Sexual Differences in Contemporary Selection Acting on Size in the Hermit Crab Clibanarius digueti

Abstract

The hermit crab Clibanarius digueti shows strong sexual dimorphism with respect to size, with almost no overlap between adult males and females. Both sexes gain in fitness-related parameters with increasing body size. Courting males average 35% heavier than noncourting males; males in the largest-size classes, which make up less than 1% of the total male population, accounted for 12% of the reproductive activity during the study. Female mating success apparently is not affected by body size, since breeding frequencies did not vary with size. In addition, courting pairs are formed non-assortatively, indicating that males are not choosing females on the basis of size. Females did show an increase in clutch size with body size, as do virtually all ectotherms, but the rate of increase is among the lowest recorded for decapod crustaceans. If female fecundity in C. digueti increased isometrically with body size, as is typical in decapod crustaceans, then female fitness would actually increase more rapidly than male fitness with body size. This modest size-related fecundity gain in females is unexplained (possibly resulting from increased shell limitations on larger crabs), but it leads to the conclusion that selection for large size is stronger in males than in females, a necessary condition for sexual differences in selection to have a role in sexual size dimorphism.