Correlated evolution of biparental incubation and sexual tail monomorphism in swallows and martins (Aves: Hirundinidae)

Abstract

The evolution of sexually dimorphic ornamentation should reflect sex differences in a form of parental care, incubation behavior, but empirical support is scarce after controlling for phylogenetic inertia. Here, using phylogenetic comparative methods in swallows and martins (Aves: Hirundinidae), we studied the evolution of sexual dimorphism in tail length in relation to two incubation types, female-only and biparental incubation. We found that species with biparental incubation were more likely to have sexually monomorphic tails compared to species with female-only incubation. Furthermore, an analysis of evolutionary pathways suggested interdependent evolution of biparental incubation and sexual tail dimorphism; transitions to biparental incubation and sexual tail dimorphism were less likely to occur than transitions from that state. This finding is consistent with the hypothesis that selection on male ornamentation matters during the incubation period. We also confirmed that species with biparental incubation had significantly lower rates of extra-pair paternity compared to species with female-only incubation in this clade, providing additional support for the importance of selection on male ornamentation during the incubation period. The current study is one of few phylogenetic comparative studies demonstrating the correlated evolution of biparental incubation and sexual dimorphism in ornamentation.