Abstract
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. Yet, few studies conducted on wild populations have compared sex-specific senescence trajectories outside of polygynous species, making it difficult to make generalized 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. 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 monopolizes reproduction in each group and subordinate group members help to rear offspring produced by breeders. In contrast to many polygynous societies, we find that neither the onset nor the rate of senescence in body mass or reproductive output shows 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. We suggest that the potential for intrasexual competition to increase rates of senescence in females-who are hormonally masculinized and frequently aggressive-is offset by their ability to maintain longer tenures of dominance than males, and that these processes when combined lead to similar patterns of senescence in both sexes. 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.
Highlights
Across vertebrate and invertebrate taxa there is striking variation in the extent to which rates of senescence differ between males and females (Austad, 2011; Barford & Dorling, 2006; Carroll & Sherratt, 2017)
Our study finds that in meerkats, the form and rate of senescence across three components of life history are similar in males and females
Agerelated survival probability was unaffected by sex, but unlike the former two traits, we found no evidence of survival senescence in dominant individuals as annual survival probability remained constant beyond the age of peak reproductive output
Summary
Across vertebrate and invertebrate taxa there is striking variation in the extent to which rates of senescence differ between males and females (Austad, 2011; Barford & Dorling, 2006; Carroll & Sherratt, 2017). Because sex differences in reproductive competition are often related to the mating system of a species, the comparison of senesence trajectories across mating systems has provided a useful framework for exploring some of the key predictions of the life history theories of senescence. In polygynous, sexually dimorphic mammals, where males fight frequently and display energetically costly traits that improve fighting success or help to monopolise access to females, it is often males that have shorter lifespans and show higher rates of survival (‘actuarial’) senescence than females In monogamous taxa, where levels of intrasexual competition are closer to parity, sexual dimorphism in longevity and rates of senescence is often absent, or much reduced (Allman, Rosin, Kumar, & Hasenstuab, 1998; Brownikowski et al, 2011; Clutton-Brock & Isvaran, 2007)
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