A fixed action pattern (FAP) is a specific, hard-wired sequence of behaviors that occurs in response to an external stimulus, called a sign stimulus. The behavior is “fixed” because it is essentially unchangeable—proceeding similarly across individuals of a species every time it occurs.

Three-Spined Sticklebacks

A classic example of a FAP was described in three-spined stickleback fish by Nikolaas Tinbergen in the 1930s. During the breeding season, the male fish develop a red throat and belly and build a nest. Male sticklebacks attract females to the nest with a courtship dance and encourage her to lay eggs, which the male then fertilizes. He protects the fertilized eggs and, later, the newborn young. During these phases of courtship, breeding, and parental care, the male aggressively attacks any other males who come near his nesting territory. This increases the likelihood of his genes being successfully passed on to future generations.

Tinbergen discovered that male sticklebacks would also attack models of fish—even if they are unrealistic looking—but only if the undersides are painted red like the red belly of a breeding male. When a more realistic model is introduced that does not have a red belly (appearing similar to a female stickleback), the male will not attack. It was concluded that the red belly is the sign stimulus that triggers the attack behavior—the FAP.

FAPs are innate, meaning that the animal is born with the ability to carry out the behavior, and therefore does not need to learn it. FAPs are also usually completed once initially triggered. These behaviors may be critical to increasing an animal’s fitness, for instance, by allowing them to instinctively recognize and attack prey, or—as in sticklebacks—increase the production of offspring that can survive to maturity.