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.在社会一夫一妻制物种中，面临更高婚外父权（extra-pair paternity）风险的雄性，可通过将父权让渡给亲属来获得广义适合度（inclusive fitness）。我们针对藏地地山雀（Pseudopodoces humilis）检验了这一预测：该物种是一类特殊的兼性合作繁殖类群，其中87%的育幼帮手（helpers）均为雄性，且会在配对成体即将完成产窝时加入其繁殖组。与合作育幼窝（68%的窝次、25%的幼鸟存在婚外父权）相比，双亲育幼窝的婚外父权发生率更低（仅26%的窝次、9%的幼鸟存在该现象）。在双亲育幼窝中，绝大多数婚外父本（88%）是与被婚外配雄性无亲缘关系的配对繁殖个体；而在合作育幼窝中，87%的婚外父本均为与优势雄性（dominant male）存在亲缘关系的育幼帮手。双亲繁殖组与合作繁殖组的每窝幼鸟产出量并无显著差异，但育幼帮手对群体父权的分配，降低了获得协助的雄性的实际繁殖成功率（realized reproductive success）。然而，若将广义适合度纳入考量，则两种社会系统中优势雄性的繁殖成功率并无差异。两种繁殖系统中婚外父权发生率存在差异的一个可能解释是体型差异：双亲繁殖模式下的配对雄性，体型显著大于合作繁殖模式下的配对雄性。我们提出了两种可帮助雄性最大化适合度的备选策略：若体型较大，则以配对模式繁殖，以避免来自育幼帮手的婚外父权威胁；若体型较小，则采取合作繁殖模式，不过需将部分父权让渡给亲属。本研究结果揭示了合作繁殖群体形成的一条特殊路径——该路径源于繁殖雄性自身竞争力低下带来的限制，而非外部生态或种群统计因素（demographic factors）。