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（parental care）的决策通常伴随着权衡取舍，因为投入到亲本抚育的资源往往无法再用于自身维持或交配活动。在多数动物类群中，雌性的亲本抚育投入多于雄性，但这一现象背后的成因在进化生态学（evolutionary ecology）领域仍存在争议。为更深入地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性别差异的近缘物种。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（coucal）物种，探讨了亲本抚育与雏鸟生长的关联；这两个物种栖息于食物充足、气候温和的同类生境，且繁殖物候特征相似。两种鸦鹃的孵育模式存在差异：单亲育幼的雄性黑鸦鹃对雏鸟的投喂频次是雄性与雌性白眉鸦鹃总投喂频次的两倍。此外，白眉鸦鹃的育幼停歇时段（即停留梳理羽毛的时段）多于雄性黑鸦鹃。但亲本抚育的这些差异并未体现在雏鸟的生长情况中，这可能是因为白眉鸦鹃给雏鸟投喂的食物中，蛙类占比高于昆虫。食物充足、气候温和的生境或许是单亲育幼演化的必要条件，但或许不出所料，其并非单亲育幼演化的充分驱动因素。结合此前的研究结果（Goymann等人，2015年，《进化生物学杂志》，28卷，1335-1353页），本研究数据表明，白眉鸦鹃采取双亲合作育幼的原因，可能是它们缺乏多配偶制的交配机会，而非成功抚育全部后代需要双亲共同投入。本案例研究支持了近期提出的演化理论：即宽松的环境条件结合特定的生活史（life-history）特征，可能会在雌性中引发性选择（sexual selection）；随后，性选择、体型大小与成体性比（adult sex-ratio）之间的正向反馈循环，或可稳定竞争与亲本抚育中的性别角色反转现象。