Fixed action pattern

The term fixed action pattern (FAP), or modal action pattern, is sometimes used in ethology to denote an instinctive behavioral sequence that is relatively invariant within the species and almost inevitably runs to completion.

Fixed action patterns, or similar behaviour sequences, 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. A fixed action pattern is one of the few types of behaviors which was thought to be "hard-wired" and instinctive.

The term "sign stimulus", or "releaser", is used to denote a simple feature of a complex stimulus that can elicit a FAP. For example, the red belly of a male stickleback elicits a head-down, attack behaviour in other male sticklebacks. This same response can be elicited by artificial models or objects that contain the sign stimulus of red, for example, a red coloured card.

The terms "sign stimulus" and "releaser" are sometimes used interchangeably; however, they have different meanings. The term "sign stimulus" is used to denote a feature of an animal's environment that elicits a particular response. The term "releaser" is used for a stimulus that has evolved to facilitate communication between conspecifics (animals of the same species).

Another example of fixed action patterns is the courtship and aggression behaviour of the male three-spined stickleback during the mating season, described in a series of studies by Niko Tinbergen. During spring, male sticklebacks change colour, establish a territory and build a nest. They attack male sticklebacks that enter their territory, but court females and entice them to enter the nest to lay eggs. Tinbergen used crude models of sticklebacks to investigate which features of male and female sticklebacks elicited attack and courtship behaviour from male sticklebacks. Tinbergen's main findings were that male sticklebacks responded in a relatively invariant way and attacked a model with a red belly, but in contrast, courted a model with a swollen belly.

Another example of a behaviour that has been described as a FAP is the egg-retrieval behavior of the graylag goose, reported in classic studies by Niko Tinbergen and Konrad Lorenz. Like many ground-nesting birds, if an egg becomes displaced from the nest, the greylag rolls it back to the nest with its beak. The sight of the displaced egg is the sign stimulus and elicits the egg-retrieval behaviour. If the egg is removed from the goose during the performance of egg-rolling, the bird often continues with the behavior, pulling its head back as if an imaginary egg is still being maneuvered by the underside of its beak. The greylag will also attempt to retrieve other egg-shaped objects, such as a golf ball, door knob, or even a model egg too large to have possibly been laid by the goose itself (i.e. a supernormal stimulus).

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