What are fixed action patterns in animal behaviour?

In the study of animal behaviour, a fixed action pattern (FAP) is a sequence of innate, genetically programmed behaviours that occur in an automatic and mechanical fashion when triggered by a specific external stimulus.12

These behaviours are characterized by several key features:
* Species-specific: They are generally performed by all members of a species (or at least one sex of that species).32
* Stereotyped: They occur in a highly similar, predictable manner each time they are performed.32
* Innate: They are genetically inherited and do not require prior learning or experience to be performed correctly.13
* Triggered by specific stimuli: They are initiated by a "sign stimulus" or "releaser"—such as a specific color, shape, or sound—which acts as the key to set the behavior in motion.32
* Self-contained: Once triggered, the action pattern typically runs its full course without requiring further external stimulation.3

A classic example is the courtship and territorial behavior of the three-spined stickleback, where specific visual cues, such as the red underside of an intruding male, trigger aggressive fixed action patterns.2