Sexual Selection and Exaggerated Male Tail Length in Birds

Abstract

Many species of birds demonstrate extreme sexual dimorphism in tail length. Darwin's theory of sexual selection explained the evolution of such elaborate characters, arguing that selection is stronger under polygamy than under monogamy. Alternatively, R. L. Trivers proposed that relative parental investment was the key variable influencing sexual selection To determine whether tail length dimorphism is associated with mating system and/or relative parental care, we analyzed morphological information for 1,990 species and natural history information for 745 species. Controlling for phylogenetic effects by investigating associations within taxa, we found that tail length dimorphism was significantly associated with both mating system and relative parental care, but particularly with the latter. These results suggest that, when male birds invest less in parental care and/or are polygynous, selection for exaggerated male tail length is more likely to occur.