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.

亲本抚育行为的决策通常伴随着权衡取舍——因为分配给抚育后代的资源，通常无法同时用于自身维持或求偶交配。在多数动物类群中，雌性提供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领域仍存在争议。为更深入地解析亲本抚育的性别差异及其演化后果，我们需要选取两性在后代抚育模式上存在本质差异的近缘物种开展研究。本研究针对两种亲本抚育与性角色存在根本差异的近缘鸦鹃属物种，探讨了亲本抚育与后代生长的关联；这两个物种栖息于同类食物资源充沛且气候温和的生境，且繁殖物候特征相似。两种鸦鹃的孵育模式存在差异：单雄抚育的黑鸦鹃（black coucal）对后代的投喂频次，是白眉鸦鹃（white-browed coucal）雌雄双亲投喂总频次的两倍。此外，白眉鸦鹃的抚育停歇时段显著长于雄性黑鸦鹃，停歇期间它们会停留栖木并整理羽毛。然而，亲本抚育的上述差异并未体现在后代生长速率上，这可能是因为白眉鸦鹃给雏鸟投喂的食物中，蛙类占比高于昆虫类。食物充沛且气候温和的生境，或许是单雄抚育演化的必要条件，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，《进化生物学杂志》，第28卷，1335-1353页），本研究数据表明，白眉鸦鹃选择双亲合作抚育后代，可能是因为它们缺乏多配偶繁殖的机会，而非因为需要双亲共同投入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，可能会对雌性施加性选择；而性选择、体型大小与成体性比之间的正向反馈循环，可进一步巩固竞争与亲本抚育中反转的性角色。