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.

Methods
Data was collected using standard observational methods employed at the Kalahari Meerkat Project since 1992. All details are provided in the main paper.

1. 研究哺乳动物的学者常将雄性更早或更快的衰老速率，归因于一雄多雌制（polygyny）及其伴随的繁殖竞争更高成本。
2. 然而，针对野生种群的研究中，鲜有在非一雄多雌制物种中比较两性特异性衰老轨迹的工作，这使得我们难以就繁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，也可能影响其整个生命周期内的表现变化时。
3. 本研究聚焦于优势雌雄狐獴（Suricata suricatta）的体重、繁殖输出与存活情况随年龄的变化。狐獴属于社会一夫一妻制的合作繁育物种，每个种群中仅由一对优势个体几乎独占繁殖权，种群内的从属个体则协助抚育繁殖者所产的幼崽。
4. 与诸多一雄多雌制社会不同，我们发现雌雄两性在体重或繁殖输出的衰老起始时间与速率上均无显著差异。二者在整个生命周期中的存活衰老模式也较为相似；但与多数野生脊椎动物不同，两性优势个体均未出现存活衰老（survival senescence，即随年龄增长年死亡率升高的现象），因此衰老的适合度代价完全源于中年至晚年阶段繁殖输出的下降。
5. 我们认为，尽管雌性（其激素水平呈雄性化且常具攻击性）面临的同性竞争可能加快其衰老速率，但雌性相较于雄性更长的优势统治任期抵消了这一影响；二者共同作用使得两性的衰老模式趋于一致。
6. 本研究结果表明，在探究野生种群的衰老机制时，需要同时考虑性竞争的形式与强度，以及生活史中其他两性特异性特征。

方法
本研究的数据采集采用了自1992年起卡拉哈里狐獴项目（Kalahari Meerkat Project）所沿用的标准观测方法，详细信息已在正文中给出。