lack eyelids and limbs, and have cartilaginous skeletons, gills for respiration (external gills at first, internal gills later), and tails they use for swimming.<ref name="Duellman">{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/29023/Anura/40603/From-tadpole-to-adult |title=Anura: From tadpole to adult |author1=Duellman, W. E. |author2=Zug, G. R. |encyclopedia=Encyclopædia Britannica |access-date=July 13, 2012}}</ref> As a general rule, free-living larvae are fully aquatic, but at least one species (''[[Nannophrys ceylonensis]]'') has semiterrestrial tadpoles which live among wet rocks.<ref>{{cite journal |author1=Wickramasinghe, Deepthi D. |author2=Oseen, Kerri L. |author3=Wassersug, Richard J. |year=2007 |title=Ontogenetic Changes in Diet and Intestinal Morphology in Semi-Terrestrial Tadpoles of ''Nannophrys ceylonensis'' (Dicroglossidae) |journal=Copeia |volume=2007 |issue=4 |pages=1012–1018 |jstor=25140719 |doi=10.1643/0045-8511(2007)7[1012:ocidai]2.0.co;2|s2cid=86244218 }}</ref><ref>{{cite web|url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Nannophrys&where-species=ceylonensis |title=''Nannophrys ceylonensis'' |author=Janzen, Peter |date=May 10, 2005 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 20, 2012}}</ref>
From early in its development, a gill pouch covers the tadpole's gills and front legs. The lungs soon start to develop and are used as an accessory breathing organ. Some species go through metamorphosis while still inside the egg and hatch directly into small frogs. Tadpoles lack true teeth, but the jaws in most species have two elongated, parallel rows of small, [[keratin]]ized structures called keradonts in their upper jaws. Their lower jaws usually have three rows of keradonts surrounded by a horny beak, but the number of rows can vary and the exact arrangements of mouth parts provide a means for species identification.<ref name=Stebbins2/> In the Pipidae, with the exception of ''[[Hymenochirus]]'', the tadpoles have paired anterior barbels, which make them resemble small [[catfish]].<ref name=Pipidae/> Their tails are stiffened by a [[notochord]], but does not contain any bony or cartilaginous elements except for a few vertebrae at the base which forms the urostyle during metamorphosis. This has been suggested as an adaptation to their lifestyles; because the transformation into frogs happens very fast, the tail is made of soft tissue only, as bone and cartilage take a much longer time to be broken down and absorbed. The tail fin and tip is fragile and will easily tear, which is seen as an adaptation to escape from predators which try to grasp them by the tail.<ref>{{cite journal|last1=Hoff|first1=K. vS.|last2=Wassersug|first2=R. J.|title=Tadpole Locomotion: Axial Movement and Tail Functions in a Largely Vertebraeless Vertebrate|journal=Integrative and Comparative Biology|date=February 1, 2000|volume=40|issue=1|pages=62–76|doi=10.1093/icb/40.1.62|doi-access=free}}</ref>
Tadpoles are typically [[herbivore|herbivorous]], feeding mostly on [[alga]]e, including [[diatom]]s filtered from the water through the [[gill]]s. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish. The Cuban tree frog (''Osteopilus septentrionalis'') is one of a number of species in which the tadpoles can be [[Cannibalism|cannibalistic]]. Tadpoles that develop legs early may be eaten by the others, so late developers may have better long-term survival prospects.<ref>{{cite journal |author=Crump, Martha L. |year=1986 |title=Cannibalism by younger tadpoles: another hazard of metamorphosis |journal=Copeia |volume=1986 |issue=4 |pages=1007–1009 |jstor=1445301 |doi=10.2307/1445301}}</ref>
Tadpoles are highly vulnerable to being eaten by fish, [[newt]]s, predatory [[diving beetle]]s, and birds, particularly [[water bird]]s, such as [[stork]]s and [[heron]]s and [[domestic duck]]s. Some tadpoles, including those of the [[cane toad]] (''Rhinella marina''), are poisonous. The tadpole stage may be as short as a week in explosive breeders or it may last through one or more winters followed by metamorphosis in the spring.<ref>{{cite book |title=A Natural History of Amphibians |last=Stebbins |first=Robert C. |author-link1=Robert C. Stebbins|author2=Cohen, Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=173–175 }}</ref>
====Metamorphosis====
At the end of the tadpole stage, a frog undergoes metamorphosis in which its body makes a sudden transition into the adult form. This metamorphosis typically lasts only 24 hours, and is initiated by production of the [[hormone]] [[thyroxine]]. This causes different tissues to develop in different ways. The principal changes that take place include the development of the lungs and the disappearance of the gills and gill pouch, making the front legs visible. The lower jaw transforms into the big mandible of the carnivorous adult, and the long, spiral gut of the herbivorous tadpole is replaced by the typical short gut of a predator.<ref name=Stebbins2/> Homeostatic feedback control of food intake is largely absent, making tadpoles eat constantly when food is present. But shortly before and during metamorphosis the sensation of hunger is suppressed, and they stop eating while their gut and internal organs are reorganised and prepared for a different diet.<ref>{{cite journal | pmc=5897637 | year=2018 | last1=Bender | first1=M. C. | last2=Hu | first2=C. | last3=Pelletier | first3=C. | last4=Denver | first4=R. J. | title=To eat or not to eat: Ontogeny of hypothalamic feeding controls and a role for leptin in modulating life-history transition in amphibian tadpoles | journal=Proceedings. Biological Sciences | volume=285 | issue=1875 | doi=10.1098/rspb.2017.2784 | pmid=29593109 }}</ref><ref>[https://www.futurity.org/tadpoles-leptin-metamorphosis-1720852-2/ Tadpoles eat like crazy before remodeling their guts - Futurity]</ref> Also the [[gut microbiota]] changes, from being similar to that of fish to resembling that of amniotes.<ref>{{cite journal | pmid=24249298 | year=2013 | last1=Kohl | first1=K. D. | last2=Cary | first2=T. L. | last3=Karasov | first3=W. H. | last4=Dearing | first4=M. D. | title=Restructuring of the amphibian gut microbiota through metamorphosis | journal=Environmental Microbiology Reports | volume=5 | issue=6 | pages=899–903 | doi=10.1111/1758-2229.12092 | bibcode=2013EnvMR...5..899K }}</ref> Exceptions are carnivorous tadpoles like [[Lepidobatrachus laevis]], which has a gut already adapted to a diet similar to that of adults. These continue to eat during metamorphosis.<ref>{{cite journal | doi=10.1186/s12861-020-00221-5 | title=Evolutionary and developmental considerations of the diet and gut morphology in ceratophryid tadpoles (Anura) | year=2020 | last1=Fabrezi | first1=Marissa | last2=Cruz | first2=Julio César | journal=BMC Developmental Biology | volume=20 | issue=1 | page=16 | pmid=32723314 | s2cid=255787646 | pmc=7388516 | doi-access=free }}</ref> The nervous system becomes adapted for hearing and stereoscopic vision, and for new methods of locomotion and feeding.<ref name=Stebbins2/> The eyes are repositioned higher up on the head and the eyelids and associated glands are formed. The eardrum, middle ear, and inner ear are developed. The skin becomes thicker and tougher, the lateral line system is lost, and skin glands are developed.<ref name=Stebbins2/> The final stage is the disappearance of the tail, but this takes place rather later, the tissue being used to produce a spurt of growth in the limbs.<ref>{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/25677/animal-development/63769/Metamorphosis |title=Animal development: Metamorphosis |last1=Balinsky |first1=Boris Ivan |encyclopedia=Encyclopædia Britannica |access-date=August 10, 2012}}</ref> Frogs are at their most vulnerable to predators when they are undergoing metamorphosis. At this time, the tail is being lost and locomotion by means of limbs is only just becoming established.<ref name=Britannica/>
{{gallery
|align=center
|width=220
|File:RanaTemporariaLarva2.jpg|Larva of the common frog ''Rana temporaria'' a day before metamorphosis
|File:Rana Temporaria - Larva Final Stage.jpg|Metamorphosis stage with deforming jaws, large eyes, and remains of gill pouch
|File:Juvenile Frog with tail top view (1).JPG|Young frog with a stumpy tail, metamorphosis nearly complete
}}
====Adults====
[[File:Xenopus laevis froglet swimming -1749-8104-7-13-S10.ogv|thumb|A ''[[Xenopus laevis]]'' froglet after metamorphosis]]
Adult frogs may live in or near water, but few are fully aquatic.<ref>{{cite book|author=Mattison, Chris|year=2011|title=Frogs and Toads of the Worlds|publisher=Princeton University Press|page=107|isbn=978-0-691-14968-4}}</ref> Almost all frog species are [[carnivore|carnivorous]] as adults, preying on invertebrates, including [[insect]]s, [[crab]]s, [[spider]]s, [[mite]]s, [[annelid|worm]]s, [[snails]], and [[slug]]s. A few of the larger ones may eat other frogs, small [[mammal]]s and reptiles, and [[fish]].<ref>{{cite book|author=Mattison, Chris|year=2011|title=Frogs and Toads of the Worlds|publisher=Princeton University Press|pages=65–68|isbn=978-0-691-14968-4}}</ref><ref>{{cite encyclopedia |url=https://www.britannica.com/animal/Anura/Natural-history#ref40605 |title=Anura: Feeding habits|author1=Duellman, W. E. |author2=Zug, G. R. |encyclopedia=Encyclopædia Britannica |access-date=April 9, 2022}}</ref> A few species also eat plant matter; the tree frog ''[[Xenohyla truncata]]'' is partly herbivorous, its diet including a large proportion of fruit, floral structures and nectar.<ref>{{Cite journal |last1=de-Oliveira-Nogueira |first1=Carlos Henrique |last2=Souza |first2=Ubiratã Ferreira |last3=Machado |first3=Thaynara Mendes |last4=Figueiredo-de-Andrade |first4=Caio Antônio |last5=Mônico |first5=Alexander Tamanini |last6=Sazima |first6=Ivan |last7=Sazima |first7=Marlies |last8=Toledo |first8=Luís Felipe |date=June 2023 |title=Between fruits, flowers and nectar: The extraordinary diet of the frog Xenohyla truncata |url=https://linkinghub.elsevier.com/retrieve/pii/S2352249623000101 |journal=Food Webs |language=en |volume=35 |article-number=e00281 |doi=10.1016/j.fooweb.2023.e00281|bibcode=2023FWebs..3500281D |url-access=subscription }}</ref><ref>{{cite journal |author1=Da Silva, H. R. |author2=De Britto-Pereira, X. C. |year=2006 |title=How much fruit do fruit-eating frogs eat? An investigation on the diet of ''Xenohyla truncata'' (Lissamphibia: Anura: Hylidae) |journal=Journal of Zoology |volume=270 |issue=4 |pages=692–698 |doi=10.1111/j.1469-7998.2006.00192.x }}</ref> ''[[Leptodactylus mystaceus]]'' has been found to eat plants,<ref>{{Cite web|url=http://www.herpetologynotes.seh-herpetology.org/Volume7_PDFs/Camera_HerpetologyNotes_volume7_pages31-36.pdf|title=Diet of the Neotropical frog Leptodactylus mystaceus|access-date=April 2, 2014|archive-date=April 7, 2014|archive-url=https://web.archive.org/web/20140407060239/http://www.herpetologynotes.seh-herpetology.org/Volume7_PDFs/Camera_HerpetologyNotes_volume7_pages31-36.pdf|url-status=dead}}</ref><ref>{{cite journal |first1=Bruno F. |last1=Camera |first2=Diones |last2=Krinski |first3=Isabella A. |last3=Calvo |title=Diet of the Neotropical frog ''Leptodactylus mystaceus'' (Anura: Leptodactylidae) |pages=31–36 |date=February 4, 2014 |journal=Herpetology Notes |volume=7 |url=http://www.herpetologynotes.seh-herpetology.org/Volume7_PDFs/Camera_HerpetologyNotes_volume7_pages31-36.pdf |access-date=April 26, 2015 |archive-date=September 24, 2015 |archive-url=https://web.archive.org/web/20150924030141/http://www.herpetologynotes.seh-herpetology.org/Volume7_PDFs/Camera_HerpetologyNotes_volume7_pages31-36.pdf |url-status=dead }}</ref> and [[folivory]] occurs in ''[[Euphlyctis hexadactylus]]'', with plants constituting 79.5% of its diet by volume.<ref>{{cite journal|last1=Das|first1=I.|title=Folivory and seasonal changes in diet in ''Rana hexadactyla'' (Anura: Ranidae)|journal=Journal of Zoology|date=April 1996|volume=238|issue=4|pages=785–794|doi=10.1111/j.1469-7998.1996.tb05430.x|url=http://ir.unimas.my/id/eprint/11519/1/100-%20Das%20%28Folivory%20in%20Rana%20hexadactyla%29.pdf }}</ref> Many frogs use their sticky tongues to catch prey, while others simply grab them with their mouths.<ref>{{cite book|author=Mattison, Chris|year=2011|title=Frogs and Toads of the Worlds|publisher=Princeton University Press|page=69|isbn=978-0-691-14968-4}}</ref> Adult frogs are themselves attacked by many predators. The [[northern leopard frog]] (''Rana pipiens'') is eaten by [[heron]]s, [[hawk]]s, fish, large [[salamanders]], [[snake]]s, [[raccoon]]s, [[skunk]]s, [[mink]], bullfrogs, and other animals.<ref>{{cite web |url=http://www3.northern.edu/natsource/AMPHIB1/Northe1.htm |title=Northern leopard frog (''Rana pipiens'') |author=Graham, Donna |work=An Educator's Guide to South Dakota's Natural Resources |access-date=August 4, 2012 |archive-url=https://web.archive.org/web/20120623230955/http://www3.northern.edu/natsource/AMPHIB1/Northe1.htm |archive-date=June 23, 2012 |url-status=dead }}</ref>
[[File:TrophicWeb.jpg|thumb|left|alt=|A trophic pyramid showing frogs as primary predators]]
Frogs are primary predators and an important part of the [[food web]]. Being [[Ectotherm|cold-blooded]], they make efficient use of the food they eat with little energy being used for metabolic processes, while the rest is transformed into [[biomass (ecology)|biomass]]. They are themselves eaten by secondary predators and are the primary terrestrial consumers of invertebrates, most of which feed on plants. By reducing herbivory, they play a part in increasing the growth of plants and are thus part of a delicately balanced ecosystem.<ref>{{cite journal|author=Pimm, Stuart L. |year=1979 |title=The structure of food webs |journal=Theoretical Population Biology |volume=16 |issue=2 |pages=144–158 |url=http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/12_Pimm_TPB_1979.pdf |doi=10.1016/0040-5809(79)90010-8 |pmid=538731 |bibcode=1979TPBio..16..144P |url-status=dead |archive-url=https://web.archive.org/web/20110927230926/http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/12_Pimm_TPB_1979.pdf |archive-date=September 27, 2011 }}</ref>
Little is known about the longevity of frogs and toads in the wild, but some can live for many years. [[Skeletochronology]] is a method of examining bones to determine age. Using this method, the ages of [[mountain yellow-legged frog]]s (''Rana muscosa'') were studied, the phalanges of the toes showing seasonal lines where growth slows in winter. The oldest frogs had ten bands, so their age was believed to be 14 years, including the four-year tadpole stage.<ref>{{cite journal |author1=Matthews, K. R. |author2=Miaud, C. |year=2007 |title=A skeletochronological study of the age structure, growth, and longevity of the mountain yellow-legged frog, ''Rana muscosa'', in the Sierra Nevada, California |journal=Copeia |volume=2007 |issue=4 |pages=986–993 |issn=0045-8511 |doi=10.1643/0045-8511(2007)7[986:ASSOTA]2.0.CO;2 |s2cid=86237494 }}{{subscription required}}</ref> Captive frogs and toads have been recorded as living for up to 40 years, an age achieved by a European common toad (''Bufo bufo''). The cane toad (''Rhinella marina'') has been known to survive 24 years in captivity, and the American bullfrog (''Rana catesbeiana'') 14 years.<ref>{{cite web |url=http://www.pondturtle.com/lfrog.html |title=Frog and toad index |author1=Slavens, Frank |author2=Slavens, Kate |work=Longevity |access-date=July 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120818221708/http://www.pondturtle.com/lfrog.html |archive-date=August 18, 2012 }}</ref> Frogs from temperate climates hibernate during the winter, and four species are known to be able to withstand freezing during this time, including the wood frog (''Rana sylvatica'').<ref>{{cite journal | last1=Storey | first1=KB | title=Life in a frozen state: adaptive strategies for natural freeze tolerance in amphibians and reptiles | journal=American Journal of Physiology | volume=258 | issue=3 Pt 2 | pages=559–568 | year=1990 |pmid = 2180324 | doi=10.1152/ajpregu.1990.258.3.R559 }}</ref>
===Parental care===
[[File:AlytesObstetricansMaleWithEggs.jpg|thumb|Male [[common midwife toad]] (''Alytes obstetricans'') with eggs]]
[[File:Assa darlingtoni.jpg|thumb|[[Pouched frog]] (''Assa darlingtoni'')]]
Although care of offspring is poorly understood in frogs, up to an estimated 20% of amphibian species may care for their young in some way.<ref>{{cite book|last=Crump| first=M. L.| year=1996| chapter=Parental care among the Amphibia| volume=25| pages=109–144| doi=10.1016/S0065-3454(08)60331-9|series=Advances in the Study of Behavior|isbn=978-0-12-004525-9 | title=Parental Care: Evolution, Mechanisms, and Adaptive Significance}}</ref> The [[evolution of biparental care in tropical frogs|evolution of parental care]] in frogs is driven primarily by the size of the water body in which they breed. Those that breed in smaller water bodies tend to have greater and more complex parental care behaviour.<ref name=Brown1>{{cite journal | doi = 10.1086/650727 | last1 = Brown | first1 = J. L. | last2 = Morales | first2 = V. | last3 = Summers | first3 = K. | year = 2010 | title = A key ecological trait drove the evolution of biparental care and monogamy in an amphibian | journal = American Naturalist | volume = 175 | issue = 4| pages = 436–446 | pmid = 20180700 | bibcode = 2010ANat..175..436B | s2cid = 20270737 }}</ref> Because predation of eggs and larvae is high in large water bodies, some frog species started to lay their eggs on land. Once this happened, the desiccating terrestrial environment demands that one or both parents keep them moist to ensure their survival.<ref>{{cite journal | last1= Sheridan | journal= Copeia | volume= 2008 | first1= Jennifer A. | last2= Ocock | first2= Joanne F. |pages=733–736 | title= Parental care in ''Chiromantis hansenae'' (Anura: Rhacophoridae) | year= 2008 | doi = 10.1643/CH-07-225 | issue= 4 | s2cid= 85122799 }}</ref> The subsequent need to transport hatched tadpoles to a water body required an even more intense form of parental care.<ref name=Brown1/>
In small pools, predators are mostly absent and competition between tadpoles becomes the variable that constrains their survival. Certain frog species avoid this competition by making use of smaller [[phytotelma]]ta (water-filled leaf [[wikt:axil|axils]] or small woody cavities) as sites for depositing a few tadpoles.<ref>{{cite journal | doi = 10.1111/j.1420-9101.2008.01609.x | last1 = Brown | first1 = J. L. | last2 = Morales | first2 = V. | last3 = Summers | first3 = K. | year = 2008a | title = Divergence in parental care, habitat selection and larval life history between two species of Peruvian poison frogs: An experimental analysis | journal = Journal of Evolutionary Biology | volume = 21 | issue = 6| pages = 1534–1543 | pmid = 18811668 | s2cid = 29546555 }}</ref> While these smaller rearing sites are free from competition, they also lack sufficient nutrients to support a tadpole without parental assistance. Frog species that changed from the use of larger to smaller phytotelmata have evolved a strategy of providing their offspring with nutritive but unfertilised eggs.<ref name=Brown1/> The female [[strawberry poison-dart frog]] (''Oophaga pumilio'') lays her eggs on the forest floor. The male frog guards them from predation and carries water in his cloaca to keep them moist. When they hatch, the female moves the tadpoles on her back to a water-holding [[bromeliad]] or other similar water body, depositing just one in each ___location. She visits them regularly and feeds them by laying one or two unfertilised eggs in the phytotelma, continuing to do this until the young are large enough to undergo metamorphosis.<ref>{{cite journal |author1=Grant, T. |author2=Frost, D. R. |author3=Caldwell, J. P. |author4=Gagliardo, R. |author5=Haddad, C. F. B. |author6=Kok, P. J. R. |author7=Means, D. B. |author8=Noonan, B. P. |author9=Schargel, W. E. |author10=Wheeler, W. |year=2006 |title=Phylogenetic systematics of dart-poison frogs and their relatives (Amphibia, Athesphatanura, Dendrobatidae) |journal=Bulletin of the American Museum of Natural History |volume=299 |pages=1–262 |doi=10.1206/0003-0090(2006)299[1:PSODFA]2.0.CO;2 |issn=0003-0090 |url=http://dendrobates.org/articles/Grant2006.pdf |access-date=November 13, 2017 |archive-url=https://web.archive.org/web/20160404073816/http://www.dendrobates.org/articles/Grant2006.pdf |archive-date=April 4, 2016 |url-status=dead |citeseerx=10.1.1.693.8392 |hdl=2246/5803 |s2cid=82263880 }}</ref> The [[granular poison frog]] (''Oophaga granulifera'') looks after its tadpoles in a similar way.<ref>{{cite journal |author1=van Wijngaarden, René |author2=Bolaños, Federico |year=1992 |title=Parental care in ''Dendrobates granuliferus'' (Anura: Dendrobatidae), with a description of the tadpole |journal=Journal of Herpetology |volume=26 |issue=1 |pages=102–105 |jstor=1565037 |doi=10.2307/1565037 }}</ref>
Many other diverse forms of parental care are seen in frogs. The tiny male ''[[Colostethus subpunctatus]]'' stands guard over his egg cluster, laid under a stone or log. When the eggs hatch, he transports the tadpoles on his back to a temporary pool, where he partially immerses himself in the water and one or more tadpoles drop off. He then moves on to another pool.<ref>{{cite journal |author1=Fandiño, María Claudia |author2=Lüddecke, Horst |author3=Amézquita, Adolfo |year=1997 |title=Vocalisation and larval transportation of male ''Colostethus subpunctatus'' (Anura: Dendrobatidae) |journal=Amphibia-Reptilia |volume=18 |issue=1 |pages=39–48 |doi=10.1163/156853897X00297 }}</ref> The male [[common midwife toad]] (''Alytes obstetricans'') carries the eggs around with him attached to his hind legs. He keeps them damp in dry weather by immersing himself in a pond, and prevents them from getting too wet in soggy vegetation by raising his hindquarters. After three to six weeks, he travels to a pond and the eggs hatch into tadpoles.<ref>{{cite web |url=http://www.herpfrance.com/amphibian/common_midwife_toad_alytes_obstetricans.php |title=Common midwife toad, ''Alytes obstetricans'' |work=Repties et Amphibiens de France |access-date=July 30, 2012}}</ref> The [[tungara frog]] (''Physalaemus pustulosus'') builds a floating nest from foam to protect its eggs from predation. The foam is made from [[protein]]s and [[lectin]]s, and seems to have antimicrobial properties.<ref>{{cite web |url=http://www.gla.ac.uk/schools/lifesciences/staff/malcolmkennedy/malcolmkennedy/proteinsoffrogfoamnests/ |title=Proteins of frog foam nests |publisher=University of Glasgow |access-date=August 24, 2012 |archive-date=June 3, 2013 |archive-url=https://web.archive.org/web/20130603175135/http://www.gla.ac.uk/schools/lifesciences/staff/malcolmkennedy/malcolmkennedy/proteinsoffrogfoamnests/ |url-status=dead }}</ref> Several pairs of frogs may form a colonial nest on a previously built raft. The eggs are laid in the centre, followed by alternate layers of foam and eggs, finishing with a foam capping.<ref>{{cite journal |author1=Dalgetty, Laura |author2=Kennedy, Malcolm W. |year=2010 |title=Building a home from foam—túngara frog foam nest architecture and three-phase construction process |journal=Biology Letters |volume=6 |issue=3 |pages=293–296 |doi=10.1098/rsbl.2009.0934 |doi-access=free |pmid=20106853 |pmc=2880057 }}</ref>
Some frogs protect their offspring inside their own bodies. Both male and female [[pouched frog]]s (''Assa darlingtoni'') guard their eggs, which are laid on the ground. When the eggs hatch, the male lubricates his body with the jelly surrounding them and immerses himself in the egg mass. The tadpoles wriggle into skin pouches on his side, where they develop until they metamorphose into juvenile frogs.<ref>{{cite web |url=http://frogs.org.au/frogs/species/Assa/darlingtoni/ |title=''Assa darlingtoni'' |year=2005 |work=Frogs Australia Network |access-date=August 5, 2012}}</ref> The female [[gastric-brooding frog]] (''Rheobatrachus'' sp.) from [[Australia]], now probably extinct, swallows her fertilised eggs, which then develop inside her stomach. She ceases to feed and stops secreting [[stomach acid]]. The tadpoles rely on the yolks of the eggs for nourishment. After six or seven weeks, they are ready for metamorphosis. The mother regurgitates the tiny frogs, which hop away from her mouth.<ref>{{cite web |url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Rheobatrachus_silus.html |title=''Rheobatrachus silus'' |author=Semeyn, E. |year=2002 |work=Animal Diversity Web |publisher=University of Michigan Museum of Zoology |access-date=August 5, 2012}}</ref> The female [[Darwin's frog]] (''Rhinoderma darwinii'') from [[Chile]] lays up to 40 eggs on the ground, where they are guarded by the male. When the tadpoles are about to hatch, they are engulfed by the male, which carries them around inside his much-enlarged vocal sac. Here they are immersed in a frothy, viscous liquid that contains some nourishment to supplement what they obtain from the yolks of the eggs. They remain in the sac for seven to ten weeks before undergoing metamorphosis, after which they move into the male's mouth and emerge.<ref>{{cite web |url=http://amphibiaweb.org/cgi/amphib_query?where-genus=Rhinoderma&where-species=darwinii |title=''Rhinoderma darwinii'' |author=Sandmeier, Fran |date=March 12, 2001 |work=AmphibiaWeb |access-date=August 5, 2012}}</ref>
==Defence==
[[File:R. imitator Chazuta.jpg|thumb|The mildly toxic ''[[Ranitomeya imitator]]'']]
[[File:Dendrobates pumilio.jpg|thumb|[[Strawberry poison-dart frog]] contains numerous alkaloids which deter predators.]]
At first sight, frogs seem rather defenceless because of their small size, slow movement, thin skin, and lack of defensive structures, such as spines, claws or teeth. Many use camouflage to avoid detection, the skin often being spotted or streaked in neutral colours that allow a stationary frog to merge into its surroundings. Some can make prodigious leaps, often into water, that help them to evade potential attackers, while many have other defensive adaptations and strategies.<ref name=Stebbins/>
The skin of many frogs contains mild toxic substances called [[bufotoxin]]s to make them unpalatable to potential predators. Most toads and some frogs have large poison glands, the [[parotoid gland]]s, located on the sides of their heads behind the eyes and other glands elsewhere on their bodies. These glands secrete mucus and a range of toxins that make frogs slippery to hold and distasteful or poisonous. If the noxious effect is immediate, the predator may cease its action and the frog may escape. If the effect develops more slowly, the predator may learn to avoid that species in future.<ref>{{cite journal |author1=Barthalmus, George T. |author2=Zielinski, William J. |year=1988 |title=''Xenopus'' skin mucus induces oral dyskinesias that promote escape from snakes |journal=Pharmacology Biochemistry and Behavior |volume=30 |issue=4 |pages=957–959 |doi=10.1016/0091-3057(88)90126-8 |pmid=3227042|s2cid=25434883 }}</ref> Poisonous frogs tend to advertise their toxicity with bright colours, an adaptive strategy known as [[aposematism]]. The [[poison dart frog]]s in the family Dendrobatidae do this. They are typically red, orange, or yellow, often with contrasting black markings on their bodies. ''[[Allobates zaparo]]'' is not poisonous, but mimics the appearance of two different toxic species with which it shares a common range in an effort to deceive predators.<ref>{{cite journal |author1=Darst, Catherine R. |author2=Cummings, Molly E. |year=2006 |title=Predator learning favours mimicry of a less-toxic model in poison frogs |journal=Nature |volume=440 |issue=7081 |pages=208–211 |doi=10.1038/nature04297 |pmid=16525472 |bibcode=2006Natur.440..208D |doi-access=free }}</ref> Other species, such as the [[European Fire-bellied Toad|European fire-bellied toad]] (''Bombina bombina''), have their warning colour underneath. They "flash" this when attacked, adopting a pose that exposes the vivid colouring on their bellies.<ref name=Bb/>
[[File:Bufo bufo-defensive reaction1.JPG|thumb|left|A common toad adopting a defensive stance]]
Some frogs, such as the [[poison dart frog]]s, are especially toxic. The native peoples of South America extract poison from these frogs to apply to their [[Dart (missile)|weapons]] for hunting,<ref>{{cite journal| doi=10.1038/scientificamerican0283-120| last=Myers| first=C. W.|author2=Daly, J. W.| year=1983| title=Dart-poison frogs| journal=Scientific American| volume=248| pages=120–133| pmid=6836257| issue=2| bibcode=1983SciAm.248b.120M}}</ref> although few species are toxic enough to be used for this purpose. At least two non-poisonous frog species in tropical America (''[[Eleutherodactylus|Eleutherodactylus gaigei]]'' and ''[[Lithodytes lineatus]]'') [[Batesian mimicry|mimic]] the colouration of dart poison frogs for self-protection.<ref>{{cite book| last=Savage| first=J. M.| year=2002| title=The Amphibians and Reptiles of Costa Rica| url=https://archive.org/details/amphibiansreptil0000sava| url-access=registration| publisher=University of Chicago Press| isbn=978-0-916984-16-8}}</ref><ref>{{cite journal|last=Duellman |first=W. E. |year=1978 |title=The Biology of an Equatorial Herpetofauna in Amazonian Ecuador |journal=University of Kansas Museum of Natural History Miscellaneous Publication |volume=65 |url=http://200.31.31.2/Recursos/publicaciones/Cientifica/dulleman1978.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110704040530/http://200.31.31.2/Recursos/publicaciones/Cientifica/dulleman1978.pdf |archive-date=July 4, 2011 }}</ref> Some frogs obtain poisons from the ants and other arthropods they eat.<ref>{{cite journal|last=Saporito| first=R.A.|author2=Garraffo, H. M.|author3= Donnelly, M. A.|author4= Edwards, A. L.|author5= Longino, J. T.|author6= Daly, J. W.| year=2004| title=Formicine ants: an arthropod source for the pumiliotoxin alkaloids of dendrobatid poison frogs| journal=Proceedings of the National Academy of Sciences| volume=101| pages=8045–8050| doi=10.1073/pnas.0402365101| pmid=15128938|issue=21|pmc=419554|bibcode=2004PNAS..101.8045S| doi-access=free}}</ref> Others, such as the Australian [[Corroboree frog (disambiguation)|corroboree frog]]s (''Pseudophryne corroboree'' and ''Pseudophryne pengilleyi''), can synthesize the [[alkaloid]]s themselves.<ref>{{cite journal| last=Smith| first=B. P.|author2=Tyler, M. J.|author3= Kaneko, T.|author4= Garraffo, H. M.|author5= Spande, T. F.|author6= Daly, J. W. | year=2002| title=Evidence for biosynthesis of pseudophrynamine alkaloids by an Australian myobatrachid frog (''Pseudophryne'') and for sequestration of dietary pumiliotoxins| journal=Journal of Natural Products| volume=65| issue=4| pages=439–47| doi=10.1021/np010506a| pmid=11975476| bibcode=2002JNAtP..65..439S}}</ref> The chemicals involved may be irritants, [[hallucinogen]]s, [[seizure|convulsants]], [[neurotoxin|nerve poisons]] or [[vasoconstrictor]]s. Many predators of frogs have become adapted to tolerate high levels of these poisons, but other creatures, including humans who handle the frogs, may be severely affected.<ref>{{cite web |url=http://www.amnh.org/exhibitions/past-exhibitions/frogs-a-chorus-of-colors/poison-dart-frog-vivarium |title=Poison Dart Frog Vivarium |author=Grant, T. |publisher=American Museum of Natural History |access-date=July 7, 2012}}</ref>
Some frogs use bluff or deception. The European common toad (''Bufo bufo'') adopts a characteristic stance when attacked, inflating its body and standing with its hindquarters raised and its head lowered.<ref>{{cite book |title=Reptiles and Amphibians of Britain and Europe |last=Arnold |first=Nicholas |author2=Ovenden, Denys |year=2002 |publisher=Harper Collins Publishers |isbn=978-0-00-219964-3 |pages=73–74 }}</ref> The bullfrog (''Rana catesbeiana'') crouches down with eyes closed and head tipped forward when threatened. This places the parotoid glands in the most effective position, the other glands on its back begin to ooze noxious secretions and the most vulnerable parts of its body are protected.<ref name=Stebbins/> Another tactic used by some frogs is to "scream", the sudden loud noise tending to startle the predator. The grey tree frog (''[[Hyla versicolor]]'') makes an explosive sound that sometimes repels the shrew ''[[Blarina brevicauda]]''.<ref name=Stebbins/> Although toads are avoided by many predators, the [[Common Garter Snake|common garter snake]] (''Thamnophis sirtalis'') regularly feeds on them. The strategy employed by juvenile American toads (''[[Bufo americanus]]'') on being approached by a snake is to crouch down and remain immobile. This is usually successful, with the snake passing by and the toad remaining undetected. If it is encountered by the snake's head, however, the toad hops away before crouching defensively.<ref>{{cite journal |author=Hayes, Floyd E. |year=1989 |title=Antipredator behavior of recently metamorphosed toads (''Bufo a. americanus'') during encounters with garter snakes (''Thamnophis s. sirtalis'') |journal=Copeia |volume=1989 |issue=4 |pages=1011–1015 |jstor=1445987 |doi=10.2307/1445987 }}</ref>
==Distribution==
[[File:Wood Frog (Rana sylvatica) (25234151669).jpg|thumb|Although frogs are most diverse in warm regions, a few species like the [[wood frog]] live at the [[Arctic Circle]].]]
Frogs live on every continent except Antarctica, but they are not present on certain islands, especially those far away from continental land masses.<ref>{{cite web |url=http://magma.nationalgeographic.com/ngexplorer/0403/articles/mainarticle.html |archive-url=https://web.archive.org/web/20040306180404/http://magma.nationalgeographic.com/ngexplorer/0403/articles/mainarticle.html |url-status=dead |archive-date=March 6, 2004 |title=Freaky Frogs |publisher=National Geographic Explorer |access-date=July 13, 2012}}</ref><ref>{{cite encyclopedia |url=http://evolution-facts.org/Ev-V3/3evlch27.htm |title=Geographical Distribution |last1=Ferrell |first1=Vance |date=March 4, 2012 |encyclopedia=Evolution Encyclopedia |volume=3 |publisher=Evolution Facts |access-date=July 13, 2012 |archive-date=May 5, 2012 |archive-url=https://web.archive.org/web/20120505220038/http://www.evolution-facts.org/Ev-V3/3evlch27.htm |url-status=dead }}</ref> Many species are isolated in restricted ranges by changes of climate or inhospitable territory, such as stretches of sea, mountain ridges, deserts, forest clearance, road construction, or other human-made barriers.<ref>{{cite encyclopedia |url=http://www.teara.govt.nz/en/frogs/ |title=Story: Frogs |last1=Ryan |first1=Paddy |date=September 25, 2011 |encyclopedia=The Encyclopedia of New Zealand |access-date=August 20, 2012}}</ref> Usually, a greater diversity of frogs occurs in tropical areas than in temperate regions, such as Europe.<ref>{{cite journal |author1=Dahl, Chris |author2=Novotny, Vojtech |author3=Moravec, Jiri |author4=Richards, Stephen J. |year=2009 |title=Beta diversity of frogs in the forests of New Guinea, Amazonia and Europe: contrasting tropical and temperate communities |journal=Journal of Biogeography |volume=36 |issue=5 |pages=896–904 |doi=10.1111/j.1365-2699.2008.02042.x |bibcode=2009JBiog..36..896D |s2cid=13067241 }}</ref> Some frogs inhabit arid areas, such as deserts, and rely on specific adaptations to survive. Members of the Australian genus ''[[Cyclorana]]'' bury themselves underground where they create a water-impervious cocoon in which to [[aestivation|aestivate]] during dry periods. Once it rains, they emerge, find a temporary pool, and breed. Egg and tadpole development is very fast compared with those of most other frogs, so breeding can be completed before the pond dries up.<ref>{{cite web |url=http://frogs.org.au/frogs/species/Cyclorana/platycephala/ |title=''Cyclorana platycephala'' |date=February 23, 2005 |publisher=Frogs Australia Network |access-date=July 20, 2012}}</ref> Some frog species are adapted to a cold environment. The [[wood frog]] (''Rana sylvatica''), whose habitat extends into the [[Arctic Circle]], buries itself in the ground during winter. Although much of its body freezes during this time, it maintains a high concentration of glucose in its vital organs, which protects them from damage.<ref name=Exploratorium/>
== Conservation ==
{{See also|Decline in amphibian populations}}
[[File:Bufo periglenes2.jpg|thumb|[[Golden toad]] (''Bufo periglenes'') – last seen in 1989]]
In 2006, of 4,035 species of amphibians that depend on water during some lifecycle stage, 1,356 (33.6%) were considered to be threatened. This is likely to be an underestimate because it excludes 1,427 species for which evidence was insufficient to assess their status.<ref>{{cite web |url=http://app.databasin.org/app/pages/datasetPage.jsp?id=461e58214aa54ad79382066ab829c05f |title=Number of Globally Threatened Amphibian Species by Freshwater Ecoregion |author1=Hoekstra, J. M. |author2=Molnar, J. L. |author3=Jennings, M. |author4=Revenga, C. |author5=Spalding, M. D. |author6=Boucher, T. M. |author7=Robertson, J. C. |author8=Heibel, T. J. |author9=Ellison, K. |year=2010 |work=The Atlas of Global Conservation: Changes, Challenges, and Opportunities to Make a Difference. |publisher=The Nature Conservancy |access-date=September 5, 2012}}</ref> Frog populations have declined dramatically since the 1950s. More than one-third of frog species are considered to be threatened with [[extinction]], and more than 120 species are believed to have become extinct since the 1980s.<ref name="Stuart">{{cite journal| last=Stuart| first=S. N.| author2=Chanson, J. S.| author3=Cox, N. A.| author4=Young, B. E.| author5=Rodrigues, A. S. L.| author6=Fischman, D. L.| author7=Waller, R. W.| year=2004| title=Status and trends of amphibian declines and extinctions worldwide| journal=Science| volume=306| pages=1783–1786| doi=10.1126/science.1103538| pmid=15486254| issue=5702| url=http://www.natureserve.org/library/gaa_science120304.pdf| bibcode=2004Sci...306.1783S| access-date=October 27, 2017| archive-url=https://web.archive.org/web/20171027232036/http://www.natureserve.org/library/gaa_science120304.pdf| archive-date=October 27, 2017| url-status=dead| citeseerx=10.1.1.225.9620| s2cid=86238651}}</ref> Among these species are the gastric-brooding frogs of Australia and the [[golden toad]] of Costa Rica. The latter is of particular concern to scientists because it inhabited the pristine [[Monteverde Cloud Forest Reserve]] and its population crashed in 1987, along with about 20 other frog species in the area. This could not be linked directly to human activities, such as deforestation, and was outside the range of normal fluctuations in population size.<ref>{{cite journal |author1=Pounds, J. Alan |author2=Fogden, Michael P. L. |author3=Savage, Jay M. |author4=Gorman, George C. |year=1997 |title=Tests of null models for amphibian declines on a tropical mountain |journal=Conservation Biology |volume=11 |issue=6 |pages=1307–1322 |doi=10.1046/j.1523-1739.1997.95485.x |bibcode=1997ConBi..11.1307P |s2cid=85382659 }}</ref> Elsewhere, habitat loss is a significant cause of frog population decline, as are pollutants, climate change, increased [[ultraviolet|UVB]] radiation, and the introduction of [[Indigenous (ecology)|non-native]] predators and competitors.<ref>{{cite web |url=http://amphibiaweb.org/declines/declines.html |title=Worldwide Amphibian Declines: How big is the problem, what are the causes and what can be done? |date=January 22, 2009 |publisher=AmphibiaWeb |access-date=October 15, 2012}}</ref> A Canadian study conducted in 2006 suggested heavy traffic in their environment was a larger threat to frog populations than was habitat loss.<ref>{{cite news |title=Frog population decrease mostly due to traffic |url=https://www.newscientist.com/article/dn9506-frogs-toads-and-automobiles--a-fatal-combination.html |newspaper=New Scientist |date=July 7, 2006 |access-date=July 13, 2012}}</ref> Emerging infectious diseases, including [[chytridiomycosis]] and [[ranavirus]], are also devastating populations.<ref>{{cite journal |author1=Voordouw, M. J. |author2=Adama, D. |author3=Houston, B. |author4=Govindarajulu, P. |year=2010 |title=Prevalence of the pathogenic chytrid fungus, ''Batrachochytrium dendrobatidis'', in an endangered population of northern leopard frogs, ''Rana Pipiens'' |journal=BMC Ecology |volume=10 |issue=6 |pages=6 |doi=10.1186/1472-6785-10-6 |pmid=20202208 |pmc=2846871 |doi-access=free |bibcode=2010BMCE...10....6V }}</ref><ref>{{cite journal |author1=Harp, Elizabeth M. |author2=Petranka, James W. |year=2006 |title=Ranavirus in wood frogs (''Rana sylvatica''): Potential sources of transmission within and between ponds |journal=Journal of Wildlife Diseases |volume=42 |issue=2 |pages=307–318 |pmid=16870853 |doi=10.7589/0090-3558-42.2.307 |doi-access=free}}</ref>
Many environmental scientists believe amphibians, including frogs, are good biological [[indicator species|indicators]] of broader [[ecosystem]] health because of their intermediate positions in food chains, their permeable skins, and typically biphasic lives (aquatic larvae and terrestrial adults).<ref>{{cite book| last=Phillips| first=Kathryn| title=Tracking the Vanishing Frogs| publisher=Penguin Books| year=1994| isbn=978-0-14-024646-9}}</ref> It appears that species with both aquatic eggs and larvae are most affected by the decline, while those with direct development are the most resistant.<ref>{{cite journal | last1= Lips | pages= 106–117 | first1= Karen R. | volume= 12 | issue= 1 | year= 2008 | title= Decline of a tropical montane amphibian fauna | doi = 10.1111/j.1523-1739.1998.96359.x | journal= Conservation Biology | jstor=2387466 | s2cid= 55870526 }}</ref>
[[File:Deformed Frog.gif|left|thumb|Deformed [[mink frog]] with an extra left leg]]
[[Frog mutations and genetic defects]] have increased since the 1990s. These often include missing legs or extra legs. Various causes have been identified or hypothesized, including an increase in [[Ultraviolet|ultraviolet radiation]] affecting the spawn on the surface of ponds, chemical contamination from pesticides and fertilizers, and parasites such as the [[trematode]] ''[[Ribeiroia ondatrae]]''. Probably all these are involved in a complex way as [[stressor]]s, environmental factors contributing to rates of disease, and vulnerability to attack by parasites. Malformations impair mobility and the individuals may not survive to adulthood. An increase in the number of frogs eaten by birds may actually increase the likelihood of parasitism of other frogs, because the trematode's complex lifecycle includes the [[Planorbidae|ramshorn snail]] and several intermediate hosts such as birds.<ref name=Blaustein>{{cite journal|author1=Blaustein, Andrew R. |author2=Johnson, Pieter T. J. |year=2003 |title=The complexity of deformed amphibians |journal=Frontiers in Ecology and the Environment |volume=1 |issue=2 |pages=87–94 |url=http://tiee.ecoed.net/vol/v2/issues/frontier_sets/amphibians/pdf/Frontiers-Blaustein-Johnson.pdf |doi=10.1890/1540-9295(2003)001[0087:TCODA]2.0.CO;2 |url-status=dead |archive-url=https://web.archive.org/web/20131029213623/http://tiee.ecoed.net/vol/v2/issues/frontier_sets/amphibians/pdf/Frontiers-Blaustein-Johnson.pdf |archive-date=October 29, 2013 }}</ref><ref>{{cite journal|title=Strategies for assessing the implications of malformed frogs for environmental health |author1=Burkhart, J. G. |author2=Ankley, G. |author3=Bell, H. |author4=Carpenter, H. |author5=Fort, D. |author6=Gardiner, D. |author7=Gardner, H. |author8=Hale, R. |author9=Helgen, J. C. |author10=Jepson, P. |author11=Johnson, D. |author12=Lannoo, M. |author13=Lee, D. |author14=Lary, J. |author15=Levey, R. |author16=Magner, J. |author17=Meteyer, C. |author18=Shelby, M. D. |author19=Lucier, G. |journal=Environmental Health Perspectives |volume=108 |issue=1 |year=2000 |doi=10.2307/3454299 |doi-access=free |pmid=10620528 |pmc=1637865 |jstor=3454299 |pages=83–90}}</ref>
In a few cases, captive breeding programs have been established and have largely been successful.<ref>{{cite news|url=http://news.bbc.co.uk/1/hi/sci/tech/4298050.stm |title=New frog centre for London Zoo |work=BBC News |author=Black, Richard |access-date=November 3, 2008 |date=October 2, 2005}}</ref><ref>{{cite web|url=http://www.environment.gov.au/biodiversity/threatened/publications/recovery/p-corroboree/part4.html|archive-url=https://web.archive.org/web/20080512065637/http://www.environment.gov.au/biodiversity/threatened/publications/recovery/p-corroboree/part4.html|archive-date=May 12, 2008 |title=National recovery plan for the Southern Corroboree Frog (Pseudophryne corroboree): 5. Previous recovery actions |publisher=Environment.gov.au |access-date=November 3, 2008}}</ref> The [[World Association of Zoos and Aquariums]] named 2008 as the "Year of the Frog" in order to draw attention to the conservation issues faced by them.<ref>{{cite web |url=http://www.bgci.org/ourwork/news/0477/ |title=2008: Year of the Frog |date=January 15, 2008 |access-date=July 13, 2012 |archive-url=https://web.archive.org/web/20120421180510/http://www.bgci.org/ourwork/news/0477 |archive-date=April 21, 2012 |url-status=dead }}</ref>
The [[cane toad]] (''Rhinella marina'') is a very adaptable species native to South and Central America. In the 1930s, it was introduced into Puerto Rico, and later various other islands in the Pacific and Caribbean region, as a [[biological pest control]] agent.<ref>{{cite book | last=Tyler | first=Michael J. | year=1989 | title=Australian Frogs | publisher=Penguin Books | page=111 | isbn=978-0-670-90123-4 }}</ref> In 1935, 3000 toads were liberated in the [[sugar cane]] fields of Queensland, Australia, in an attempt to control [[cane beetle]]s such as ''Dermolepida albohirtum'', the larvae of which damage and kill the canes. Initial results in many of these countries were positive, but it later became apparent that the toads upset the ecological balance in their new environments. They bred freely, competed with native frog species, ate bees and other harmless native invertebrates, had few predators in their adopted habitats, and poisoned pets, carnivorous birds, and mammals. In many of these countries, they are now regarded both as pests and [[invasive species]], and scientists are looking for a biological method to control them.<ref>{{cite web |url=https://australian.museum/learn/animals/frogs/cane-toad/ |title=Cane toad |author=Cameron, Elizabeth |date=September 6, 2012 |publisher=Australian Museum |access-date=September 12, 2012}}</ref>
==
===Culinary===
{{Cookbook|Frog}}{{Main|Frog legs}}
[[File:2012 Froschschenkel anagoria.JPG|thumb|French ''cuisses de grenouille'']]
[[Frog legs]] are eaten by humans in many parts of the world. Indonesia is the world's largest exporter of frog meat, exporting more than 5,000 tonnes of frog meat each year, mostly to France, Belgium and Luxembourg.<ref name="abc news">̺{{cite news|url =https://abcnews.go.com/Technology/story?id=6688391&page=1 |newspaper =abc news |title =Appetite For Frogs' Legs Harming Wild Populations|date =January 20, 2009| first= Catherine |last =Brahic}}</ref> Originally, they were supplied from local wild populations, but overexploitation led to a diminution in the supply. This resulted in the development of [[Aquaculture|frog farming]] and a global trade in frogs. The main importing countries are France, Belgium, Luxembourg, and the United States, while the chief exporting nations are Indonesia and China.<ref name=Warkentin>{{cite journal |author1=Warkentin, I. G. |author2=Bickford, D. |author3=Sodhi, N. S. |author4=Corey, J. A. |year=2009 |title=Eating frogs to extinction |journal=Conservation Biology |volume=23 |issue=4 |pages=1056–1059 |doi=10.1111/j.1523-1739.2008.01165.x |pmid=19210303 |bibcode=2009ConBi..23.1056W |s2cid=1837255 }}</ref> The annual global trade in the [[American bullfrog]] (''Rana catesbeiana''), mostly farmed in China, varies between 1200 and 2400 tonnes.<ref>{{cite web |url=http://www.fao.org/fishery/culturedspecies/Rana_catesbeiana/en |title=Cultured Aquatic Species Information Programme: ''Rana catesbeiana'' |publisher=FAO: Fisheries and Aquaculture Department |access-date=July 5, 2012}}</ref>
The [[Leptodactylus fallax|mountain chicken frog]], so-called as it tastes of chicken, is now endangered, in part due to human consumption, and was a major food choice of the [[Dominica]]ns.<ref>{{cite web|url=http://news.nationalgeographic.com/2016/01/160128-mountain-chicken-frog-endangered-animals/|archive-url=https://web.archive.org/web/20160129185426/http://news.nationalgeographic.com/2016/01/160128-mountain-chicken-frog-endangered-animals/|url-status=dead|archive-date=January 29, 2016|title=The Mountain Chicken Frog's First Problem: It Tastes Like...|author= Ryan Schuessler|date=January 28, 2016|work=National Geographic News}}</ref> [[Raccoon]], [[Virginia opossum|opossum]], [[partridges]], [[Greater prairie chicken|prairie chicken]], and frogs were among the fare [[Mark Twain]] recorded as part of American cuisine.<ref name="TwainWarner1904">{{cite book|author1=Mark Twain|author2=Charles Dudley Warner|title=The Writings of Mark Twain [pseud.].: A tramp abroad|url=https://books.google.com/books?id=sKdGAQAAMAAJ&q=mark+twain++possum,+coon+and+prairie+hen&pg=PA263|year=1904|publisher=Harper & Bros.|page=263}}</ref>
===Scientific research===
{{See also|Animal testing on frogs|Frogs in captivity}}
In November 1970, [[NASA]] sent two bullfrogs into space for six days during the [[Orbiting Frog Otolith]] mission to test weightlessness.
Frogs are used for [[dissection]]s in high school and university anatomy classes, often first being injected with coloured substances to enhance contrasts among the [[biological system]]s. This practice is declining due to [[animal welfare]] concerns, and "digital frogs" are now available for virtual dissection.<ref>{{cite news |title=California Schools Leading Race to Stop Dissections |url=http://awionline.org/content/california-schools-leading-race-stop-dissections |newspaper=Animal Welfare Institute |date=April 25, 2011 |access-date=June 17, 2012}}</ref>
Frogs have served as [[Animal testing|experimental animals]] throughout the history of science. Eighteenth-century biologist [[Luigi Galvani]] discovered the link between [[electricity]] and the [[nervous system]] by studying frogs. He created [[Frog galvanometer|one of the first tools]] for measuring [[electric current]] out of a frog leg.<ref>{{cite book |title=The science of common things: a familiar explanation of the first principles of physical science. For schools, families, and young students |last=Wells |first=David Ames |year=1859 |publisher=Ivison, Phinney, Blakeman |page=290 |url={{Google books|ajQAAAAAYAAJ|page=290|plainurl=yes}} }}</ref> In 1852, H. F. Stannius used a frog's heart in a procedure called a [[Stannius ligature]] to demonstrate the ventricle and atria beat independently of each other and at different rates.<ref>{{cite web |url=http://www.biology-online.org/dictionary/Stannius_ligature |title=Stannius ligature |date=October 3, 2005 |work=Biology online |access-date=August 5, 2012}}</ref> The [[African clawed frog]] or platanna (''Xenopus laevis'') was first widely used in laboratories in pregnancy tests in the first half of the 20th century. A sample of urine from a pregnant woman injected into a female frog induces it to lay [[egg]]s, a discovery made by English zoologist [[Lancelot Hogben]]. This is because a hormone, [[human chorionic gonadotropin]], is present in substantial quantities in the urine of women during pregnancy.<ref>{{cite journal | last1 = Sarkar | first1 = S. | title = Lancelot Hogben, 1895–1975 | journal = Genetics | volume = 142 | issue = 3 | pages = 655–660 | year = 1996 | doi = 10.1093/genetics/142.3.655 | pmid = 8849876 | pmc = 1207007}}</ref> In 1952, [[Robert William Briggs|Robert Briggs]] and [[Thomas J. King]] cloned a frog by [[somatic cell nuclear transfer]]. This same technique was later used to create [[Dolly (sheep)|Dolly the sheep]], and their experiment was the first time a successful nuclear transplantation had been accomplished in higher animals.<ref>{{cite web |url=http://www.pnas.org/site/misc/classics4.shtml |archive-url=https://web.archive.org/web/20121014105435/http://www.pnas.org/site/misc/classics4.shtml |archive-date=October 14, 2012 |title=Nuclear Transfer: Bringing in the Clones |author=Brownlee, Christen |work=Proceedings of the National Academy of Sciences of the United States of America |access-date=October 21, 2012}}</ref>
Frogs are used in cloning research and other branches of [[embryology]]. Frogs of the genus ''[[Xenopus]]'' are used as a [[model organism]] in [[developmental biology]] because their embryos are large and easy to manipulate, they are readily obtainable, and can easily be kept in the laboratory.<ref>{{cite web |url=http://people.ucalgary.ca/~browder/frogsrus.html |title=''Xenopus'' as a Model System in Developmental Biology |editor1=Browder, L.|editor2=Iten, L.|year=1998 |work=Dynamic Development |publisher=University of Calgary |access-date=June 17, 2012}}</ref> ''[[Xenopus laevis]]'' is increasingly being displaced by its smaller relative, ''[[Xenopus tropicalis]]'', which reaches its reproductive age in five months rather than the one to two years for ''X. laevis'',<ref>{{cite web | url=https://grants.nih.gov/grants/guide/rfa-files/RFA-HD-01-008.html | title=Developing the potential of ''Xenopus tropicalis'' as a genetic model |author=Klein, S.|publisher=Trans-NIH Xenopus Working Group | access-date=March 9, 2006 }}</ref> thus facilitating faster studies across generations.
Genomes of ''Xenopus laevis'', ''X. tropicalis'', ''Rana catesbeiana'', ''Rhinella marina'', and ''Nanorana parkeri'' have been sequenced and deposited in the [[National Center for Biotechnology Information|NCBI]] Genome database.<ref>{{Cite web|url=https://www.ncbi.nlm.nih.gov/genome/browse/#!/overview/Amphibia|title=Genome List – Genome |website=NCBI |access-date=April 7, 2019 |url-status=dead |archive-url=https://web.archive.org/web/20190713120919/https://www.ncbi.nlm.nih.gov/genome/browse/#!/overview/Amphibia |archive-date= July 13, 2019 }}</ref>
===Pharmaceutical===
[[File:Schrecklicherpfeilgiftfrosch-01.jpg|thumb|left|upright|[[Golden poison frog]] (''Phyllobates terribilis'')]]
Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid [[epibatidine]], a painkiller 200 times more potent than [[morphine]], is made by some species of [[poison dart frog]]s. Other chemicals isolated from the skins of frogs may offer resistance to [[HIV]] infection.<ref>{{cite journal |last1=VanCompernolle |first1=S. E. |last2=Taylor |first2=R. J. |last3=Oswald-Richter |first3=K. |last4=Jiang |first4=J. |last5=Youree |first5=B. E. |last6=Bowie |first6=J. H. |last7=Tyler |first7=M. J. |last8=Conlon |first8=M. |last9=Wade |first9=D. |last10=Aiken |first10=C. |last11=Dermody |first11=T. S. |last12=KewalRamani |first12=V. N. |last13=Rollins-Smith |first13=L. A. |last14=Unutmaz |first14=D. |display-authors=9 |date=2005 |title=Antimicrobial peptides from amphibian skin potently inhibit human immunodeficiency virus infection and transfer of virus from dendritic cells to T cells |journal=Journal of Virology |volume=79 |issue=18 |pages=11598–11606 |doi=10.1128/JVI.79.18.11598-11606.2005 |doi-access=free |pmid=16140737 |pmc=1212620}}</ref> Dart poisons are under active investigation for their potential as therapeutic drugs.<ref>{{cite journal |last1=Phillipe |first1=G. |last2=Angenot |first2=L. |date=2005 |title=Recent developments in the field of arrow and dart poisons |journal=Journal of Ethnopharmacology |pmid=15993556 |volume=100 |issue=1–2 |pages=85–91 |doi=10.1016/j.jep.2005.05.022|url=http://orbi.ulg.ac.be/handle/2268/16897 }}</ref>
It has long been suspected that pre-Columbian [[Mesoamerica]]ns used a toxic secretion produced by the cane toad as a [[hallucinogen]], but more likely they used substances secreted by the [[Colorado River toad]] (''Bufo alvarius''). These contain [[bufotenin]] (5-MeO-DMT), a [[psychoactive drug|psychoactive compound]] that has been used in modern times as a [[recreational drug]]. Typically, the skin secretions are dried and then smoked.<ref>{{cite journal |author1=Lyttle, T. |author2=Goldstein, D. |author3=Gartz, J. |year=1996 |title=''Bufo'' toads and bufotenine: fact and fiction surrounding an alleged psychedelic |journal=Journal of Psychoactive Drugs |volume=28 |issue=3 |pages=267–290 |pmid=8895112 |doi=10.1080/02791072.1996.10472488 |citeseerx=10.1.1.688.5926 }}</ref> Illicit drug use by licking the skin of a toad has been reported in the media, but this may be an [[urban myth]].<ref>{{cite journal |author=Lyttle, T. |year=1993 |title=Misuse and legend in the "toad licking" phenomenon |journal=International Journal of the Addictions |volume=28 |issue=6 |pages=521–538 |pmid=8486435 |doi=10.3109/10826089309039645}}</ref>
Exudations from the skin of the [[golden poison frog]] (''Phyllobates terribilis'') are traditionally used by native Colombians to poison the darts they use for hunting. The tip of the projectile is rubbed over the back of the frog and the dart is launched from a [[blowgun]]. The combination of the two alkaloid toxins [[batrachotoxin]] and [[homobatrachotoxin]] is so powerful, one frog contains enough poison to kill an estimated 22,000 mice.<ref name=dartpoison>{{cite journal |author1=Myers, Charles W. |author2=Daly, John W. |author3=Malkin, Borys |year=1978 |title=A dangerously toxic new frog (''Phyllobates'') used by Emberá Indians of western Colombia, with discussion of blowgun fabrication and dart poisoning |journal=Bulletin of the American Museum of Natural History |volume=161 |pages=307–366 |hdl=2246/1286 |hdl-access=free}}</ref> Two other species, the [[Kokoe poison dart frog]] (''Phyllobates aurotaenia'') and the [[black-legged dart frog]] (''Phyllobates bicolor'') are also used for this purpose. These are less toxic and less abundant than the golden poison frog. They are impaled on pointed sticks and may be heated over a fire to maximise the quantity of poison that can be transferred to the dart.<ref name=dartpoison/>
[[File:Toadlarcomuseum.jpg|thumb|right|upright|[[Moche culture|Moche]] frog sculpture]]
=== Cultural significance ===
{{Main|Frogs in culture}}
Frogs have been featured in mythology, [[fairy tale]]s and popular culture. In traditional Chinese myths, the world rests on a giant frog, who would try to swallow the moon, causing the [[lunar eclipse]]. Frogs have been featured in religion, folklore, and popular culture. The [[ancient Egyptian]]s depicted the god [[Heqet]], protector of newborns, with the head of a frog. For the [[Mayan civilization|Mayans]], frogs represented water, crops, fertility and birth and were associated with the god [[Chaac]]. In the [[Bible]], [[Moses]] unleashes a [[Plagues of Egypt|plague of frog]]s on the Egyptians. Medieval Europeans associated frogs and toads with evil and [[witchcraft]].<ref name=culture>{{cite book|author=Mattison, Chris|year=2011|title=Frogs and Toads of the Worlds|publisher=Princeton University Press|pages=91–92|isbn=978-0-691-14968-4}}</ref> The [[Brothers Grimm]] fairy tale ''[[The Frog Prince]]'' features a princess taking in a frog and it turning into a handsome prince.<ref>{{cite book|author=Sleigh, Charlotee|year=2012|title=Frog|publisher=Reaktion Books|pages=40–42|isbn=978-1-86189-920-0}}</ref> In modern culture, frogs may take a comedic or hapless role, such as [[Mr. Toad]] of the 1908 novel ''[[The Wind in the Willows]]'', [[Michigan J. Frog]] of [[Warner Bros. Cartoons]], the [[Muppet]] [[Kermit the Frog]] and in the game ''[[Frogger]]''.<ref>{{cite book|author=Sleigh, Charlotee|year=2012|title=Frog|publisher=Reaktion Books|pages=167–175|isbn=978-1-86189-920-0}}</ref>
{{clear}}
==References==
{{Reflist|30em}}
==Further reading==
* {{cite book |last=Beltz |first=Ellin |year=2005 |title=Frogs: Inside their Remarkable World |publisher=Firefly Books |isbn=978-1-55297-869-6 |url=https://archive.org/details/frogs00elli }}
* {{cite book |last1=Cogger |first1=H. G. |last2=Zweifel |first2=R. G. |last3=Kirschner |first3=D. |date=2004 |title=Encyclopedia of Reptiles & Amphibians |edition=2nd |publisher=Fog City Press |isbn=978-1-877019-69-2}}
* Estes, R., and O. A. Reig. (1973). "The early fossil record of frogs: a review of the evidence." pp. 11–63 In J. L. Vial (Ed.), ''Evolutionary Biology of the Anurans: Contemporary Research on Major Problems''. University of Missouri Press, Columbia.
* {{cite journal |last1=Gissi |first1=Carmela |last2=San Mauro |first2=Diego |last3=Pesole |first3=Graziano |last4=Zardoya |first4=Rafael |title=Mitochondrial phylogeny of Anura (Amphibia): A case study of congruent phylogenetic reconstruction using amino acid and nucleotide characters |date=February 2006 |doi=10.1016/j.gene.2005.07.034 |journal=Gene |volume=366 |issue=2 |pmid=16307849 |pages=228–237}}
* {{cite book |last=Holman |first=J. A. |year=2004 |title=Fossil Frogs and Toads of North America |publisher=Indiana University Press |isbn=978-0-253-34280-5}}
* {{cite journal |last1=San Mauro |first1=Diego |last2=Vences |first2=Miguel |last3=Alcobendas |first3=Marina |last4=Zardoya |first4=Rafael |last5=Meyer |first5=Axel |title=Initial diversification of living amphibians predated the breakup of Pangaea |date=May 2005 |journal=American Naturalist |volume=165 |issue=5 |pages=590–599 |doi=10.1086/429523 |pmid=15795855 |bibcode=2005ANat..165..590S |s2cid=17021360 |url=http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-33053 }}
* {{cite book |last=Tyler |first=M. J. |year=1994 |title=Australian Frogs A Natural History |publisher=Reed Books |isbn=978-0-7301-0468-1}}
==External links==
{{Commons category|Frogs}}
{{Commons category|Anura}}
{{cookbook}}
{{Wikibooks|School Science|Frog dissection}}
{{Wikiquote|Frogs}}
{{Wikispecies|Anura}}
* [http://amphibiaweb.org/ AmphibiaWeb]
* [http://www.theinformationarchives.com/frogs/ Gallery of Frogs] – Photography and images of various frog species
* [http://froggy.lbl.gov/ The Whole Frog Project] – Virtual frog dissection and anatomy
* [http://raysweb.net/specialplaces/pages/frogsdecline.html "Disappearance of toads, frogs has some scientists worried"] ''San Francisco Chronicle'', April 20, 1992
* [http://calphotos.berkeley.edu/browse_imgs/amphibian_sci_1.html Amphibian photo gallery by scientific name] – Features many unusual frogs
* [http://www.scientificamerican.com/article/researchers-pinpoint-sour/ ''Scientific American'': Researchers Pinpoint Source of Poison Frogs' Deadly Defenses]
===Media===
* [http://www.thinkoholic.com/2005/03/31/time-lapse-part-5-frog-spawn/ Time-lapse video showing the egg's development until hatching]
* [http://sounds.bl.uk/Environment/Amphibians Frog vocalisations from around the world] – From the [[British Library Sound Archive]]
* [http://www.naturenorth.com/spring/sound/shfrsnd.html Frog calls] – From [[Manitoba]], Canada
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