Animal Behavior: Instinct Stimulus and Response Key Stimuli Certain innate behaviors can be produced in all individuals of the same species. Usually, these behaviors are triggered by a key stimulus (KS). The key stimulus triggers an innate releasing mechanism (IRM), a sort of middleman, which produces a fixed action pattern (FAP), a definite, constant response. The IRM can be any visual, hormonal, or muscular mechanism that results in the FAP. An important feature of the FAP is that once started it cannot be stopped until the entire action sequence is completed. An example of an innate behavior can be seen in Herring gull chicks, which were studied by Tinbergen. Adult females have a red spot on their beak. Chicks instinctually peck at this spot, which stimulates the female to regurgitate and feed the young chick. The red spot is the KS, and it releases the pecking FAP in the chick. A given KS can be important in triggering several different FAPs. The red bellies of male stickleback fish are one well-studied example. In these fish, males defend territories by attacking
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