Sex-specific aging: sex differences in survival and health in a wild primate population
Final Report Abstract
Senescence is thought to result from weakened selection against deleterious mutations at old age, and optimized resource allocation to reproduction versus self‐maintenance. Life history theory predicts that a longer lifespan should lead to delayed or slower senescence. Likewise, the shorter‐lived sex (males in most mammals) should show earlier and more pronounced senescence. However, sex‐specific selective pressures for lifespan and reproductive rate might be expected to promote sex‐specific investment in different traits, optimizing self‐maintenance in females and competitive ability in males. Beyond comparative studies of age‐related mortality rates, these predictions remain largely untested under natural conditions. The aim of this project was to evaluate evidence for these hypotheses by studying sex‐specific functional senescence (within‐individual deterioration in physiological or physical functioning) in a sexually monomorphic primate species, the gray mouse lemur Microcebus murinus that experiences high extrinsic mortality in its natural environment. We used longitudinal and cross‐sectional data from a wild mouse lemur population to examine sex‐specific age trajectories in four components of health that broadly indicate the functioning of the individual and its ability to cope with environmental demands: body mass, physical strength, gastrointestinal parasite burden and allostatic load (baseline glucocorticoid level). Comparative data on body mass and physical strength were obtained from a captive breeding colony to evaluate the importance of extrinsic mortality on patterns of senescence. The evidence for senescence differed among the components of health and showed sex‐, season‐, and environment‐specific patterns. Body mass and parasite resistance were preserved until old age in the wild, likely reflecting their importance for survival, whereas senescence was observed in physical strength and allostatic load. Unlike in the wild, senescence was observed in body mass and strength in both sexes in the captive population, suggesting that extrinsic mortality masks senescence in the wild. However, mortality was biased towards individuals in poor condition in both captive and wild populations. Despite their early mortality, wild males did not appear to experience earlier or faster functional senescence, perhaps due to strong selective pressures for a “robust” male phenotype required for male reproductive success. Our results do not support the fundamental prediction of earlier or more pronounced senescence in the shorter‐lived sex. Furthermore, the onset of senescence in the study species occurs much later than the average age at disappearance in the wild, contradicting the prediction that somatic maintenance should rapidly decline after sexual maturity under conditions of high extrinsic mortality. Overall, our results suggest that rapid, selective mortality of individuals in a declining condition and the associated positive selection for better self‐maintenance may lead to delayed or negligible senescence of traits that enhance fitness.
Publications
- (2014) Senescence or selective disappearance? Age trajectories of body mass in wild and captive populations of a small‐bodied primate. Proceedings of the Royal Society B 281: 20140830
Hämäläinen A, Dammhahn M, Aujard F, Eberle M, Hardy I, Kappeler P, Perret M, Schliehe‐Diecks S, Kraus C
(See online at https://doi.org/10.1098/rspb.2014.0830) - (2014). Evaluating capture stress in wild gray mouse lemurs via repeated fecal sampling: method validation and the influence of prior experience and handling protocols on stress responses. General Comparative Endocrinology 195: 68‐79
Hämäläinen A, Heistermann M, Fenosoa ZSE, Kraus C
(See online at https://doi.org/10.1016/j.ygcen.2013.10.017)
