Sign stimuli, also called releasers or key stimuli, are those stimuli that are capable of releasing Fixed Action Pattern (FAP) or consummatory behaviour of the animal. They are signals that evoke instinctive patterns of behaviour in animals, such as fighting behaviour in the territorial animals, triggered by the entry of another male. Lehrman found that courting male dove began to bow and coo to a stuffed model of female in the absence of a living female.
Konrad Lorenz (1972) was the first biologist to identify sign stimuli which he called key stimuli because they function as keys to release and unlock the fixed action pattern of the animal. He proposed the Innate Release Mechanism in response to sign stimuli.
Niko Tingbergen (1952) conducted experiments on stickleback fish in which bright red colour of the belly and neck is a sign stimulus for the other males to attack it, while it elicits attraction in female fish.
Bird chicks respond to jerks in the nest, which is a sign stimulus for them to open their beak for eating food. Similarly distress calls given by chicks are a sign stimuli for hens to release rescuing behaviour.
Bright red colour of the oral cavity of a cuckoo chick is a sign stimulus for the foster parent, warbler to feed it. Otherwise the chick is distinctly different from the foster parents.
Sign stimulus is often not one character but a combination of many stimuli, namely, shape of the bill, colour and patches on body or even actions and auditory signals.
Sign stimuli or releasers can be of three types:
Visual Releasers. They are morphological characters that are displayed to elicit response, as display of feathers or dancing in birds. Nest making behaviour in birds such as weaver bird, not only attracts females but also repels other males. Fire flies emit light signals that bring about response from females. In three-spined stickleback fish (Gasterosteus) males get nuptial coloration during breeding season in which neck and belly become bright red in colour to attract females and also to warn other males not to enter the territory.
Auditory Releasers. The sign stimulus is auditory signal like humming sound of male insects. Song birds such as cuckoos, nightingales, starlings establish their territory and attract the female for mating by singing, often for several days. The song is a signature stimulus of individual bird as it is specific. Birds of different areas sing in different dialect and elicit different response.
Stridulatory organs of some insects, such as cicadas and crickets are strong sign stimuli during breeding season.
Co-qui calls given by the tree frog, Eleutherodactylus are strong auditory releaser for males as well as female frogs of the same species.
Chemical Releasers. Pheromones are different types of volatile chemicals released in the atmosphere that act as releasers on the individuals of the same species or other species. Pheromones affect the individuals of the same species while allomones affect other species.
Sex pheromones in insects are produced by the females and perceived by the enlarged antennae of males of the same species, sometimes from the distance of a couple of kilometres.
In vertebrates, the sex pheromone, Copulin is secreted by the female in estrus.
Alarm pheromones are produced by skin cells of several species of fishes that have schooling or shoaling behaviour. Alarm pheromones are also released by the stinging honey bees and wasps which attract other individuals of hive to attack.
Ants and termites produce trail pheromones from the posterior end of abdomen which help them to follow each other.
Sign stimuli bring about responsiveness in target individuals which show consummatory behaviour. The responsiveness diminishes as the consumption proceeds and energy is released.