Interspecific Competition and the Abundance of Nest Sites: Factors Affecting Sexual Selection

The abundance of suitable nest sites and competition with other species for such sites appears to affect the intensity of intersexual selection in Coryphopterus nicholsi, a temperate goby that is a protogynous hermaphrodite. Field manipulations demonstrated that, within a population, the proportion of males that breed, and therefore the intensity of sexual selection, depends on the number of suitable nest rocks. The abundance of nest sites may also affect the timing of sex change in this species. Collections from areas with many suitable nest sites contained a higher proportion of males than collections from areas with few suitable nest sites. Aggression among males probably determines which individuals within the population can monopolize a suitable nest site and therefore are able to mate as a male. Field observations indicated that aggressive interactions occur frequently and that the larger individual of an interacting pair almost inevitably chases off the smaller individual during aggressive encounters. In field collections, males guarding eggs averaged larger in size than males not guarding eggs, and large males guarded nest sites that were superior to those guarded by small males. Competitors, including a large fish and several invertebrates, monopolize rocks otherwise suitable for Coryphopterus' use and thereby limit the number and proportion of breeding males in the goby's population. Field and laboratory observations and experiments indicated that a larger fish, Porichthys notatus, displaces Coryphopterus from nest sites and prevents Coryphopterus from using many of the large rocks on the reef. Other animals, including encrusting invertebrates and sea urchins, may also reduce the suitability and availability of rocks as nest sites. Such competition for nest sites reduces the opportunity of breeding by small males and increases the number of mates per large breeding male. Interspecific competition should, therefore, increase selection for protogynous hermaphroditism in Coryphopterus by increasing the reproductive advantage of large over small males. I discuss the possibility that interspecific competition may be an important selective force in resource—based mating systems of other animals and present potential examples from the literature.