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A fixed action pattern is an instinctive, stereotyped sequence of behavior that, once triggered by a specific external stimulus, usually runs to completion. These behaviors are innate—animals are born with the ability to perform them without learning. In male three-spined sticklebacks, the red belly of a rival male triggers an aggressive attack response that helps protect their nesting territory and offspring.

A sign stimulus is the specific external trigger that activates a fixed action pattern. In sticklebacks, the red belly is the sign stimulus that elicits aggressive attack behavior. Remarkably, even unrealistic fish models with red undersides trigger the same response, while realistic models without red coloring do not, demonstrating that the red color alone is the critical trigger.

Male sticklebacks attack rivals to defend their nesting territory and protect fertilized eggs and young from being harmed or having their genes displaced. This aggressive behavior increases the likelihood that the male's genes are successfully passed to future generations. The hardwired attack response ensures males instinctively recognize and eliminate threats to their reproductive success without needing to learn this behavior.

Fixed action patterns are innate behaviors, meaning animals are born with the ability to perform them and do not need to learn them through experience. In sticklebacks, males automatically attack red-bellied rivals without prior training. This hardwired nature ensures the behavior is performed consistently and correctly across all individuals of the species every time the sign stimulus is encountered.

Tinbergen conducted experiments using fish models to isolate which features triggered male stickleback attacks. He found that unrealistic models with red undersides provoked aggression, while realistic models lacking red coloring did not. This demonstrated that the red belly alone—not overall appearance or realism—was the sign stimulus responsible for activating the aggressive fixed action pattern.

Once triggered, a fixed action pattern typically runs to completion without interruption. The behavior proceeds in a stereotyped, unchangeable sequence that is essentially the same across all individuals of a species. In sticklebacks, once a male detects the red belly sign stimulus, the attack sequence unfolds automatically until the rival is driven away or the pattern naturally concludes.

Fixed action patterns support parental care by enabling males to instinctively protect eggs and young through territorial defense. The aggressive attack response triggered by rival males is part of a broader suite of innate behaviors—including nest building and courtship—that ensure offspring survival. These hardwired behaviors increase fitness by allowing males to focus energy on reproduction rather than learning complex care strategies.