In ethology, a fixed action pattern (FAP) is an instinctive behavioral sequence that is indivisible and runs to completion. Fixed action patterns are invariant and are produced by a neural network known as the innate releasing mechanism.
In response to an external sensory stimulus known as a sign stimulus or releaser, which is a signal from one individual to another. A fixed action pattern is one of the few types of behaviors that can be considered hard-wired and instinctive.
Examples of Fixed Action Patterns in Animals
Many mating dances, commonly carried out by birds, are examples of fixed action patterns. In these cases, the sign stimulus is typically the presence of the female.
Another example is the aggression shown by male red-bellied sticklebacks during mating season. Experiments conducted by Niko Tinbergen demonstrated that this aggressive behavior is a FAP triggered by anything red, the sign stimulus. The threat display of male sticklebacks is also a fixed action pattern triggered by a specific stimulus.
Another well-known example comes from classic experiments conducted by Tinbergen and Lorenz on the Graylag Goose. Like similar waterfowl, the goose rolls a displaced egg near its nest back to the others with its beak.
The sight of the displaced egg triggers this behavior. Even if the egg is removed, the goose continues the behavior, as if an imaginary egg is still being maneuvered. Interestingly, it will also attempt to move other egg-shaped objects, such as a golf ball, doorknob, or even an object too large to have been laid by the goose itself, which is a supernormal stimulus.
Other Examples
1. Kelp Gull chicks are stimulated by a red spot on the mother’s beak to peck at the spot, which induces regurgitation.
2. Some moths instantly fold their wings and drop to the ground if they encounter ultrasonic signals, such as those produced by bats, demonstrating ultrasound avoidance.
3. Mayflies drop their eggs when they encounter a certain pattern of light polarization, which indicates they are over water.
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Importance of Fixed Action Patterns in Animal Behavior
A FAP is significant in animal behavior because it represents the simplest type of behavior, in which a well-defined stimulus almost always leads to a consistent behavioral response.
FAPs are unusual in that they are hard-wired, unlike most behaviors, which are modulated by environmental factors. While a fixed response can lead to maladaptive results, flexible behaviors are generally more beneficial.
For example, the egg-rolling behavior of the Greylag Goose is crucial to the survival of its chicks, making this hard-wired behavior essential for fitness. In the case of moths, a rapid response to echolocation helps them avoid predation.
However, because these behaviors are hard-wired, they are predictable, which can lead to exploitation by other species or humans.
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Exploitation of Fixed Action Patterns
Some species have evolved to exploit the FAPs of other species by mimicking their sign stimuli, a phenomenon known as code-breaking. A well-known example is brood parasitism, where one species lays its eggs in the nest of another, which will then raise the offspring.
For instance, a young North American cowbird provides a supernormal stimulus to its foster parent, prompting the parent to forage rapidly to satisfy the larger bird’s demands.
In natural situations, nestlings provide higher levels of stimulus with noisier, more energetic behavior, communicating their need for food. Parents, in turn, work harder to meet these demands, or risk their offspring starving.
In conclusion, a fixed action pattern is one of the few behaviors that can be said to be hard-wired and instinctive. However, FAPs are modulated by the environment, and while a fixed response can sometimes lead to maladaptive outcomes, flexible behaviors are often more advantageous for animal survival.
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