Life history evolution, reproduction, and the origins of sex-dependent aging and longevity

Males and females in many species differ in how they age and how long they live. These differences have motivated much research, concerning both their evolution and the underlying mechanisms that cause them. We review how differences in male and female life histories have evolved to shape patterns of aging and some of the mechanisms and pathways involved. We pay particular attention to three areas where considerable potential for synergy between mechanistic and evolutionary research exists: (1) the role of estrogens, androgens, the growth hormone/insulin-like growth factor 1 pathway, and the mechanistic target of rapamycin signaling pathway in sex-dependent growth and reproduction; (2) sexual conflict over mating rate and fertility, and how mate presence or mating can become an avenue for males and females to directly affect each other's life span; and (3) the link between dietary restriction and aging, and the emerging understanding that only the restriction of certain nutrients is involved and that this is linked to reproduction. We suggest that ideas about life histories, sex-dependent selection, and sexual conflict can inform and be informed by the ever more refined and complex understanding of the mechanisms that cause aging.

Comparative analyses of longevity and senescence reveal variable survival benefits of living in zoos across mammals

While it is commonly believed that animals live longer in zoos than in the wild, this assumption has rarely been tested. We compared four survival metrics (longevity, baseline mortality, onset of senescence and rate of senescence) between both sexes of free-ranging and zoo populations of more than 50 mammal species. We found that mammals from zoo populations generally lived longer than their wild counterparts (84% of species). The effect was most notable in species with a faster pace of life (i.e. a short life span, high reproductive rate and high mortality in the wild) because zoos evidently offer protection against a number of relevant conditions like predation, intraspecific competition and diseases. Species with a slower pace of life (i.e. a long life span, low reproduction rate and low mortality in the wild) benefit less from captivity in terms of longevity; in such species, there is probably less potential for a reduction in mortality. These findings provide a first general explanation about the different magnitude of zoo environment benefits among mammalian species, and thereby highlight the effort that is needed to improve captive conditions for slow-living species that are particularly susceptible to extinction in the wild.

Reproductive costs in terrestrial male vertebrates: insights from bird studies

Reproduction requires resources that cannot be allocated to other functions resulting in direct reproductive costs (i.e. trade-offs between current reproduction and subsequent survival/reproduction). In wild vertebrates, direct reproductive costs have been widely described in females, but their occurrence in males remains to be explored. To fill this gap, we gathered 53 studies on 48 species testing direct reproductive costs in male vertebrates. We found a trade-off between current reproduction and subsequent performances in 29% of the species and in every clade. As 73% of the studied species are birds, we focused on that clade to investigate whether such trade-offs are associated with (i) levels of paternal care, (ii) polygyny or (iii) pace of life. More precisely for this third question, it is expected that fast species (i.e. short lifespan, early maturity, high fecundity) pay a cost in terms of survival, whereas slow species (with opposite characteristics) do so in terms of fecundity. Our findings tend to support this hypothesis. Finally, we pointed out the potential confounding effects that should be accounted for when investigating reproductive costs in males and strongly encourage the investigation of such costs in more clades to understand to what extent our results are relevant for other vertebrates.