Evidence for sex-specific reproductive senescence in monogamous cooperatively breeding red wolves

Sperm morphology and forward motility are indicators of reproductive success and are not age- or condition-dependent in a captive breeding population of endangered snake

The relationship between male ejaculate traits and reproductive success is an important consideration for captive breeding programs. A recovery plan for the endangered Louisiana pinesnake includes captive breeding for the release of young to the wild. Semen was collected from twenty captive breeding male snakes and ejaculate traits of motility, morphology, and membrane viability were measured for each male. Semen traits were analyzed in relation to the fertilization rate of eggs produced from pairings of each male with a single female (% fertility) to determine the ejaculate factors contributing to reproductive success. In addition, we investigated the age- and condition-dependence of each ejaculate trait. We found significant variation in the ejaculate traits of males and normal sperm morphology ([Formula: see text] = 44.4 ± 13.6%, n = 19) and forward motility ([Formula: see text] = 61.0 ± 13.4%, n = 18) were found to be the best predictors of fertility. No ejaculate traits were found to be condition-dependent (P > 0.05). Forward progressive movement (FPM) ([Formula: see text] = 4 ± 0.5, n = 18) was determined to be age-dependent (r2 = 0.27, P = 0.028), but FPM was not included in the best model for rate of fertilization. Male Louisiana pinesnakes do not appear to experience a significant decline in reproductive potential with age (P > 0.05). The observed average rate of fertilization in the captive breeding colony was below 50% and only those pairings with a male having >51% normal sperm morphology avoided a 0% rate of fertilization. Identification of the factors contributing to the reproductive success of captive breeding Louisiana pinesnakes is of considerable conservation value in the recovery of the species, and captive breeding programs should use assessments of ejaculate traits to plan breeding pairs for maximum reproductive output.

Sex-specific aging in animals: Perspective and future directions

Sex differences in aging occur in many animal species, and they include sex differences in lifespan, in the onset and progression of age-associated decline, and in physiological and molecular markers of aging. Sex differences in aging vary greatly across the animal kingdom. For example, there are species with longer-lived females, species where males live longer, and species lacking sex differences in lifespan. The underlying causes of sex differences in aging remain mostly unknown. Currently, we do not understand the molecular drivers of sex differences in aging, or whether they are related to the accepted hallmarks or pillars of aging or linked to other well-characterized processes. In particular, understanding the role of sex-determination mechanisms and sex differences in aging is relatively understudied. Here, we take a comparative, interdisciplinary approach to explore various hypotheses about how sex differences in aging arise. We discuss genomic, morphological, and environmental differences between the sexes and how these relate to sex differences in aging. Finally, we present some suggestions for future research in this area and provide recommendations for promising experimental designs.

Understanding Reproductive Aging in Wildlife to Improve Animal Conservation and Human Reproductive Health

Similar to humans and laboratory animals, reproductive aging is observed in wild species-from small invertebrates to large mammals. Aging issues are also prevalent in rare and endangered species under human care as their life expectancy is longer than in the wild. The objectives of this review are to (1) present conserved as well as distinctive traits of reproductive aging in different wild animal species (2) highlight the value of comparative studies to address aging issues in conservation breeding as well as in human reproductive medicine, and (3) suggest next steps forward in that research area. From social insects to mega-vertebrates, reproductive aging studies as well as observations in the wild or in breeding centers often remain at the physiological or organismal scale (senescence) rather than at the germ cell level. Overall, multiple traits are conserved across very different species (depletion of the ovarian reserve or no decline in testicular functions), but unique features also exist (endless reproductive life or unaltered quality of germ cells). There is a broad consensus about the need to fill research gaps because many cellular and molecular processes during reproductive aging remain undescribed. More research in male aging is particularly needed across all species. Furthermore, studies on reproductive aging of target species in their natural habitat (sentinel species) are crucial to define more accurate reproductive indicators relevant to other species, including humans, sharing the same environment. Wild species can significantly contribute to our general knowledge of a crucial phenomenon and provide new approaches to extend the reproductive lifespan.

Female reproductive senescence across mammals: A high diversity of patterns modulated by life history and mating traits

Senescence patterns are highly variable across the animal kingdom. However, while empirical evidence of actuarial senescence in vertebrates is accumulating in the wild and life history correlates of actuarial senescence are increasingly identified, both the extent and variation of reproductive senescence across species remain poorly studied. Here, we performed the first large-scale analysis of female reproductive senescence across 101 mammalian species that encompassed a wide range of Orders. We found evidence of reproductive senescence in 68.31 % of the species, which demonstrates that reproductive senescence is pervasive in mammals. As expected from allometric rules, the onset of reproductive senescence occurs later and the rate of reproductive senescence decreases with increasing body mass and delayed age at first reproduction. Moreover, for a given pace of life, females displaying a high level of multiple mating and/or with induced ovulation senesce earlier than females displaying a low level of multiple mating and/or with spontaneous ovulation. These results suggest that both female mating behavior and reproductive physiology shape the diversity of reproductive senescence patterns across mammals. We propose future avenues of research regarding the role played by environmental conditions or reproductive features (e.g. type of placentation) on the evolution of reproductive senescence.

Sex-independent senescence in a cooperatively breeding mammal

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.

Sex-specific patterns of reproductive senescence in a long-lived reintroduced raptor

For many species, there is evidence that breeding performance changes as an individual ages. In iteroparous species, breeding performance often increases through early life and is expected to level out or even decline (senesce) later in life. An individual's sex and conditions experienced in early life may also affect breeding performance and how this changes with age. Long-term monitoring of individuals from reintroduced populations can provide unique opportunities to explore age-related trends in breeding performance that might otherwise be logistically challenging. We used a unique dataset from a reintroduced population of white-tailed eagles Haliaeetus albicilla in Scotland, which has been intensively monitored since their initial reintroduction in 1975, to study age- and sex-specific trends in two measures of breeding performance. This monitoring provided data on the breeding performance of known individuals ranging in age from 3 to 26 years. We also explored changes in breeding performance in relation to early life experience (i.e., whether they were released or fledged in the wild). Breeding performance increased with age in early life in a similar manner for both sexes. We found stronger evidence for senescence in breeding performance in males than females. However, late-life female breeding success was associated with early life experience, while male senescent trends were not apparently impacted by conditions experienced during early life. Sexual differences in senescence mean that older males are less likely to breed successfully compared to older females, and this may influence females' mate changes later in life. This difference may suggest a linked sexual difference in survival rates or the possibility of proactive partner change by females in later life in this typically monogamous biparental species.