Data from: The effects of life history and sexual selection on male and female plumage colouration

Classical sexual selection theory provides a well-supported conceptual framework for understanding the evolution and signalling function of male ornaments. It predicts that males obtain greater fitness benefits than females through multiple mating because sperm are cheaper to produce than eggs. Sexual selection should therefore lead to the evolution of male-biased secondary sexual characters. However, females of many species are also highly ornamented. The view that this is due to a correlated genetic response to selection on males was widely accepted as an explanation for female ornamentation for over 100 years and current theoretical and empirical evidence suggests that genetic constraints can limit sex-specific trait evolution. Alternatively, female ornamentation can be the outcome of direct selection for signalling needs. Since few studies have explored interspecific patterns of both male and female elaboration, our understanding of the evolution of animal ornamentation remains incomplete, especially over broad taxonomic scales. Here we use a new method to quantify plumage colour of all ~6,000 species of passerine birds to determine the main evolutionary drivers of ornamental colouration in both sexes. We found that conspecific male and female colour elaboration are strongly correlated, suggesting that evolutionary changes in one sex are constrained by changes in the other sex. Both sexes are more ornamented in larger species and in species living in tropical environments. Ornamentation in females (but not males) is increased in cooperative breeders—species in which female–female competition for reproductive opportunities and other resources related to breeding may be high. Finally, strong sexual selection on males has antagonistic effects, causing an increase in male colouration but a considerably more pronounced reduction in female ornamentation. Our results indicate that although there may be genetic constraints to sexually independent colour evolution, both female and male ornamentation are strongly and often differentially related to morphological, social and life-history variables.

经典性选择理论为理解雄性装饰性特征的演化及其信号功能提供了得到充分支持的概念框架。该理论预测，由于精子的生产成本远低于卵子，雄性通过多次交配获得的适合度收益高于雌性。因此，性选择应推动雄性偏向型第二性征的演化。然而，诸多物种的雌性同样具有高度发达的装饰性特征。超过一个世纪以来，“雌性装饰性特征是对雄性选择的相关遗传响应”这一观点被广泛接受为雌性装饰性特征的合理解释；当前的理论与实证证据均表明，遗传约束会限制性特异性性状的演化。另一种可能性是，雌性装饰性特征可直接由对信号需求的选择所塑造。由于鲜有研究探讨雌雄两性装饰性特征的种间演化模式，我们对动物装饰性特征演化的认知仍不完善，尤其是在宽泛的分类学尺度下。本研究采用全新方法量化了约6000种雀形目鸟类的羽色，以明确驱动雌雄两性装饰性羽色演化的主要因素。我们发现，同种雌雄个体的羽色发达程度呈显著相关，提示某一性别的演化变化会受另一性别的演化变化约束。在体型更大的物种以及栖息于热带环境的物种中，雌雄两性的装饰性特征均更为发达。合作繁殖物种中，雌性（而非雄性）的装饰性特征更为显著——这类物种中，雌性间为繁殖机会及其他繁殖相关资源展开的竞争往往较为激烈。最后，对雄性的强性选择会产生拮抗效应：既会提升雄性的羽色发达程度，却会显著降低雌性的装饰性特征。我们的研究结果表明，尽管性独立的羽色演化可能存在遗传约束，但雌雄两性的装饰性特征均与形态、社会及生活史变量密切相关，且二者的关联模式往往存在差异。