Effects of competitors and time on the evolution of sexual dimorphism

Background/Question/Methods

Sexual dimorphism is a common theme in nature and one of the most intriguing subjects in evolutionary biology. There are two main hypotheses addressing its evolution: female choice, and intraspecific niche divergence. In the latter, sexes are hypothesized to diverge in morphology to avoid competition and more efficiently utilize available resources. In addition, animals with similar ecological requirements can potentially compete if they don’t exploit resources in different ways. In birds, this divergence is typically observed in trophic structures (bill). Thus the predictions are (1) sexes feed differently, (2) there should be enough evolutionary time for such differentiation to arise, and (3) sexual dimorphism is higher in the absence of heterospecific competitors. We tested these predictions in a woodpecker radiation with representatives in North America, South America, and the West Indies. We built a time-calibrated multilocus phylogeny of all species in the genus Melanerpes, and reconstructed sexual dimorphism in bill morphology and number of sympatric congeners. We used these data to test whether sexual dimorphism is correlated to presence or absence of competitors.

Results/Conclusions

We found that sexually dimorphic woodpecker species occur primarily on islands with no ecological competitors. Continental species of Melanerpes living in sympatry with other woodpeckers do not exhibit marked sexual dimorphism in bill dimensions, with one exception. The rise of sexual dimorphism occurred between 15 million years ago (mya) and 2.5 mya. More recent island colonists exhibit lower degrees of sexual dimorphism. The West Indies are inferred to be the ancestral area of distribution for this genus. However, the majority of species radiated north of the Isthmus of Panama with relatively few invasions to South America predating the isthmus formation. Our findings indicate that presence of congeners drives the degree of sexual dimorphism, but that the degree of dimorphism increases with time since speciation.