On the evolution of body size and of sexual dimorphism in corbiculate bees

Abstract

Abstract Corbiculate bees offer the opportunity to analyse the evolution of differences in body size between sexes in relationship to the social environment, because this group of phylogenetically related taxa have contrasting levels of social organization, ranging from solitary to highly eusocial. Sexual size dimorphism evolves because body size is usually related to reproductive success through different pathways in females and males. Female body size is strongly correlated with fecundity, whereas in males the body size is correlated with mating success. Within a clade, if sexual selection on males has been stronger than selection on female fecundity, a sexual size dimorphism bias to males would be expected (Rensch’s rule), or an opposite trend when the fecundity of females has had a stronger effect. We tested these predictions by taking an allometric approach that considered differences in the capacity of a reproductive female to start a nest on her own or not. Phylogenetic analysis of the whole clade and another analysis including only species in which the females start nests on their own showed that the corbiculate bees do not seem to follow Rensch’s rule. The males and reproductive females of the species in which the females have lost their capacity to start nests were smaller than the reproductive individuals of species in which the females start nests on their own, suggesting that selective pressures linked to the evolution of sociality might constrain the evolution of a large body size in both males and reproductive females.