The Midas cichlid, Cichlasoma citrinellum, is monogamous and biparental. Because of competition for limited spawning sites and intense predation on their young, vigorous defense of their territory is essential. Although both sexes engage in defense, they differ in aggressiveness.
The aggressive responses of both sexes were measured by counting the number of bites and bumps each fish directed toward its own mirror image. The size of the fish's genital papilla was also recorded to estimate its reproductive state.
Compared with females, males had higher median mirror scores with greater variance. The scores of individual males were also more consistent through time than were those of females. Females close to spawning had the highest mirror scores, whereas male scores were highest early in the reproductive cycle.
Selection has apparently favored aggressiveness in both sexes. We argue, however, that differences in aggression are the result of selection acting dissimilarly on the two sexes.