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（melanin）是鸟类及其他脊椎动物羽毛与裸露皮肤中最常见的着色色素。诸多鸟类在成鸟或定型羽毛阶段呈现两性二色性，但这类性二色性（sexual dichromatism）——即某一性别通常较另一性别更深色——的演化方向尚未得到充分研究。本研究通过彩色图谱，分析了西古北界（Western Palearctic）69科、共666种常在此区域繁殖的鸟类中基于黑色素的着色斑块分布情况。结果显示，至少有一处体被部位存在基于黑色素的性二色性的物种共计205种，占比30.7%。在两性二色性中贡献占比更高的身体部位依次为背部、头部与胸部，而着色差异最不显著的部位则是腹部以及裸露的体被结构（即喙与腿部）。就二色性的系统发育分布而言，37个科（占比53.6%）至少拥有1种在成鸟定型羽毛中存在基于黑色素的性二态性的物种。关于着色差异的方向，多数物种的雄性体色较雌性更深，这意味着雄性通常会生成更多的真黑色素（eumelanin），而雌性则倾向于合成更多的褐黑色素（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——至少在西古北界区域是如此。上述模式究竟是源于性选择对更显眼雄性的偏好，还是自然选择对更具隐蔽性雌性的青睐，仍有待进一步研究。鉴于褐黑色素的合成过程会同时消耗抗氧化剂谷胱甘肽（glutathione）并降低有毒半胱氨酸（cysteine）的水平，鸟类羽毛演化的研究中也应纳入性别偏向性生理因素的考量。