At the start of the breeding season, male stickleback fish undergo a number of significant changes. Their bellies become bright red and they become aggressive toward other males of the species, establishing and defending territories in which they build their nests. Although most male-male encounters consist solely of the resident lunging at an intruder, if this fails to drive off the intruder, the resident may bite. Females choose to mate with males that have large territories and well-constructed nests. What would you expect to happen as nest sites become scarcer?

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As nest sites become scarcer, male sticklebacks will become more aggressive and may resort to biting in order to defend their territories and access to females. This is because a larger and better-constructed nest will become even more of a valuable resource, and males will need to compete fiercely in order to attract mates.

At the Start of the Breeding Season, Male Stickleback Fish