Data from: Female ground tits prefer relatives as extra-pair partners: driven by kin-selection?

Socially monogamous female birds routinely mate with males outside the pair bond. Three alternative hypotheses consider genetic benefits as the major driver behind the female strategy. The inbreeding avoidance hypothesis predicts that females paired with closely related males should seek copulations with distantly related extra-pair partners to avoid fitness loss from inbreeding depression; the outbreeding avoidance hypothesis predicts the opposite; the kin-selection hypothesis suggests that regardless of social mate relatedness, females should give related males extra-pair fertilization opportunities to gain inclusive fitness if the costs from inbreeding are minor. We test these hypotheses with a facultative cooperative breeder, the ground tit (Parus humilis). Social pairs of ground tits formed randomly with respect to genetic relatedness. In both bi-parental and cooperative groups, a female’s engaging in extra-pair mating was independent of relatedness to her social mate; however, females preferred extra-pair sires to which they were more related than to their social mates. Moreover, females had higher relatedness with either their extra-group extra-pair sires in both bi-parental and cooperative groups, or within-group helper sires in cooperative groups, than expected by chance. When more than one potential extra-pair partner was available around a female’s nest, she tended to select a relative. There was no indication of fitness reduction from extra-pair mating, which occurred at an intermediate level of inbreeding. These data support the kin-selection hypothesis, although there might be alternative non-genetic reasons associated with the extra-pair mating preference. Our finding offers a new explanation for why female birds pursue extra-pair mating. It also may broaden our understanding of the role of kin-selection in the evolution of cooperative society.实行社会单配制的雌性鸟类常会与配偶联结之外的雄性进行交配。现有三种以遗传收益为该雌性策略核心驱动因素的备选假说：近交回避假说（inbreeding avoidance hypothesis）预测，若雌性与亲缘关系紧密的雄性结成配偶，应会寻求与亲缘关系疏远的婚外雄性交配，以规避近交衰退带来的适合度损失；远交回避假说（outbreeding avoidance hypothesis）则预测相反的结果；亲缘选择假说（kin-selection hypothesis）提出，若近交带来的成本较低，则无论社会配偶的亲缘关系如何，雌性都应为亲缘雄性提供婚外受精机会，以获得广义适合度收益。 我们以兼性合作繁殖者——地山雀（Parus humilis）为研究对象，对上述假说进行检验。结果显示，地山雀的社会配偶配对与遗传亲缘关系并无关联。无论是双亲家庭还是合作繁殖群体中，雌性发生婚外配的概率均与其社会配偶的亲缘关系无关；但相较于社会配偶，雌性更偏好与其亲缘关系更近的婚外父本。此外，无论在双亲家庭还是合作繁殖群体中，雌性与其群外婚外父本的亲缘关系，或是在合作繁殖群体中与其群内帮手父本的亲缘关系，均显著高于随机预期水平。当雌性巢穴周边存在多个潜在婚外配对象时，其倾向于选择亲缘个体。本研究未发现婚外配会导致适合度下降，且该类婚外配发生在中等程度的近交水平下。 上述数据支持亲缘选择假说，尽管雌性的婚外配偏好可能还存在其他非遗传层面的诱因。本研究为雌性鸟类为何会采取婚外配策略提供了全新解释，也有助于拓展我们对亲缘选择在合作社会演化中所发挥作用的认知。