Sex-limited expression of ornamental feathers in birds

Abstract

Extravagant secondary sexual characters show sexual size dimorphism in some species but are completely sex limited in others. Sexual ornamentation has been hypothesized to benefit mainly males through sexual selection, but the costs of secondary sexual characters initially would be experienced by both sexes. The evolution of sexual size dimorphism of ornaments and, eventually, the complete sex-limited expression of these characters, will depend on the effects of sexual and natural selection on the two sexes. A phylogenetic analysis controlling for similarities due to common ancestry of 60 independent evolutionary origins of feather ornamentation in birds was used to investigate ecological factors correlated with sexual size dimorphism and sex-limited expression of secondary sexual characters. When the size of an ornament is large relative to body size, the trait will be particularly costly for females, resulting in selection for increased sexual size dimorphism of the ornament. Indeed, sexual size dimorphism of ornaments was positively related to the relative size of male ornaments but was unrelated to relative size of female ornaments. Species with polygynous and lekking mating systems with little or no male parental care (in particular nest building and incubation) demonstrated sex-limited expression of ornaments as compared to monogamous species. Species with no food provisioning of offspring by the male showed a trend for increased sexual size dimorphism of ornaments. Therefore, large natural selection costs during reproduction imposed by the expression of secondary sexual characters are related to the evolution of sexual size dimorphism of ornaments and eventually their complete loss from females.