Evolution of sexual dichromatism: contribution of carotenoid- versus melanin-based coloration

In birds, carotenoid-based plumage coloration is more dependent on physical condition and foraging abilities and less constrained developmentally than is melanin-based coloration. Thus, female mate choice for honest signals s hould result in more intense sexual selection on carotenoid- than on melani n-based plumage coloration. using variation in sexual dimorphism as an indi rect measure of the intensity of sexual selection, we tested the prediction that variation in sexual dimorphism is driven more by change in carotenoid -based coloration between males and females than by change in melanin-based coloration. Examination of historical changes in carotenoid- versus melani n-based pigmentation in 126 extant species of Cardueline finches supported this prediction. We found that carotenoid-derived coloration changed more f requently among congeners than melanin-based coloration. In both sexes, inc rease in carotenoid-based coloration score, but not in melanin-based colora tion score, was strongly associated with increase in sexual dichromatism. I n addition, sexual dimorphism in carotenoid-based coloration contributed mo re to overall dichromatism than dimorphism in melanin-based plumage. Our re sults supported the hypothesis that melanin-based and carotenoid-based colo ration have fundamentally different signal content and suggest that combini ng melanin-based and carotenoid-based coloration in comparative analysis is not appropriate. (C) 2000 The Linnean Society of London.