Does extra-pair paternity drive sexual dichromatism and sexual size dimorphism in passerines?

Abstract

Male-biased sexual selection is considered a main evolutionary driver of the sexual dimorphism frequently observed in species exhibiting polygyny or female-only parental care. Correspondingly, it is hypothesized that extra-pair paternity (EPP) can select for sexual dimorphism in monogamous species. This is because EPP can increase variance in male reproductive success in a similar way to social polygyny. Here we used two common indicators of extra-pair mating (species EPP rates and residual testes mass - a presumed indicator of the level of sperm competition) to examine the relationship between EPP and two forms of sexual dimorphism in passerine birds: sexual dichromatism and sexual size dimorphism (SSD). First, we confirm that species with high EPP rates are more sexually dichromatic and this effect was much stronger than the effects of mating system and parental care. Furthermore, by modelling male and female colouration independently, we show that while social mating system has primarily acted on female colouration, EPP has instead driven on male colouration. In contrast, testes size was only weakly related to sexual dichromatism, and this effect was instead associated with reduced female colouration in species with large testes size. Second, we found a strong positive relationship between EPP and SSD, but surprisingly no relationship between testes size and SSD. The contrasting results using either EPP rates or testes size suggests that there are fundamental differences in the traits measured by the two variables, but further study is needed to determine why these two variables produce different findings. Our study provides evidence that EPP has been an important selective force in the evolution of both sexual dichromatism and SSD. These results highlight the importance of considering the impact of both social and genetic mating systems in evolutionary studies of sexual dimorphism.