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（extra-pair paternity）风险的雄性或可通过将父权让渡给亲属来获得广义适合度（inclusive fitness）。我们针对褐背拟地鸦（Pseudopodoces humilis）——一种特殊的兼性合作繁殖物种，其87%的繁殖帮手均为雄性，且多数帮手会在配对完成产卵前不久加入已配对的繁殖对——检验了这一预测。双亲育雏巢群的婚外父权发生率（26%的巢群、9%的幼鸟）显著低于合作育雏巢群（68%的巢群、25%的幼鸟）；其中，双亲育雏巢群内的多数婚外父本（88%）为与遭遇婚外父权的雄鸟无亲缘关系的结对繁殖者，而合作育雏巢群内的婚外父本（87%）则主要为与优势雄鸟存在亲缘关系的繁殖帮手。双亲育雏与合作育雏的窝繁殖成功率并无差异，但繁殖帮手对群体父权的分配降低了获得帮手的雄鸟的实际繁殖成功；然而在纳入广义适合度考量后，两种社会体系下优势雄鸟的繁殖成功并无显著差异。导致两种体系婚外父权率存在差异的一个可能解释是体型差异：处于双亲育雏情境下的结对繁殖雄鸟，其体型显著大于合作育雏情境中的个体。我们提出两种可最大化雄性适合度的备选繁殖策略：若体型较大，则以结对方式繁殖以规避来自繁殖帮手的婚外父权；若体型较小，则选择合作繁殖，但需将部分父权让渡给亲缘个体。本研究结果揭示了合作繁殖群体形成的一条特殊路径，该路径源于繁殖雄鸟竞争能力低下带来的约束，而非外部生态或种群统计因素。