Sex-independent senescence in a cooperatively breeding mammal

1. Researchers studying mammals have frequently interpreted earlier or faster rates of ageing in males as resulting from polygyny and the associated higher costs of reproductive competition. 2. Yet few studies conducted on wild populations have compared sex-specific senescence trajectories outside of polygynous species, making it difficult to make generalised inferences on the role of reproductive competition in driving senescence, particularly when other differences between males and females might also contribute to sex-specific changes in performance across lifespan. 3. Here, we examine age-related variation in body mass, reproductive output and survival in dominant male and female meerkats, Suricata suricatta. Meerkats are socially monogamous cooperative breeders where a single dominant pair virtually monopolize reproduction in each group and subordinate group members help to rear offspring produced by breeders. 4. In contrast to many polygynous societies, we find that neither the onset nor the rate of senescence in body mass or reproductive output show clear differences between males and females. Both sexes also display similar patterns of age-related survival across lifespan, but unlike most wild vertebrates, survival senescence (increases in annual mortality with rising age) was absent in dominants of both sexes, and as a result, the fitness costs of senescence were entirely attributable to declines in reproductive output from mid- to late-life. 5. We suggest that the potential for intrasexual competition to increase rates of senescence in females – who are hormonally masculinised and frequently aggressive – is offset by their ability to maintain longer tenures of dominance than males, and that these processes combined lead to similar patterns of senescence in both sexes. 6. Our results stress the need to consider the form and intensity of sexual competition as well as other sex-specific features of life history when investigating the operation of senescence in wild populations.1. 从事哺乳动物研究的学者常将雄性更早或更快的衰老（senescence）速率，归因于一夫多妻制（polygyny）及其伴随的生殖竞争更高成本。 2. 然而，针对野生种群的相关研究工作中，极少有在非一夫多妻物种类群中比较两性特异性衰老轨迹的研究，这使得我们难以对生殖竞争在驱动衰老中的作用作出普遍性推论——尤其是当雌雄两性间的其他差异，也可能对其整个生命周期内的性能性别特异性变化产生影响时。 3. 本研究针对优势雌雄狐獴（Suricata suricatta）的体重、繁殖产出与存活情况的年龄相关变异展开分析。狐獴属于社会一夫一妻制的合作繁育类群，每个种群中仅由一对优势个体几乎完全独占繁殖权，其余群体成员均会协助抚育繁殖个体所产的后代。 4. 与诸多一夫多妻制社会不同，本研究发现雌雄两性在体重或繁殖产出方面的衰老起始时间与速率均无显著差异。两性在整个生命周期内的年龄相关存活模式也较为相似；但与多数野生脊椎动物不同的是，两性优势个体均未出现存活衰老（survival senescence）现象，即随年龄增长年死亡率上升，因此衰老的适合度（fitness）成本完全源自中年至晚年阶段繁殖产出的下降。 5. 我们提出，尽管雌性会因激素作用出现雄性化特征且常具攻击性，其面临的种内竞争加剧衰老速率的潜在可能，却被它们相较于雄性更长的优势地位任期所抵消；二者共同作用最终导致两性呈现相似的衰老模式。 6. 本研究结果强调，在探究野生种群的衰老机制时，不仅需要考虑性竞争的形式与强度，还需纳入生活史中其他性别特异性特征。