Data from: Incomplete control and concessions explain mating skew in male chimpanzees

Sexual selection theory predicts that because male reproductive success in mammals is limited by access to females, males will attempt to defend access to mates and exclude rivals from mating. In mammals, dominance rank is correlated with male reproductive success; however, the highest-ranking (alpha) male rarely monopolizes reproduction completely. To explain why, incomplete control models propose that alpha males simply cannot control others’ access to mates. If true, then dominance rank should be a key factor influencing subordinate male mating success. Alternatively, the concession model states that alpha males can prevent other males from gaining access to mates but posits that they concede matings to subordinates in exchange for social favours. This predicts that a male’s grooming interactions with the alpha should mediate his access to females. We test these predictions using 36 years of data, encompassing the tenures of eight alpha male chimpanzees at Gombe National Park, Tanzania. Incomplete control models were most strongly supported. At a given copulation event, the probability that the alpha was the male that mated was negatively associated with the number of males and sexually receptive females in the party. Additionally, as the number of males increased, high dominance rank was associated with an increased likelihood that a particular non-alpha male mated. The concession model, however, was also supported: The amount of time a male spent grooming with the alpha was positively associated with his likelihood of mating when the alpha was present in the party. As grooming is a major affiliative component of male social relationships, our results suggest that social bonds with dominant individuals are leveraged for mating access, particularly in species in which males form coalitions.

性选择理论（Sexual selection theory）预测，哺乳类雄性的繁殖成功率受雌性可获得性限制，雄性会试图守护交配权并排除竞争对手的交配行为。在哺乳类中，优势等级（dominance rank）与雄性繁殖成功率相关；然而，等级最高的阿尔法雄性（alpha male）极少能完全垄断交配权。为解释这一现象，不完全控制模型（incomplete control models）提出，阿尔法雄性实际上无法管控其他个体的交配途径。若该模型成立，则优势等级应是影响从属雄性交配成功率的核心因素。与之相对，让步模型（concession model）则认为，阿尔法雄性虽可阻止其他雄性获得交配机会，但会向从属雄性让步交配权以换取社交支持。该模型预测，雄性与阿尔法雄性的理毛互动（grooming interactions）会介导其获取雌性的途径。我们利用坦桑尼亚贡贝国家公园（Gombe National Park）8只阿尔法雄性在位任期内的36年观测数据，对上述预测进行了检验。研究结果强烈支持不完全控制模型：在单次交配事件中，阿尔法雄性参与交配的概率，与临时群（party）内的雄性数量及性接受态雌性（sexually receptive females）的数量呈负相关。此外，随着群内雄性数量增加，较高的优势等级会提升特定非阿尔法雄性的交配概率。但让步模型同样得到了支持：当阿尔法雄性存在于临时群中时，雄性为其理毛的时长与其交配概率呈正相关。由于理毛行为是雄性社交关系中核心的亲和性（affiliative）组成部分，我们的研究结果表明，与优势个体建立社交纽带可用于获取交配机会，这一现象在雄性组建联盟的物种中尤为显著。