Data from: Melanin-based sexual dichromatism in the Western Palearctic avifauna implies darker males and lighter females

Melanins are the most common pigments providing coloration in the plumage and bare skin of birds and other vertebrates. Numerous species are dichromatic in the adult or definitive plumage, but the direction of this type of sexual dichromatism (i.e., whether one sex tends to be darker than the other ones) has not been thoroughly investigated. Using color plates, we analysed the presence of melanin-based color patches in 666 species belonging to 69 families regularly breeding in the Western Palearctic. Sexual dichromatism based on melanins in at least one integumentary part involved 205 (30.7%) species. The body parts contributing more frequently to dichromatism were the dorsal areas, head and breast, whereas the less dichromatic body parts were the belly and the exposed integumentary parts (i.e., bill and legs). Regarding the phylogenetic spread of dichromatisms, 37 (53.6%) families contained at least one species with melanin-based sexual dimorphism in the definitive adult plumage. As for the direction of the color difference, males are darker than females in a majority of species, meaning that males tend to produce more eumelanin and females tend to synthesize more pheomelanin. This survey has revealed the high prevalence of melanins in the emergence of sexual dichromatism in birds, at least in the Western Palearctic. Whether the described pattern is due to sexual selection promoting more conspicuous males or to natural selection for more cryptic females remains to be determined. Given that pheomelanin synthesis concurrently consumes the antioxidant glutathione but also reduces toxic cysteine, sex-biased physiological factors should also be given consideration in the evolution of bird plumages.

黑色素是赋予鸟类及其他脊椎动物羽毛（plumage）与裸露皮肤颜色的最常见色素。诸多鸟类物种的成鸟（定型羽衣阶段）存在性二色性（sexual dichromatism），但这类性二色性的方向——即是否某一性别整体体色较另一性更深——尚未得到充分研究。本研究采用彩色图版，对69科、共666种在西古北界（Western Palearctic）定期繁殖的鸟类的黑色素依赖型色斑分布情况展开分析。结果显示，至少在一处体被结构（integumentary part）存在黑色素介导的性二色性的物种共计205种，占比30.7%。在性二色性中贡献占比最高的体被部位为背部、头部与胸部，而性二色性最弱的部位则为腹部与裸露体被结构（即喙与腿部）。从性二色性的系统发育分布来看，37科（占比53.6%）至少存在1个物种在成鸟定型羽衣中表现出黑色素介导的性二态性（sexual dimorphism）。关于体色差异的方向，多数物种中雄性体色较雌性更深，这意味着雄性倾向于合成更多真黑色素（eumelanin），而雌性则更多合成褐黑色素（pheomelanin）。本调查揭示了黑色素在鸟类性二色性演化中的高普遍性，至少在西古北界范围内是如此。上述模式究竟是由性选择驱动雄性体色更醒目，还是由自然选择促使雌性体色更具隐蔽性，仍有待进一步探明。鉴于褐黑色素的合成过程会同时消耗抗氧化谷胱甘肽（glutathione），并降低有毒半胱氨酸（cysteine）的水平，鸟类羽衣演化中还应考虑性别偏向的生理因素。