Data from: Bi-parental versus cooperative breeding in a passerine: fitness-maximizing strategies of males in response to risk of extra-pair paternity?

In socially monogamous species, males that risk cuckoldry more than others might gain inclusive fitness by yielding paternity to relatives. We tested this prediction in the Tibetan ground tit Pseudopodoces humilis, an unusual facultative cooperative breeder wherein most helpers (87% males) join a mated pair shortly before clutch completion. Extra-pair paternity among bi-parental broods occurred less often (26% of broods, 9% of young) compared to cooperative broods (68%, 25%). In the former, most extra-pair sires (88%) were pair breeders unrelated to the cuckolded males whereas in the latter, sires (87%) were mainly helpers related to the dominant male. Brood productivity did not differ between the bi-parental and cooperative breeders, but helpers' partitioning over group paternity reduced the realized reproductive success of helped males. After taking inclusive fitness into account, however, there was no difference in success of dominant males between the two social systems. One possible explanation for the differences in the rates of cuckoldry in the two systems was body size, for pair-bond males in bi-parental situations were significantly larger than those in cooperative ones. We propose two alternative strategies for males to maximize fitness: breed as a pair if large to avoid cuckoldry from helpers, or breed cooperatively if small but compromise some paternity to relatives. Our results provide an unusual route to the incidence of cooperative groups, based on constraints imposed by low competitive ability of breeding males rather than some external ecological or demographic factors.在社会性单配制物种中，相较于其他雄性，面临更高婚外父权（cuckoldry）风险的雄性可通过将父权让渡给亲缘个体来获得广义适合度（inclusive fitness）。我们在藏地山雀（Pseudopodoces humilis）——一种特殊的兼性合作繁殖者，其中绝大多数协助者（87%为雄性）会在配对繁殖对产卵完成前不久加入——中验证了这一理论预测。双亲育雏巢中的婚外父权（extra-pair paternity）发生率（26%的巢、9%的幼鸟）远低于合作育雏巢（68%的巢、25%的幼鸟）。前者中，绝大多数婚外父本（88%）为与父权受侵害雄性无亲缘关系的单配制繁殖者；而后者中，父本（87%）主要是与优势雄性存在亲缘关系的协助者。双亲繁殖与合作繁殖的巢繁殖成功率并无显著差异，但协助者对群体父权的分配降低了获得协助的雄性的实际繁殖成功率。然而，当纳入广义适合度考量后，两种社会系统中优势雄性的繁殖成功率并无差异。两种系统中婚外父权发生率存在差异的一个可能解释是体型：双亲繁殖场景下的结对雄性体型显著大于合作繁殖场景中的雄性。我们提出了两种可让雄性最大化适合度的备选策略：若体型较大，则以单配制方式繁殖，以避免来自协助者的婚外父权；若体型较小，则采用合作繁殖策略，但需将部分父权让渡给亲缘个体。本研究结果揭示了合作群体形成的一条非常规路径：其驱动因素为繁殖雄性竞争能力不足所带来的限制，而非外部生态或种群统计因素。