Fixed action pattern

A fixed action pattern (FAP) is an instinctive behavioral sequence that, once initiated, runs to completion with minimal modulation by feedback. First identified by classical ethologists studying species-typical behaviors, FAPs are triggered by specific external stimuli called sign stimuli — a red belly on a rival stickleback, a beak shape in a begging chick — and proceed through a rigid choreography regardless of environmental change. The concept was central to the early ethological program but has been challenged by research showing that even supposedly 'fixed' patterns exhibit context-dependent modulation through motor program plasticity and feedback from the nervous system. The question is no longer whether FAPs exist — they do — but whether 'fixed' is the right descriptor for behaviors that are stereotyped yet not immutable.

The fixed action pattern is not a robotic reflex. It is an evolved solution to the problem of producing reliable behavior in unreliable environments — and the reliability itself is the adaptation, not the rigidity.