Sex differences in song and plumage color do not evolve through sexual selection alone: new insights from recent research

Recent studies show that traits long recognized as sexually selected ornaments, such as the bright plumage colors and melodious vocal displays of songbirds, have evolved in surprising ways. Male plumage colors are generally more elaborate and diverse than female colors, and in many species only males sing, so both of these dimorphisms are widely presumed to have evolved through sexual selection on males. Yet, in multiple widely divergent songbird clades, sexual differences in colors and songs have evolved through losses in females rather than gains in males. Indeed, changes in female phenotypes may be the principle driving factor affecting such sexual dimorphisms in most songbirds. Sexual dichromatism and song dimorphism also tend to be associated with life history traits other than mating system, such as seasonal migration or territorial behaviors. Altogether, these recent findings support the view that sexual dimorphism is an evolutionary byproduct of multiple, different selection pressures acting on each sex rather than just sexual selection on males. Researchers should therefore be cautious to avoid assuming that dimorphism itself is a character solely reflecting levels of sexual selection in species.