Wired-In Behavior
A Fixed Action Pattern (FAP) is an innate, stereotyped sequence of behavior that runs to completion once triggered, regardless of environmental changes. Think of it as ballistic, like a fired projectile.
FAPs are triggered by a sign stimulus (or releaser), a specific environmental feature the animal is tuned to detect. Konrad Lorenz and Niko Tinbergen proposed the Innate Releasing Mechanism (IRM) as the neural filter that unleashes the matching FAP.
| Concept | Definition & Classic Example |
|---|---|
| Sign Stimulus | A specific trigger, such as the red underside of a rival male stickleback. |
| Supernormal Stimulus | An exaggerated trigger that yields a stronger response than reality. |
Common pitfall: Assuming a FAP can be stopped partway. Once the sign stimulus triggers it, it cannot be aborted, allowing supernormal stimuli to completely hijack the mechanism.
Mechanics & Supernormal Cues
Tinbergen discovered that herring gull chicks peck harder at a stick with three red stripes than at a realistic gull head. Exaggerated cues simply out-compete reality for the releasing mechanism's attention.
This proportionality means that as the cue is exaggerated into a supernormal stimulus, the innate response grows stronger.
| Model Type | Stickleback Response |
|---|---|
| Neutral grey model (no red) | No lunge or attack |
| Model with red belly patch | Lunge to attack |
| Exaggerated bright-red model | Aggressive attack response |
Distinction: A standard sign stimulus elicits normal species-typical behavior, whereas a supernormal stimulus exploits the IRM to produce a response that exceeds natural biological limits.