Data from: Kin selection, not group augmentation, predicts helping in an obligate cooperatively breeding bird

Kin selection theory has been the central model for understanding the evolution of cooperative breeding, where non-breeders help bear the cost of rearing young. Recently the dominance of this idea has been questioned; particularly in obligate cooperative breeders where breeding without help is uncommon and seldom successful. In such systems, the direct benefits gained through augmenting current group size have been hypothesised to provide a tractable alternative (or addition) to kin selection. However, clear empirical tests of the opposing predictions are lacking. Here, we provide convincing evidence to suggest that kin selection and not group augmentation accounts for decisions of whether, where and how often to help in an obligate cooperative breeder, the chestnut-crowned babbler (Pomatostomus ruficeps). We found no evidence that group members base helping decisions on the size of breeding units available in their social group, despite both correlational and experimental data showing substantial variation in the degree to which helpers affect productivity in units of different size. By contrast, 98% of group members with kin present helped, 100% directed their care towards the most related brood in the social group and those rearing half/full-sibs helped approximately 3 times harder than those rearing less/non-related broods. We conclude that kin selection plays a central role in the maintenance of cooperative breeding in this species, despite the apparent importance of living in large groups.

亲缘选择（kin selection）理论长期以来都是阐释合作繁殖（cooperative breeding）演化的核心模型——在合作繁殖系统中，未参与繁殖的个体将协助承担育幼成本。近年来，这一理论的主导地位受到了质疑，尤其针对专性合作繁殖者（obligate cooperative breeders）类群：此类群中，无协助的繁殖行为极为罕见且鲜有成功案例。在此类系统中，有假说提出，通过扩大当前群体规模所获得的直接收益（direct benefits），可作为亲缘选择之外的一种可行替代（或补充）机制。然而，目前仍缺乏针对这两种对立预测的明确实证检验。本研究以专性合作繁殖物种栗冠鸫鹛（chestnut-crowned babbler, Pomatostomus ruficeps）为研究对象，提供了确凿证据表明：决定个体是否提供协助、在何处提供协助以及协助频率的核心机制是亲缘选择，而非群体扩增（group augmentation）。尽管相关分析与实验数据均显示，在不同规模的繁殖单元中，协助者对繁殖成效的影响程度存在显著差异，但我们并未发现群体成员会依据社会群体中可用繁殖单元的规模来调整育幼协助行为。与之形成鲜明对比的是：当群体中存在亲缘个体时，98%的群体成员会参与育幼协助；所有协助者均会将照料行为投向社会群体中亲缘关系最近的一窝幼鸟；相较于照料亲缘关系较远或无亲缘关系幼鸟的个体，照料半同胞/全同胞幼鸟的个体，其育幼协助强度约为后者的3倍。综上，尽管群居大群体看似具有重要意义，但亲缘选择仍是维持该物种合作繁殖行为的核心机制。