What Are Fixed Action Patterns?
A fixed action pattern (FAP) is a sequence of coordinated motor acts that is innate, stereotyped, and species-specific. Once initiated by an appropriate sign stimulus, the behavior runs to completion even if the stimulus is removed. Konrad Lorenz famously demonstrated this with the egg-rolling behavior of greylag geese: when an egg is placed outside the nest, the goose extends its neck and carefully rolls the egg back. If the egg is removed mid-roll, the goose continues the rolling motion to completion with an imaginary egg.
The Innate Releasing Mechanism
Lorenz and Tinbergen proposed that each FAP is controlled by an innate releasing mechanism (IRM) — a neural circuit that acts as a lock, opened only by the correct key (the sign stimulus). The IRM accumulates action-specific energy over time, lowering the threshold for triggering. In extreme cases, a behavior may fire spontaneously in a vacuum activity when no stimulus is present but motivation is overwhelming. This simulator models the relationship between stimulus intensity, threshold, and the resulting behavioral response.
Habituation and Sensitization
Repeated exposure to the same sign stimulus typically leads to habituation — a progressive decrease in response magnitude. This is adaptive: an animal that continues responding maximally to every stimulus wastes energy and time. The rate of habituation depends on stimulus intensity, inter-stimulus interval, and biological relevance. Spontaneous recovery occurs when the stimulus is withheld, gradually restoring the original response level. This simulator tracks how the effective threshold rises with repeated stimulation and recovers over time.
Supernormal Stimuli and Modern Implications
Tinbergen discovered that exaggerated sign stimuli often produce exaggerated responses — a phenomenon called supernormal stimulation. Oystercatchers prefer to incubate giant artificial eggs over their own smaller ones. Herring gull chicks peck more vigorously at a red-striped stick than at a realistic model of an adult gull's beak. This principle extends to human behavior: junk food, social media, and pornography may function as supernormal stimuli that hijack evolved behavioral circuits, triggering responses far stronger than those produced by the natural stimuli our brains evolved to process.