SEXUAL SELECTION AND EXAGGERATED MALE TAIL LENGTH IN BIRDS

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