Why is the coral-reef fish Valenciennea strigata (Gobiidae) monogamous?

Abstract

Parental-investment theory predicts that monogamy should be rare, and empirical evidence supports this prediction. Monogamy has generally been explained by either (1) a need for biparental care, or (2) a uniform distribution of limiting resources. By contrast, monogamy has evolved in several coral-reef fishes without biparental care, and many coral-reef fishes may not be limited by resources. Monogamy in these fishes might be explained by either (3) a low population density that favors mate fidelity, or (4) an abundance of resources that allows all males to breed and thus lowers the cost-benefit ratio for females that mate guard. We tested predictions of these hypotheses in the coral-reef fish, Valenciennea strigata, except that (1) biparental care had previously been rejected. We found no evidence of resource limitation (2a): food densities within territories did not differ from the density of food in unused habitat adjacent to territories; potential nest sites also existed in this unused habitat. Similarly, (2b) if resources limit the population, territory defense might require the coordinated efforts of a pair; however, widowed fish maintained their territories and (3) remated rapidly. Finally, (4) all males maintained a nest, both sexes enforced monogamy by mate guarding, and females benefited from guarding a high-quality (large) mate. Females mated to large males fed more than when they paired with small males. The results support the hypothesis that abundant and uniformly distributed resources can lead to monogamy by lowering the cost of guarding a mate when mate guarding provides benefits.