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，哺乳动物的雄性繁殖成功率受雌性可获得性限制，因此雄性会试图捍卫配偶接触权并排斥竞争对手交配。在哺乳动物类群中，优势等级（dominance rank）与雄性繁殖成功率存在相关性，但最高等级的阿尔法雄性（alpha male）极少能完全独占交配权。为解释这一现象，不完全控制模型（incomplete control models）提出，阿尔法雄性根本无法管控其他个体对配偶的接触。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关键因素。另一种让步模型（concession model）则认为，阿尔法雄性能够阻止其他雄性接触配偶，但主张它们会向从属雄性让步交配机会以换取社交收益。该模型预测，雄性与阿尔法雄性的理毛互动（grooming interactions）会介导其获取配偶的途径。本研究使用36年的观测数据检验了上述两种预测，数据涵盖了坦桑尼亚贡贝国家公园（Gombe National Park）内8只阿尔法雄性黑猩猩的在位任期。研究结果最强支持不完全控制模型：在单次交配事件中，阿尔法雄性成功交配的概率与群队（party）中的雄性数量及性接受期雌性的数量呈负相关。此外，随着群队内雄性数量增加，高优势等级与特定非阿尔法雄性的交配概率提升呈正相关。但让步模型同样得到支持：当阿尔法雄性身处群队时，雄性花费在为阿尔法雄性理毛上的时长与其交配概率呈正相关。鉴于理毛是雄性社交关系中重要的亲和性（affiliative）组成部分，本研究结果表明，与优势个体建立社交纽带可被用于获取交配机会，这一现象在雄性结成联盟的物种中尤为显著。