Data from: Social monogamy versus polyandry: ecological factors associated with sex-roles in two closely related birds within the same habitat

Why mainly males compete and females take a larger share in parental care remains an exciting question in evolutionary biology. Role-reversed species are of particular interest, because such exceptions′ help to test the rule. Using mating systems theory as a framework, we compared the reproductive ecology of the two most contrasting coucals with regard to sexual dimorphism and parental care: the black coucal with male-only care and the bi-parental white-browed coucal. Both species occur in the same lush habitat and face similar ecological conditions, but drastically differ in mating system and sexual dimorphism. Black coucals were migratory and occurred at high breeding densities. With females being obligatory polyandrous and almost twice as heavy as males, black coucals belong to the most extreme vertebrates with reversed sexual dimorphism. Higher variance in reproductive success in fiercely competing females suggests that sexual selection is stronger in females than males. In contrast, resident white-browed coucals bred at low densities and invariably in pairs. They were almost monomorphic and the variance in reproductive success was similar between the sexes. Black coucals were more likely to lose nests than white-browed coucals, probably facilitating female emancipation of parental care in black coucals. We propose that a combination of high food abundance, high population density, high degree of nest loss, and male bias in the adult sex ratio are ecological conditions that facilitate role reversal and polyandry in coucals and terrestrial vertebrates in general.在进化生物学领域，为何通常雄性竞争更激烈而雌性承担更多亲本抚育（parental care）职责，始终是一个引人关注的核心议题。性角色反转的物种尤为值得研究，因为这类‘例外’能够为验证演化规律提供关键支撑。本研究以婚配制度理论（mating systems theory）为框架，对比了两种在性二态性（sexual dimorphism）与亲本抚育模式上差异最为显著的鸦鹃（coucal）物种：仅由雄性承担抚育职责的黑鸦鹃（black coucal），以及双亲共同抚育的白眉鸦鹃（white-browed coucal）。两种鸦鹃栖息于同类茂密生境，面临相似的生态条件，但在婚配制度与性二态性上却存在显著差异。黑鸦鹃为候鸟，繁殖种群密度较高。雌性黑鸦鹃为专性一妻多夫（polyandry），体重几乎是雄性的两倍，该物种属于性二态性反转最极端的脊椎动物类群之一。竞争激烈的雌性个体间繁殖成功率（reproductive success）的差异更大，这表明雌性所承受的性选择（sexual selection）强度高于雄性。与之相反，白眉鸦鹃为留鸟，繁殖种群密度较低，且始终以配对形式繁殖。该物种几乎不存在性二态性，且雌雄个体间的繁殖成功率差异程度相近。黑鸦鹃的巢更易遭到破坏，这可能促使雌性黑鸦鹃摆脱了亲本抚育的职责。我们提出，高食物丰度、高种群密度、高巢损失率以及成年性比偏雄的综合生态条件，是推动鸦鹃类群乃至陆生脊椎动物发生性角色反转与一妻多夫演化的关键因素。