Male size predicts extrapair paternity in a socially monogamous bird with extreme sexual size dimorphism

Abstract

Lay Summary The New Zealand tui is an iconic bird exhibiting extreme sexual size dimorphism. This is an unusual trait for a monogamous species and so we tested whether a male’s body size was related to his paternity success. First, we found that tūī had very high rates of extrapair paternity compared to other monogamous species. Second, larger males had higher paternity success. Thus, in contrast with previous studies, our results show that extrapair paternity can lead to the evolution of sexual size dimorphism.