Sex-limited expression of ornamental feathers in birds

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 th e costs of secondary sexual characters initially would be experienced by bo th sexes. The evolution of sexual size dimorphism of ornaments and, eventua lly, the complete sex-limited expression of these characters, will depend o n the effects of sexual and natural selection on the two sexes. A phylogene tic analysis controlling for similarities due to common ancestry of 60 inde pendent evolutionary origins of feather ornamentation in birds was used to investigate ecological factors correlated with sexual size dimorphism and s ex-limited expression of secondary sexual characters. When the size of an o rnament is large relative to body size, the trait will be particularly cost ly for females, resulting in selection for increased sexual size dimorphism of the ornament, indeed, sexual size dimorphism of ornaments was positivel y related to the relative size of male ornaments but was unrelated to relat ive size of female ornaments. Species with polygynous and lekking mating sy stems with little or no male parental, care (in particular nest building an d incubation) demonstrated sex-limited expression of ornaments as compared to monogamous species. Species with no food provisioning of offspring by th e male showed a trend for increased sexual size dimorphism of ornaments. Th erefore, large natural selection costs during reproduction imposed by the e xpression of secondary sexual characters are related to the evolution of se xual size dimorphism of ornaments and eventually their complete loss from f emales.