Data from: Carotenoid-dependent signals and the evolution of plasma carotenoid levels in birds

Sexual selection has resulted in a wide array of ornaments used in mate choice, and such indicator traits signal quality honestly when they bear costs, precluding cheating. Carotenoid-dependent coloration has attracted considerable attention in this context, because investing carotenoids in coloration has to be traded off against its physiological functions; carotenoids are antioxidants and increase immunocompetence. This trade-off is hypothesized to underlie the honesty of carotenoid-dependent coloration, signaling the "handicap" of allocating carotenoids away from somatic maintenance towards sexual display. Utilizing recent advances in modeling adaptive evolution, we used a comparative approach to investigate the evolution of plasma carotenoid levels using a species-level phylogeny of 178 bird species. We find that the evolutionary optimum for carotenoid levels is higher in lineages that evolved carotenoid-dependent coloration, with strong attraction towards this optimum. Hence carotenoids do not appear limiting given that higher carotenoid levels readily evolve in response to the evolution of carotenoid-dependent coloration. These findings challenge the assumption that carotenoids are a scarce resource, and thus the hypothesis that physiological resource value of carotenoids underlies honesty of carotenoid-dependent traits. Therefore the comparative evidence suggests that other factors such as the acquisition and incorporation of carotenoids are involved in maintaining signal honesty.