Data from: Sex roles, parental care and offspring growth in two contrasting coucal species

The decision to provide parental care is often associated with trade-offs, because resources allocated to parental care typically cannot be invested in self-maintenance or mating. In most animals, females provide more parental care than males, but the reason for this pattern is still debated in evolutionary ecology. To better understand sex differences in parental care and its consequences we need to study closely related species where the sexes differ in offspring care. We investigated parental care in relation to offspring growth in two closely related coucal species that fundamentally differ in sex roles and parental care, but live in the same food-rich habitat with a benign climate, and have a similar breeding phenology. Incubation patterns differed and uniparental male black coucals fed their offspring two times more often than female and male white-browed coucals combined. Also, white-browed coucals had more `off-times´ than male black coucals, during which they perched and preened. However, these differences in parental care were not reflected in offspring growth, likely because white-browed coucals fed their nestlings a larger proportion of frogs than insects. A food-rich habitat with a benign climate may be a necessary, but – perhaps unsurprisingly – is not a sufficient factor for the evolution of uniparental care. In combination with previous results (Goymann et al. (2015) J. Evol. Biol. 28, 1335-1353) these data suggest that white-browed coucals may cooperate in parental care because they lack opportunities to become polygamous rather than because both parents were needed to successfully raise all offspring. Our case study supports recent theory suggesting that permissive environmental conditions in combination with a particular life-history may induce sexual selection in females. A positive feed-back loop between sexual selection, body size, and adult sex-ratio may then stabilize reversed sex-roles in competition and parental care.亲本抚育的决策通常伴随着权衡取舍，因为分配给亲本抚育的资源通常无法再投入到自身维持或求偶繁殖活动中。在绝大多数动物类群中，雌性的亲本抚育投入要多于雄性，但该现象背后的成因在进化生态学领域仍存在广泛争议。为深入理解亲本抚育的性别差异及其演化后果，我们需要研究那些在育幼行为上存在明确性别分化的近缘物种。本研究针对两种育幼行为与性别角色存在本质差异的近缘鸦鹃物种，探讨了其亲本抚育模式与雏鸟生长的关联；这两个物种栖息于气候温和、食物资源充裕的相同生境，且繁殖物候特征高度相似。研究发现，二者的孵卵模式存在显著差异：单亲育幼的雄性黑鸦鹃的育雏投喂频率，是雄性与雌性白眉鸦鹃总投喂频率的两倍以上。此外，白眉鸦鹃的育雏间隙时长显著长于雄性黑鸦鹃，期间它们会停留休憩并整理羽毛。但亲本抚育的上述差异并未体现在雏鸟的生长表现中，这可能是因为白眉鸦鹃给雏鸟投喂的食物中，蛙类占比远高于昆虫类。气候温和、食物充裕的生境或许是单亲育幼演化的必要条件，但显然并非充分条件——这一结论并不出人意料。结合此前的研究结果（Goymann等人，2015年，*J. Evol. Biol.*，第28卷，1335-1353页），本研究数据表明，白眉鸦鹃采取双亲合作育幼，可能是因为它们缺乏一夫多妻的繁殖机会，而非因为需要双亲共同参与才能成功抚育全部雏鸟。本案例研究支持了近期的演化理论：宽松的环境条件结合特定的生活史特征，可能会在雌性中引发性选择；而性选择、体型大小与成体性比之间的正向反馈环路，或许会巩固竞争与亲本抚育中的性别角色反转现象。