Data from: Direct fitness benefits and kinship of social foraging groups in an Old World tropical babbler

Molecular studies have revealed that social groups composed mainly of non-relatives may be widespread in group-living vertebrates, but the benefits favoring such sociality are not well understood. In the Old World, birds often form conspecific foraging groups that are maintained year-round and offspring usually disperse to other social groups. We tested the hypothesis that non-breeding group members are largely unrelated and gain direct fitness benefits through breeding opportunities (males) and brood parasitism (females) in the tropical grey-throated babbler, Stachyris nigriceps, in Malaysian Borneo. Babblers foraged in social groups containing one or more breeding pairs (median = 8 group members of equal sex ratio), but group members rarely assisted with breeding (9% of 67 breeding pairs had a third helper; exhibiting facultative cooperative breeding). Although 20% of 266 group member dyads were first-order relatives of one or both members of the breeding pairs, 80% were unrelated. Male group members gained direct fitness benefits through extra-pair and extra-group paternity (25% of 73 offspring), which was independent of their relatedness to the breeding pair and increased with decreasing group size. In contrast, females did not gain direct fitness benefits through brood parasitism. The low levels of relatedness and helping in social groups suggest that most group members do not gain indirect fitness benefits by helping to raise unrelated offspring. These findings highlight the importance of examining benefits of sociality for unrelated individuals that largely do not help and broaden the direct fitness benefits of group foraging beyond assumed survival benefits.分子研究表明，以非亲缘关系个体为主的社会群体可能广泛存在于群居脊椎动物中，但促成这类社会性的选择收益尚未得到充分阐明。在旧大陆，鸟类常形成全年存续的同种觅食群体，后代通常会扩散至其他社会群体。我们针对马来西亚婆罗洲区域的热带灰胸穗鹛（*Stachyris nigriceps*）检验了相关假说：非繁殖群体成员大多无亲缘关系，并可通过繁殖机会（雄性）与巢寄生（brood parasitism）获得直接适合度收益（direct fitness benefits）。该物种的群体觅食规模通常包含1对及以上繁殖对，群体规模中位数为8，性别比例均衡，但群体成员极少协助繁殖——67个繁殖对中仅9%存在第三只协助者，属于兼性合作繁殖（facultative cooperative breeding）。尽管266个群体成员配对中，有20%为繁殖对一方或双方的一级亲属，但剩余80%均无亲缘关系。雄性群体成员可通过婚外父权（extra-pair paternity）与群外父权（extra-group paternity）获得直接适合度收益，73只子代中有25%由此产生，该收益与它们和繁殖对的亲缘关系无关，且随群体规模缩小而提升。与之相反，雌性个体并未通过巢寄生获得直接适合度收益。社会群体中较低的亲缘关系水平与协助繁殖比例表明，绝大多数群体成员无法通过抚育非亲缘后代获得间接适合度收益（indirect fitness benefits）。本研究结果凸显了探究基本不参与协助的无亲缘个体的社会性收益的重要性，并将群体觅食的直接适合度收益从假定的生存收益范畴进一步拓展。