Fixed action pattern

Fixed action pattern. In ethology, a fixed action pattern is a sequence of unlearned, innate behaviors that is invariant within a species and runs to completion. Once triggered by a specific external sensory stimulus known as a sign stimulus or releaser, the pattern is carried out in a stereotyped, predictable manner, largely independent of further external feedback. These behaviors are considered fundamental units of instinctive behavior and are crucial for survival and reproduction across the animal kingdom.

Definition and characteristics

A fixed action pattern is defined by several key characteristics. It is stereotyped, meaning the pattern of motor actions is performed in the same rigid sequence each time by all members of the species. The behavior is ballistic, or consummatory, proceeding to its end point once initiated without requiring additional sensory input. The initiation is dependent on a highly specific sign stimulus, such as the red belly of a male stickleback or the rolling egg-like object that triggers retrieval in greylag geese. These behaviors are also species-typical, being essentially identical across all normal individuals of a species, and are under strong genetic influence, requiring little to no learning for their expression.

Ethological context

The concept was developed and refined by the founding figures of classical ethology, most notably Konrad Lorenz, Niko Tinbergen, and Karl von Frisch. Their work, for which Tinbergen and Lorenz shared the Nobel Prize in Physiology or Medicine in 1973, established fixed action patterns as central components of instinct. Ethologists viewed behavior as being organized hierarchically, with fixed action patterns as the consummatory acts at the end of an action-specific energy drive, released by innate releasing mechanisms in response to key stimuli. This framework was foundational for the field and was extensively explored in Tinbergen's studies of the three-spined stickleback and the egg-retrieval behavior in geese.

Examples in animals

Classic examples of fixed action patterns are abundant in ethological literature. The male three-spined stickleback aggressively attacks any red-bellied intruder, a sign stimulus representing a rival male, even if the model is crude. The greylag goose will continue the rolling motion to retrieve an egg that has rolled from its nest, completing the sequence even if the egg is removed mid-action. In songbirds, such as the chaffinch, the full species-specific song is a fixed action pattern, though its development may require exposure to a tutor. Other examples include the web-spinning sequence of orb-weaver spiders, the regurgitation feeding of herring gull chicks pecking at a red spot on the parent's bill, and the complex waggle dance of the honey bee communicated to nestmates.

Neural and genetic basis

The invariant nature of fixed action patterns suggests they are generated by dedicated neural circuits often described as innate motor programs or central pattern generators. Research in model organisms like the sea slug Aplysia and the fruit fly Drosophila melanogaster has identified specific genes and neural pathways that control stereotyped action sequences, such as escape responses or courtship rituals. Studies in zebra finches have shown that while the learning of song syllables involves the high vocal center, the production of the mature song pattern is a fixed motor program. The field of neuroethology seeks to map these behaviors directly onto the activity of specific neurons and neural ensembles.

Role in learning and evolution

Fixed action patterns interact with learning through processes like imprinting and ritualization. They form the innate substrate upon which learned modifications can be built, as seen in bird song development. In evolution, fixed action patterns are subject to natural selection and can become ritualized into communication signals, such as the inciting posture in ducks originating from intention movements for flight. Comparative studies across related species, like the diverse mating dances of cichlid fish in Lake Victoria or displays in birds of paradise, reveal how these innate patterns diversify and adapt, providing clear evidence for the evolution of behavior.

Comparison to other behaviors

Fixed action patterns are distinct from other behavioral categories. Unlike reflexes, which are simple, rapid, and involve few synapses (e.g., the knee-jerk reflex), fixed action patterns are complex, longer sequences involving the entire organism. They differ from kineses and taxes, which are undirected or directed orientation movements in response to stimuli. Most importantly, they contrast with operantly conditioned behaviors and other forms of learning, which are flexible and modified by consequences. While modal action pattern is a modern term acknowledging some within-species variability, it largely describes the same core phenomenon of stereotyped, released innate behavior.