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。诸多鸟类在成鸟或终羽阶段呈现性别二态色（sexual dichromatism），但这类性别二态色的方向（即某一性别通常较另一性别体色更深）尚未得到充分研究。本研究通过彩色图谱，对西古北界（Western Palearctic）69科、共计666种在该区域常规繁殖的鸟类中基于黑色素的着色斑块分布情况进行了分析。至少在一处被膜部位（integumentary part）存在基于黑色素的性别二态色的物种共计205种，占比30.7%。在性别二态色中占比更高的身体部位依次为背部、头部与胸部，而二态性最弱的部位则是腹部以及裸露的被膜部位（即喙与腿部）。从性别二态色的系统发育分布来看，37科（占比53.6%）至少存在1种在终羽阶段呈现基于黑色素的性别二态性的物种。关于颜色差异的方向，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）。本调查表明，至少在西古北界的鸟类中，黑色素在性别二态色的演化中具有极高的普遍性。目前尚不清楚该观测到的模式是源于性选择促使雄性体色更醒目，还是自然选择令雌性体色更具隐蔽性。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），同时还会还原有毒的半胱氨酸（cysteine），因此在鸟类羽毛的演化研究中，也应考虑到存在性别偏向的生理因素。