Abstract
Sex roles were examined in a cardinalfish, Apogon notatus, in which males alone mouthbrood the eggs received from a single female. Before spawning, a male and female formed a pair, within which the female is more active in courtship and attacks against conspecifics. Females had a higher potential for reproduction and on average produced more than twice as many clutches as those mouthbrooded by a male in a season by changing mates after spawning. Animals in which mating competition is more intense among females and, ultimately, sexual selection is more strongly acting on females are defined as sex-role reversed. Sex-role reversal is expected where the operational sex ratio (OSR) is female biased. In A. notatus, however, the OSR was male biased throughout the breeding season. This was due primarily to a higher mortality in females. The theory predicts that sexual selection operates more strongly on the sex toward which the OSR is biased. The facts that the variance in reproductive success was greater in males and the males developed a sexual trait suggest that sexual selection is acting more strongly on males than on females. Accordingly, this fish is not sex-role reversed.
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