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.亲缘选择理论（Kin selection theory）一直是阐释合作繁殖（cooperative breeding）演化的核心模型：在合作繁殖体系中，非繁殖个体需承担抚育幼崽的相关成本。近期，这一理论的主导地位受到了质疑，尤以专性合作繁殖类群（obligate cooperative breeders）为甚——这类类群若无同类协助，繁殖行为极少发生且成功率极低。在这类体系中，有假说提出，通过扩大当前群体规模所获得的直接收益，可作为亲缘选择之外一种易于验证的替代（或补充）机制。然而，目前仍缺乏针对这两种对立假说的明确实证检验。本研究以专性合作繁殖的栗冠弯嘴鹛（Pomatostomus ruficeps）为研究对象，提供了令人信服的证据，表明协助行为的发生与否、发生地点及协助频率，均由亲缘选择而非群体扩增效应所决定。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内可获得的繁殖单元规模来调整协助决策。与之相反，存在亲缘个体的群体成员中，有98%会开展协助行为；所有协助者均会将抚育行为指向社会群体中亲缘关系最近的一窝幼崽；且相较于抚育亲缘关系较远或无亲缘关系幼崽的个体，抚育半同胞或全同胞幼崽的个体，其协助强度约为前者的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驱动力。