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 toward 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 toward this optimum. Hence, carotenoids do not appear to be 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 also challenge 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.性选择（sexual selection）催生了诸多用于配偶选择（mate choice）的装饰性状，此类指示性状（indicator traits）在承载成本、杜绝欺骗行为的前提下，可诚实传递个体质量信号。类胡萝卜素依赖体色（carotenoid-dependent coloration）在此研究方向中受到广泛关注，因为将类胡萝卜素投入体色表达，必须与其生理功能（physiological functions）形成权衡：类胡萝卜素作为抗氧化剂（antioxidants），可提升机体免疫能力（immunocompetence）。该权衡被假说认为是类胡萝卜素依赖体色诚实性的核心基础，即类胡萝卜素从躯体维护转向性展示的分配过程构成了“累赘”（handicap）。依托适应性进化（adaptive evolution）建模领域的最新研究进展，我们采用比较研究方法（comparative approach），基于涵盖178种鸟类的物种水平系统发育（species-level phylogeny），探究血浆类胡萝卜素水平（plasma carotenoid levels）的演化规律。研究发现，演化出类胡萝卜素依赖体色的演化支（lineages）中，类胡萝卜素水平的进化最优值（evolutionary optimum）更高，且类群会强烈向该最优值收敛。因此，类胡萝卜素似乎并非限制性资源——毕竟在类胡萝卜素依赖体色演化后，更高的类胡萝卜素水平可快速演化而来。上述研究结论对“类胡萝卜素为稀缺资源（scarce resource）”的假设提出了挑战，同时也对“类胡萝卜素的生理资源价值（physiological resource value）是类胡萝卜素依赖性状诚实性基础”这一假说构成质疑。因此，本比较研究的证据表明，类胡萝卜素的获取与整合过程等其他因素，参与维持了信号诚实性（signal honesty）。