The term '''sign stimulus''' also known as a '''[[:simple:Releaser|releaser]]''', is the determining feature of a stimulus that produces a response. Sign stimuli are often found when observing a fixed action pattern (FAP) that is an innate behaviorbehaviour with very little variance in the manner in which the actions are executed. Several examples of sign stimuli can be seen through the observation of animal behaviorbehaviour in their natural environment. Experimenters have gone into these natural environments to better assess the stimuli and determine the key features of them that elicit a fixed action pattern. Scientists have also observed direct exploitation of sign stimuli in nature among different species of birds.
Fixed action patterns are released due to certain external stimuli.<ref name=":03"/> These stimuli are single or a small group of attributes of an object, not the object as a whole.<ref name=":3">{{Cite web |url=http://www.flyfishingdevon.co.uk/salmon/year1/psy128ethology_experiments/ethexpt.htm#FAP |title=Ethological studies of sign stimuli and motivation |website=www.flyfishingdevon.co.uk |access-date=2019-10-22}}</ref> These attributes may include color, shape, odor, and sound.<ref name=":03"/><ref name=":15"/>
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=== Male stickleback mating behavior ===
[[File:3-spined_stickleback.jpg|alt=|thumb|A 3-spined stickleback like those used in Tinbergen's experiments.]]
One example of fixed action patterns is the courtship and aggression behaviorsbehaviours of the male [[stickleback]], particularly the [[three-spined stickleback]], during mating season, described in a series of studies by Niko Tinbergen.<ref name=":82"/><ref name=":2">{{cite journal |last1=Rowland |first1=William |title=Habituation and development of response specificity to a sign stimulus: male preference for female courtship posture in stickleback |journal=Animal Behaviour |date=February 3, 2000 |volume=60 |issue=1 |pages=63–68 |doi=10.1006/anbe.2000.1462 |pmid=10924204 |s2cid=23095310 }}</ref> During the spring mating season, male sticklebacks ventrum turns red and they establish a territory to build a nest.<ref name=":82"/> They attack other male sticklebacks that enter their territory, but court females and entice them to enter the nest to lay their eggs.<ref name=":82"/> Tinbergen used models of sticklebacks to investigate which features of male and female sticklebacks elicited attack and courtship behavior from male sticklebacks.<ref name=":82"/> Tinbergen's main findings were that male sticklebacks responded in a relatively invariant way and attacked even the most crude of models with a red belly, but in contrast, courted a model with a swollen belly.<ref name=":82"/> He also found that when presented with both a real male stickleback and a crude model, if the model's stomach was more red, the stickleback would attack the model as opposed to the other real male stickleback.<ref name=":82"/>
