How do sign stimuli trigger fixed action patterns in animals?

In the field of ethology, a sign stimulus (also known as a key stimulus or releaser) acts as a specific external trigger that initiates a fixed action pattern—a genetically programmed, stereotyped behavior.12

These behaviors are released in an automatic and mechanical fashion when an animal encounters the appropriate sign stimulus, such as a particular color, shape, or sound. Once this stimulus triggers the behavior, the action pattern typically runs its course without requiring further external stimulation.21

A classic example is the aggressive behavior of the male three-spined stickleback. During the breeding season, a male will attack other males that enter his territory. Research has shown that this attack is triggered specifically by the red coloration on the intruder's belly; models lacking this red color are largely ignored, while even crude models possessing a red belly are vigorously attacked.23

Essentially, these stimuli serve as highly specific inputs that the animal's nervous system is evolved to recognize, allowing for efficient, adaptive responses to critical ecological challenges like territory defense or prey capture.23