Broad-scale variation in sexual dichromatism in songbirds is not explained by sex differences in exposure to predators during incubation

The evolution of sexual dichromatism provoked one of the greatest disagreements between Charles Darwin and Alfred Russel Wallace. According to Darwin the main driving force is sexual selection, whereby choosy females prefer showy males, leading to the evolution of conspicuous male plumage. On the other hand, Wallace suggested that dichromatism may arise because nest predation favors more cryptic females. To test the role of natural selection in the evolution of dichromatism we combined quantitative data on differences in parental share in nest attentiveness (representing the strength of natural selection on males vs. females) with spectrophotometric measurements of dichromatism in 412 species of songbirds from 69 families. We expected to find stronger dichromatism in open-nesting species with more divergent parental roles and in body parts exposed during incubation. Dichromatism was not related to the differences in parental share during incubation, but it was most pronounced in lekking species, migrants, and small species. Our results thus suggest that Wallace's hypothesis is not able to explain broad-scale variation in the dichromatism of songbirds, but point to a role for sexual selection, mutual mate choice, and migration strategy in shaping the extraordinary variation in dichromatism exhibited by songbirds.

性二色性（sexual dichromatism）的演化曾引发查尔斯·达尔文（Charles Darwin）与阿尔弗雷德·拉塞尔·华莱士（Alfred Russel Wallace）之间最具影响力的学术分歧之一。达尔文主张，性选择是其核心驱动机制：具有择偶偏好的雌性更倾向于选择体色艳丽的雄性，由此推动雄性显眼羽色的演化。与之相对，华莱士则提出，性二色性的产生可能源于巢捕食压力更青睐具有隐蔽性的雌性。
为验证自然选择在性二色性演化中的作用，本研究整合了两类定量数据：其一为亲鸟巢值守投入比例的相关数据（该指标可表征自然选择对雌雄两性的选择强度差异），其二为涵盖69个科的412种鸣禽的性二色性分光光度测量结果。我们曾预设，在开放式筑巢且亲鸟育雏分工差异更显著的类群，以及孵卵期间暴露的身体部位中，性二色性会更为突出。
研究结果表明，性二色性与孵卵期亲鸟投入比例的差异并无关联，但在求偶场繁殖物种、迁徙物种以及小型物种中，性二色性的表现最为显著。据此，本研究认为华莱士的假说无法解释鸣禽性二色性的大范围变异，而研究结果提示，性选择、双向配偶选择以及迁徙策略在塑造鸣禽显著的性二色性变异中扮演了重要角色。