Demography and epidemiology of captive former biomedical research chimpanzees (Pan troglodytes) 1: Survival and mortality

Accurate and up-to-date data on longevity and mortality are essential for describing, analyzing, and managing animal populations in captivity. We assembled a comprehensive demography data set and analyzed survival and mortality patterns in a population of captive former biomedical research chimpanzees. The study synthesized over 51,000 life-years of demographic data collected on 2349 individuals between 1923 and 2014. Our goal was to assess the population's current age-sex composition, estimate rates of survivorship, mortality and life expectancy, and compare findings with other chimpanzee populations of interest. Results indicated an increasingly geriatric contemporary population declining in size. The median life expectancy (MLE) of the entire population was 32.6 years (males 29.1, females 36.1). For chimpanzees who reached 1 year of age, the MLE increased to 34.9 years (males 31.0, females 38.8). Survival probability was influenced by both sex and birth type. Females exhibited greater survivorship than males (β₁  = -0.34, z = -5.74, p < 0.001) and wild-born individuals exhibited greater survivorship than captive-born individuals (β₂  = -0.55, z = -5.89, p < 0.001). There was also a seasonal trend in mortality, wherein more individuals died during the winter months (December-February) compared with other seasons. Analyses of life expectancy over time showed continual increases in both median age of living individuals and median age at death, suggesting that these chimpanzees have yet to reach their full aging potential in a postresearch environment. As they continue to age, ongoing monitoring and analysis of demographic changes will be necessary for science-based population and program management until extinction occurs some decades in the future.

Sex differences in adult lifespan and aging rate across mammals: a test of the 'Mother Curse hypothesis'

In many animal species, including humans, males have shorter lifespan and show faster survival aging than females. This differential increase in mortality between sexes could result from the accumulation of deleterious mutations in the mitochondrial genome of males due to the maternal mode of mtDNA inheritance. To date, empirical evidence supporting the existence of this mechanism - called the Mother Curse hypothesis - remains largely limited to a few study cases in humans and Drosophila. In this study, we tested whether the Mother Curse hypothesis accounts for sex differences in lifespan and aging rate across 128 populations of mammals (60 and 68 populations studied in wild and captive conditions, respectively) encompassing 104 species. We found that adult lifespan decreases with increasing mtDNA neutral substitution rate in both sexes in a similar way in the wild - but not in captivity. Moreover, the aging rate marginally increased with neutral substitution rate in males and females in the wild. Overall, these results indicate that the Mother Curse hypothesis is not supported across mammals. We further discuss the implication of these findings for our understanding of the evolution of sex differences in mortality and aging.

Adult sex ratios: causes of variation and implications for animal and human societies

Converging lines of inquiry from across the social and biological sciences target the adult sex ratio (ASR; the proportion of males in the adult population) as a fundamental population-level determinant of behavior. The ASR, which indicates the relative number of potential mates to competitors in a population, frames the selective arena for competition, mate choice, and social interactions. Here we review a growing literature, focusing on methodological developments that sharpen knowledge of the demographic variables underlying ASR variation, experiments that enhance understanding of the consequences of ASR imbalance across societies, and phylogenetic analyses that provide novel insights into social evolution. We additionally highlight areas where research advances are expected to make accelerating contributions across the social sciences, evolutionary biology, and biodiversity conservation.

Sex differences are insufficient evidence of ecological adaptations in human females

Benenson et al. postulate that human females evolved unique survival adaptations to facilitate maternal and grandmaternal care. This hypothesis is consistent with the broader hypothesis that female phenotypes are more ecologically optimal, but further evidence is needed to make a compelling case that sex differences in self-protection are not primarily the result of more intense sexual selection on males.

Sex chromosomes, sex ratios and sex gaps in longevity in plants

In animals, males and females can display markedly different longevity (also called sex gaps in longevity, SGL). Sex chromosomes contribute to establishing these SGLs. X-hemizygosity and toxicity of the Y chromosomes are two mechanisms that have been suggested to reduce male longevity (Z-hemizygosity and W toxicity in females in ZW systems). In plants, SGLs are known to exist, but the role of sex chromosomes remains to be established. Here, by using adult sex ratio as a proxy for measuring SGLs, we explored the relationship between sex chromosomes and SGLs across 43 plant species. Based on the knowledge accumulated in animals, we specifically asked whether: (i) species with XY systems tend to have female-biased sex ratios (reduced male longevity) and species with ZW ones tend to have male-biased sex ratios (reduced female longevity); and (ii) this pattern was stronger in heteromorphic systems compared to homomorphic ones. Our results tend to support these predictions although we lack statistical power because of a small number of ZW systems and the absence of any heteromorphic ZW system in the dataset. We discuss the implications of these findings, which we hope will stimulate further research on sex differences in lifespan and ageing across plants. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Sex-specific body mass ageing trajectories in adult Asian elephants

In species with marked sexual dimorphism, the classic prediction is that the sex which undergoes stronger intrasexual competition ages earlier or quicker. However, more recently, alternative hypotheses have been put forward, showing that this association can be disrupted. Here, we utilize a unique, longitudinal data set of a semi-captive population of Asian elephants (Elephas maximus), a species with marked male-biased intrasexual competition, with males being larger and having shorter lifespans, and investigate whether males show earlier and/or faster body mass ageing than females. We found evidence of sex-specific body mass ageing trajectories: adult males gained weight up to the age of 48 years old, followed by a decrease in body mass until natural death. In contrast, adult females gained body mass with age until a body mass decline in the last year of life. Our study shows sex-specific ageing patterns, with an earlier onset of body mass declines in males than females, which is consistent with the predictions of the classical theory of ageing.

Sex-specific regulation of development, growth and metabolism

Adult females and males of most species differ in many aspects of their morphology, physiology and behavior, in response to sex-specific selective pressures that maximize fitness. While we have an increasingly good understanding of the genetic mechanisms that initiate these differences, the sex-specific developmental trajectories that generate them are much less well understood. Here we review recent advances in the sex-specific regulation of development focusing on two models where this development is increasingly well understood: Sexual dimorphism of body size in the fruit fly Drosophila melanogaster and sexual dimorphism of horns in the horned beetle Onthophagus taurus. Because growth and development are also supported by metabolism, the regulation of sex-specific metabolism during and after development is an important aspect of the generation of female and male phenotypes. Hitherto, the study of sex-specific development has largely been independent of the study of sex-specific metabolism. Nevertheless, as we discuss in this review, recent research has begun to reveal considerable overlap in the cellular and physiological mechanisms that regulate sex-specific development and metabolism.

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.

Sex-related differences in aging rate are associated with sex chromosome system in amphibians

Sex‐related differences in mortality are widespread in the animal kingdom. Although studies have shown that sex determination systems might drive lifespan evolution, sex chromosome influence on aging rates have not been investigated so far, likely due to an apparent lack of demographic data from clades including both XY (with heterogametic males) and ZW (heterogametic females) systems. Taking advantage of a unique collection of capture–recapture datasets in amphibians, a vertebrate group where XY and ZW systems have repeatedly evolved over the past 200 million years, we examined whether sex heterogamy can predict sex differences in aging rates and lifespans. We showed that the strength and direction of sex differences in aging rates (and not lifespan) differ between XY and ZW systems. Sex‐specific variation in aging rates was moderate within each system, but aging rates tended to be consistently higher in the heterogametic sex. This led to small but detectable effects of sex chromosome system on sex differences in aging rates in our models. Although preliminary, our results suggest that exposed recessive deleterious mutations on the X/Z chromosome (the “unguarded X/Z effect”) or repeat‐rich Y/W chromosome (the “toxic Y/W effect”) could accelerate aging in the heterogametic sex in some vertebrate clades.

Thermal conditions predict intraspecific variation in senescence rate in frogs and toads

Variation in temperature is known to influence mortality patterns in ectotherms. Even though a few experimental studies on model organisms have reported a positive relationship between temperature and actuarial senescence (i.e., the increase in mortality risk with age), how variation in climate influences the senescence rate across the range of a species is still poorly understood in free-ranging animals. We filled this knowledge gap by investigating the relationships linking senescence rate, adult lifespan, and climatic conditions using long-term capture-recapture data from multiple amphibian populations. We considered two pairs of related anuran species from the Ranidae (Rana luteiventris and Rana temporaria) and Bufonidae (Anaxyrus boreas and Bufo bufo) families, which diverged more than 100 Mya and are broadly distributed in North America and Europe. Senescence rates were positively associated with mean annual temperature in all species. In addition, lifespan was negatively correlated with mean annual temperature in all species except A. boreas In both R. luteiventris and A. boreas, mean annual precipitation and human environmental footprint both had negligible effects on senescence rates or lifespans. Overall, our findings demonstrate the critical influence of thermal conditions on mortality patterns across anuran species from temperate regions. In the current context of further global temperature increases predicted by Intergovernmental Panel on Climate Change scenarios, a widespread acceleration of aging in amphibians is expected to occur in the decades to come, which might threaten even more seriously the viability of populations and exacerbate global decline.

Immunosenescence in the wild? A longitudinal study in a long-lived seabird

Longitudinal studies of various vertebrate populations have demonstrated senescent declines in reproductive performance and survival probability to be almost ubiquitous. Longitudinal studies of potential underlying proximate mechanisms, however, are still scarce. Due to its critical function in the maintenance of health and viability, the immune system is among the potential (mediators of) proximate mechanisms that could underlie senescence. Here, we studied three innate immune parameters-haemagglutination titre, haemolysis titre and haptoglobin concentration-in a population of common terns (Sterna hirundo) known to undergo actuarial senescence. We repeatedly sampled birds of known sex and age across 11 years and used random regression models to (a) quantify how immune parameters vary among individuals and (b) describe within-individual age-specific changes in, and potential trade-offs between, immune parameters. Our models revealed no differences between males and females in haemagglutination titre and haptoglobin concentration, and very low among-individual variation in these parameters in general. Within individuals, haemagglutination titre increased with age, while haptoglobin concentration did not change. We found no indication for selective (dis)appearance in relation to haemagglutination titre or haptoglobin concentration, nor for the existence of a trade-off between them. Haemolysis was absent in the majority (76%) of samples. Common terns do not exhibit clear senescence in haemagglutination titre and haptoglobin concentration and show very little among-individual variation in these parameters in general. This may be explained by canalisation of the immune parameters or by the colonial breeding behaviour of our study species, but more longitudinal studies are needed to facilitate investigation of links between species' characteristics and immunosenescence in wild animals.

Does female resistance to mating select for live-fast-die-young strategies in males? A comparative analysis in the genus Drosophila

Female promiscuity is a pervasive selective force on male reproductive traits, and the strength of sexual selection is predicted to influence the trade-off between lifespan and reproduction. In species where sexual selection is intense, males are predicted to invest in sexual strategies that shorten their lifespan, potentially resulting in female-biased sexual dimorphism in longevity. However, comparative analyses have provided contrasting results, potentially due to the use of broad mating system categories or sexual size dimorphism as a proxy for sexual selection. Here, we used female remating rate (i.e. female promiscuity) as a more direct measure of sexual selection strength and conducted a phylogenetic comparative analysis of the relationship between female remating rate and sexual dimorphism in lifespan in 29 species of Drosophila. We did not find strong evidence that female remating rate was correlated with sexual dimorphism in lifespan. However, we found that male and female lifespans are positively correlated among species and that phylogeny and residual variance (i.e. variation in non-phylogenetic factors) are important in determining female remating rate, male and female lifespans separately, and the correlation between male and female lifespan. We suggest that variation in the nature of sexual competition and variation between studies could account for some of the unexplained variation among species in the relation between female remating rate and sexual dimorphism in lifespan.

Castration delays epigenetic aging and feminizes DNA methylation at androgen-regulated loci

In mammals, females generally live longer than males. Nevertheless, the mechanisms underpinning sex-dependent longevity are currently unclear. Epigenetic clocks are powerful biological biomarkers capable of precisely estimating chronological age and identifying novel factors influencing the aging rate using only DNA methylation data. In this study, we developed the first epigenetic clock for domesticated sheep (Ovis aries), which can predict chronological age with a median absolute error of 5.1 months. We have discovered that castrated male sheep have a decelerated aging rate compared to intact males, mediated at least in part by the removal of androgens. Furthermore, we identified several androgen-sensitive CpG dinucleotides that become progressively hypomethylated with age in intact males, but remain stable in castrated males and females. Comparable sex-specific methylation differences in MKLN1 also exist in bat skin and a range of mouse tissues that have high androgen receptor expression, indicating that it may drive androgen-dependent hypomethylation in divergent mammalian species. In characterizing these sites, we identify biologically plausible mechanisms explaining how androgens drive male-accelerated aging.

Evolution of large males is associated with female-skewed adult sex ratios in amniotes

Body size often differs between the sexes (leading to sexual size dimorphism, SSD), as a consequence of differential responses by males and females to selection pressures. Adult sex ratio (ASR, the proportion of males in the adult population) should influence SSD because ASR relates to both the number of competitors and available mates, which shape the intensity of mating competition and thereby promotes SSD evolution. However, whether ASR correlates with SSD variation among species has not been yet tested across a broad range of taxa. Using phylogenetic comparative analyses of 462 amniotes (i.e., reptiles, birds, and mammals), we fill this knowledge gap by showing that male bias in SSD increases with increasingly female-skewed ASRs in both mammals and birds. This relationship is not explained by the higher mortality of the larger sex because SSD is not associated with sex differences in either juvenile or adult mortality. Phylogenetic path analysis indicates that higher mortality in one sex leads to skewed ASR, which in turn may generate selection for SSD biased toward the rare sex. Taken together, our findings provide evidence that skewed ASRs in amniote populations can result in the rarer sex evolving large size to capitalize on enhanced mating opportunities.

Challenges and opportunities for comparative studies of survival rates: An example with male pinnipeds

Survival rates are a central component of life-history strategies of large vertebrate species. However, comparative studies seldom investigate interspecific variation in survival rates with respect to other life-history traits, especially for males. The lack of such studies could be due to the challenges associated with obtaining reliable datasets, incorporating information on the 0-1 probability scale, or dealing with several types of measurement error in life-history traits, which can be a computationally intensive process that is often absent in comparative studies. We present a quantitative approach using a Bayesian phylogenetically controlled regression with the flexibility to incorporate uncertainty in estimated survival rates and quantitative life-history traits while considering genetic similarity among species and uncertainty in relatedness. As with any comparative analysis, our approach makes several assumptions regarding the generalizability and comparability of empirical data from separate studies. Our model is versatile in that it can be applied to any species group of interest and include any life-history traits as covariates. We used an unbiased simulation framework to provide "proof of concept" for our model and applied a slightly richer model to a real data example for pinnipeds. Pinnipeds are an excellent taxonomic group for comparative analysis, but survival rate data are scarce. Our work elucidates the challenges associated with addressing important questions related to broader ecological life-history patterns and how survival-reproduction trade-offs might shape evolutionary histories of extant taxa. Specifically, we underscore the importance of having high-quality estimates of age-specific survival rates and information on other life-history traits that reasonably characterize a species for accurately comparing across species.

The evolution of monogamy is associated with reversals from male to female bias in the survival cost of parasitism

The extent to which parasites reduce host survival should depend upon how hosts balance trade-offs between reproduction and survival. For example, parasites are predicted to impose greater survival costs under polygynous or promiscuous mating systems in which competition for mates favours increased reproductive investment, particularly in males. We provide, to our knowledge, the first comparative test of the hypothesis that the mating system of the host is an important determinant of (i) the extent to which parasites reduce survival, and (ii) the extent to which males and females differ in the survival cost of parasitism. Using meta-analysis of 85 published estimates of the survival cost of parasitism from 72 studies of 64 species representing diverse animal lineages, we show that parasites impose a mean 3.5-fold increase in the odds of mortality on their hosts. Although this survival cost does not differ significantly across monogamous, polygynous and promiscuous mating systems, females incur a greater survival cost than males in monogamous species, whereas males incur a greater survival cost than females in polygynous and promiscuous species. Our results support the idea that mating systems shape the relative extent to which males and females invest in reproduction at the expense of defence against parasites.

Sex-specific actuarial and reproductive senescence in zoo-housed tiger (Panthera tigris): The importance of sub-species for conservation

A fifth of all known species are currently classified as threatened in the wild: the rate of biodiversity loss is rapid, continuous, and mostly due to anthropogenic activities. To slow down this decline, the accurate estimation of demographic parameters for threatened species is critical. With this aim, zoo institutions play an important role, giving access to data on zoo-housed animals, which aids researchers working on species life-history traits and intrinsic factors influencing the fitness of both sexes, such as age. While tigers (Panthera tigris) are particularly threatened in their natural environment, few of their demographic parameters have been determined because of their solitary and elusive nature as well as low population density. Using individual-based information for more than 9200 tigers (from 1938 to 2018) recorded in the International Tiger Studbook 2018, we aimed to determine sub-species and sex-specific variability of survival and reproductive parameters with age. No significant sex-difference in actuarial senescence (i.e., decline of survival probabilities with age) was observed but males tended to have a higher juvenile mortality and a faster senescence than females. Reproductive senescence (i.e., decline of reproductive parameters with age) was more pronounced in females than males. Moreover, we observed sub-species-specific variation in mortality and reproductive patterns, pointing out the necessity to consider them independently for conservation goals. Our findings can provide meaningful improvements to the husbandry of zoo-housed tigers, emphasizing the importance of adult breeding females of 7-9 years-old to control zoo-housed population size, but also providing accurate demographic estimates, crucial to set up effective conservation plans.

Aging and Senescence across Reproductive Traits and Survival in Superb Fairy-Wrens (Malurus cyaneus)

AbstractWhy do senescence rates of fitness-related traits often vary dramatically? By considering the full aging trajectories of multiple traits, we can better understand how a species' life history shapes the evolution of senescence within a population. Here, we examined age-related changes in sex-specific survival, reproduction, and several components of reproduction using a long-term study of a cooperatively breeding songbird, the superb fairy-wren (Malurus cyaneus). We compared aging patterns between traits by estimating standardized rates of maturation, age of onset of senescence, and rates of senescence while controlling for confounding factors reflecting individual variability in life history. We found striking differences in aging and senescence patterns between survival and reproduction as well as between reproductive traits. In both sexes, survival started to decline from maturity onward. In contrast, all reproductive traits showed improvements into early adulthood, and many showed little or no evidence of senescence. In females, despite senescence in clutch size, number of offspring surviving to independence did not decline in late life, possibly due to improvements in maternal care with age. Superb fairy-wrens have exceptionally high levels of extragroup paternity, and while male within-group reproductive success did not change with age, extragroup reproductive success showed a dramatic increase in early ages, followed by a senescent decline, suggesting that male reproductive aging is driven by sexual selection. We discuss how the superb fairy-wrens' complex life history may contribute to the disparate aging patterns across different traits.

Assessing the Diversity of the Form of Age-Specific Changes in Adult Mortality from Captive Mammalian Populations

Actuarial senescence (i.e., the age-specific increase in mortality rate) is pervasive across mammalian species, but our current understanding of the diversity of forms that actuarial senescence displays across species remains limited. Although several mathematical models have been proposed to model actuarial senescence, there is still no consensus on which model to use, especially when comparing mortality patterns among species. To fill this knowledge gap, we fitted and compared different forms of increase using models commonly used in senescence studies (i.e., Gompertz, Weibull, and logistic) across 61 species of mammalian captive populations using the Bayesian Survival Trajectory Analysis (BaSTA) approach. For as much as 79% of the species, a Gompertz increase of mortality with age was the most parsimonious model that satisfactorily described the shape of age-specific mortality changes in adults. This highlights that the form of the increase in mortality is mostly consistent across mammalian species and follows the Gompertz rule with some rare exceptions. The implications of that result are twofold. First, the Gompertz rate of mortality increase should be used in cross-species comparative analyses of mammals, as already done in some studies. Second, although the Gompertz model accurately describes actuarial senescence in most mammals, there are notable exceptions, and the factors causing this deviation from an exponential mortality increase during the adult stage warrant further investigation.

The hidden ageing costs of sperm competition

Ageing and sexual selection are intimately linked. There is by now compelling evidence from studies performed across diverse organisms that males allocating resources to mating competition incur substantial physiological costs, ultimately increasing ageing. However, although insightful, we argue here that to date these studies cover only part of the relationship linking sexual selection and ageing. Crucially, allocation to traits important in post-copulatory sexual selection, that is sperm competition, has been largely ignored. As we demonstrate, such allocation could potentially explain much diversity in male and female ageing patterns observed both within and among species. We first review how allocation to sperm competition traits such as sperm and seminal fluid production depends on the quality of resources available to males and can be associated with a wide range of deleterious effects affecting both somatic tissues and the germline, and thus modulate ageing in both survival and reproductive terms. We further hypothesise that common biological features such as plasticity, prudent sperm allocation and seasonality of ejaculate traits might have evolved as counter-adaptations to limit the ageing costs of sperm competition. Finally, we discuss the implications of these emerging ageing costs of sperm competition for current research on the evolutionary ecology of ageing.

Pathogens Shape Sex Differences in Mammalian Aging

Understanding the origin of sex differences in lifespan and aging patterns remains a salient challenge in both biogerontology and evolutionary biology. Different factors have been studied but the potential influence of pathogens has never been investigated. Sex differences, especially in hormones and resource allocation, generate a differential response to pathogens and thereby shape sex differences in lifespan or aging. We provide an integrative framework linking host pathogenic environment with both sex-specific selections on immune performance and mortality trajectories. We propose future directions to fill existing knowledge gaps about mechanisms that link sex differences, not only to exposition and sensitivity to pathogens, but also to mortality patterns, whilst emphasizing the urgent need to consider the role of sex in medicine.

Years of research in biomedical sciences have revealed that sex-specific immune responses to pathogens can be associated with sex-specific consequences on health.

These effects partly account for the observed sex gap in lifespan, leading women to be longer-lived than males in human populations.

Sexual selection exerted on males and the pathogenic environment may explain, at least partly, the sex-difference in lifespan generally observed across mammalian populations.

Evidence for senescence in survival but not in reproduction in a short-lived passerine

Senescence has been studied since a long time by theoreticians in ecology and evolution, but empirical support in natural population has only recently been accumulating. One of the current challenges is the investigation of senescence of multiple fitness components and the study of differences between sexes. Until now, studies have been more frequently conducted on females than on males and rather in long-lived than in short-lived species. To reach a more fundamental understanding of the evolution of senescence, it is critical to investigate age-specific survival and reproduction performance in both sexes and in a large range of species with contrasting life histories. In this study, we present results on patterns of age-specific and sex-specific variation in survival and reproduction in the whinchat Saxicola rubetra, a short-lived passerine. We compiled individual-based long-term datasets from seven populations that were jointly analyzed within a Bayesian modeling framework. We found evidence for senescence in survival with a continuous decline after the age of 1 year, but no evidence of reproductive senescence. Furthermore, we found no clear evidence for sex effects on these patterns. We discuss these results in light of previous studies documenting senescence in short-lived birds. We note that most of them have been conducted in populations breeding in nest boxes, and we question the potential effect of the nest boxes on the shape of age-reproductive trajectories.

Sex differences in adult lifespan and aging rates of mortality across wild mammals

In human populations, women consistently outlive men, which suggests profound biological foundations for sex differences in survival. Quantifying whether such sex differences are also pervasive in wild mammals is a crucial challenge in both evolutionary biology and biogerontology. Here, we compile demographic data from 134 mammal populations, encompassing 101 species, to show that the female's median lifespan is on average 18.6% longer than that of conspecific males, whereas in humans the female advantage is on average 7.8%. On the contrary, we do not find any consistent sex differences in aging rates. In addition, sex differences in median adult lifespan and aging rates are both highly variable across species. Our analyses suggest that the magnitude of sex differences in mammalian mortality patterns is likely shaped by local environmental conditions in interaction with the sex-specific costs of sexual selection.

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 senescence in artificial insect populations varying in sex-ratio to manipulate reproductive effort

BACKGROUND

The disposable soma theory of ageing assumes that organisms optimally trade-off limited resources between reproduction and longevity to maximize fitness. Early reproduction should especially trade-off against late reproduction and longevity because of reduced investment into somatic protection, including immunity. Moreover, as optimal reproductive strategies of males and females differ, sexually dimorphic patterns of senescence may evolve. In particular, as males gain fitness through mating success, sexual competition should be a major factor accelerating male senescence. In a single experiment, we examined these possibilities by establishing artificial populations of the mealworm beetle, Tenebrio molitor, in which we manipulated the sex-ratio to generate variable levels of investment into reproductive effort and sexual competition in males and females.

RESULTS

As predicted, variation in sex-ratio affected male and female reproductive efforts, with contrasted sex-specific trade-offs between lifetime reproduction, survival and immunity. High effort of reproduction accelerated mortality in females, without affecting immunity, but high early reproductive success was observed only in balanced sex-ratio condition. Male reproduction was costly on longevity and immunity, mainly because of their investment into copulations rather than in sexual competition.

CONCLUSIONS

Our results suggest that T. molitor males, like females, maximize fitness through enhanced longevity, partly explaining their comparable longevity.

Trade-offs between age-related breeding improvement and survival senescence in highly polygynous elephant seals: Dominant males always do better

Life history trade-off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding seasons, and the additional cost of successfully securing and mating females is lacking. We investigated whether trade-offs are present in the highly polygynous male southern elephant seal (Mirounga leonina) using 34-years of individual-level data. We compare age-specific survival, recruitment and future breeding success probabilities of pre-breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models. Pre-breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre-breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature. When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age-specific future breeding success (probability of being dominant at t + 1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders. The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population-level trade-off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate. Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade-off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.

Variation in actuarial senescence does not reflect life span variation across mammals

The concept of actuarial senescence (defined here as the increase in mortality hazards with age) is often confounded with life span duration, which obscures the relative role of age-dependent and age-independent processes in shaping the variation in life span. We use the opportunity afforded by the Species360 database, a collection of individual life span records in captivity, to analyze age-specific mortality patterns in relation to variation in life span. We report evidence of actuarial senescence across 96 mammal species. We identify the life stage (juvenile, prime-age, or senescent) that contributes the most to the observed variation in life span across species. Actuarial senescence only accounted for 35%-50% of the variance in life span across species, depending on the body mass category. We computed the sensitivity and elasticity of life span to five parameters that represent the three stages of the age-specific mortality curve-namely, the duration of the juvenile stage, the mean juvenile mortality, the prime-age (i.e., minimum) adult mortality, the age at the onset of actuarial senescence, and the rate of actuarial senescence. Next, we computed the between-species variance in these five parameters. Combining the two steps, we computed the relative contribution of each of the five parameters to the variance in life span across species. Variation in life span was increasingly driven by the intensity of actuarial senescence and decreasingly driven by prime-age adult mortality from small to large species because of changes in the elasticity of life span to these parameters, even if all the adult survival parameters consistently exhibited a canalization pattern of weaker variability among long-lived species than among short-lived ones. Our work unambiguously demonstrates that life span cannot be used to measure the strength of actuarial senescence, because a substantial and variable proportion of life span variation across mammals is not related to actuarial senescence metrics.

Does grandparental care select for a longer lifespan in non-human mammals?

Several non-human mammalian species provide grandparental care but remain fertile until death, unlike our species. This might call into question the ‘grandmother hypothesis’ that the ability to provide grandparental care, associated with an increase in the cost of breeding with age, promote the early cessation of reproduction. Here, we analyse individual longevity records from non-human mammals to determine whether the few species with grandparental care also stand out among mammals in terms of age-specific survival patterns. Indeed, females of species with grandparental care lived on average 43% longer than males (range: 24–61%), compared with 12% in other polygynous species (95% quantile: −8 to 30%), because of low baseline mortality rates and delayed onset of actuarial senescence. We discuss this finding with respect to the ‘stopping early’ vs. ‘living longer’ debate. We review the role of the environmental context and of the decrease in offspring performance with maternal age (Lansing effect). We formalize the idea of a continuum of parental–grandparental allocation instead of a discrete switch to grandparental care only. Lastly, we suggest that the evolution of menopause has been driven by different forces in different species.

Men Sustain Higher Dysregulation Levels Than Women Without Becoming Frail

The aging process differs in important ways between the sexes, with women living longer but at higher risk for frailty (the male-female health-survival paradox). The underlying biological mechanisms remain poorly understood, but may relate to sex differences in physiological dysregulation patterns. Here, using biomarkers from two longitudinal cohort studies (InCHIANTI and BLSA) and one cross-sectional survey (NHANES), we assess sex differences in trajectories of dysregulation globally and for five physiological systems: oxygen transport, electrolytes, hematopoiesis, lipids, and liver/kidney function. We found higher dysregulation levels in men, both globally and in the oxygen transport and hematopoietic systems (p < .001 for all), though differences for other systems were mixed (electrolytes) or absent (lipids and liver/kidney). There was no clear evidence for sex differences in rates of change in dysregulation with age. Although risk of frailty and mortality increase with dysregulation, there was no evidence for differences in these effects between sexes. These findings imply that the greater susceptibility of women to frailty is not simply due to a tolerance for higher dysregulation; rather, it may actually be men that have a greater tolerance for dysregulation, creating a male-female dysregulation-frailty paradox. However, the precise physiological mechanisms underlying the sex differences appear to be diffuse and hard to pin down.

The influence of early-life allocation to antlers on male performance during adulthood: Evidence from contrasted populations of a large herbivore

To secure mating opportunities, males often develop and maintain conspicuous traits that are involved in intrasexual and/or intersexual competition. While current models of sexual selection rely on the assumption that producing such traits is costly, quantifying the cost of allocating to secondary sexual traits remains challenging. According to the principle of allocation, high energy allocation to growth or sexual traits in males should lead to reduced energy allocation to the maintenance of cellular and physiological functions, potentially causing them to age faster, with impaired survival. We evaluated the short-term and delayed consequences of energy allocation to antlers early in life in two contrasted populations of roe deer, Capreolus capreolus. Although most males mate successfully for the first time in their fourth year, antlers are grown annually from the first year of life onwards. We tested the prediction that a high level of allocation to antler growth during the first two years of life should lead to lower body mass, antler size and survival during the early and late prime stages, as well as to reduced longevity overall. Growing and carrying long antlers during the first years of life was not associated with any detectable cost in the late prime stage. The positive association between antler growth in early life and adult body mass instead supports that fawn antler acts as an honest signal of phenotypic quality in roe deer. For a given body mass, yearling males growing longer antlers displayed impaired performance during their late prime. We also found a trend for a short-term survival cost of allocation to relative antler length during the second year of life. Yearling males that grow long antlers relative to their mass might display a fast life-history tactic. We argue that differential allocation to secondary sexual traits generates a diversity of individual trajectories that should impact population dynamics.

Actuarial senescence in a dimorphic bird: different rates of ageing in morphs with discrete reproductive strategies

It is often hypothesized that intra-sexual competition accelerates actuarial senescence, or the increase in mortality rates with age. However, an alternative hypothesis is that parental investment is more important to determining senescence rates. We used a unique model system, the white-throated sparrow (Zonotrichia albicollis), to study variation in actuarial senescence. In this species, genetically determined morphs display discrete mating strategies and disassortative pairing, providing an excellent opportunity to test the predictions of the above hypotheses. Compared to tan-striped males, white-striped males are more polygynous and aggressive, and less parental. Tan-striped females receive less parental support, and invest more into parental care than white-striped females, which are also more aggressive. Thus, higher senescence rates in males and white-striped birds would support the intra-sexual competition hypothesis, whereas higher senescence rates in females and tan-striped birds would support the parental investment hypothesis. White-striped males showed the lowest rate of actuarial senescence. Tan-striped females had the highest senescence rate, and tan-striped males and white-striped females showed intermediate, relatively equal rates. Thus, results were inconsistent with sexual selection and competitive strategies increasing senescence rates, and instead indicate that senescence may be accelerated by female-biased parental care, and lessened by sharing of parental duties.

Interactions between the sexual identity of the nervous system and the social environment mediate lifespan in Drosophila melanogaster

Sex differences in lifespan are ubiquitous, but the underlying causal factors remain poorly understood. Inter- and intrasexual social interactions are well known to influence lifespan in many taxa, but it has proved challenging to separate the role of sex-specific behaviours from wider physiological differences between the sexes. To address this problem, we genetically manipulated the sexual identity of the nervous system-and hence sexual behaviour-in Drosophila melanogaster, and measured lifespan under varying social conditions. Consistent with previous studies, masculinization of the nervous system in females induced male-specific courtship behaviour and aggression, while nervous system feminization in males induced male-male courtship and reduced aggression. Control females outlived males, but masculinized female groups displayed male-like lifespans and male-like costs of group living. By varying the mixture of control and masculinized females within social groups, we show that male-specific behaviours are costly to recipients, even when received from females. However, consistent with recent findings, our data suggest courtship expression to be surprisingly low cost. Overall, our study indicates that nervous system-mediated expression of sex-specific behaviour per se-independent of wider physiological differences between the sexes, or the receipt of aggression or courtship-plays a limited role in mediating sex differences in lifespan.

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.

Sex gap in aging and longevity: can sex chromosomes play a role?

It is well known that women live longer than men. This gap is observed in most human populations and can even reach 10-15 years. In addition, most of the known super centenarians (i.e., humans who lived for > 110 years) are women. The differences in life expectancy between men and women are often attributed to cultural differences in common thinking. However, sex hormones seem to influence differences in the prevalence of diseases, in the magnitude of aging, and in the longevity between men and women. Moreover, far from being human specific, the sex gap in longevity is extremely common in non-human animals, especially in mammals. Biological factors clearly contribute to such a sex gap in aging and longevity. Different hypotheses have been proposed to explain why males and females age and die differently. The cost of sexual selection and sexual dimorphism has long been considered the best explanation for the observed sex gap in aging/longevity. However, the way mitochondria are transmitted (i.e., through females in most species) could have an effect, called the mother's curse. Recent data suggest that sex chromosomes may also contribute to the sex gap in aging/longevity through several potential mechanisms, including the unguarded X/Z, the toxic Y/W and the loss of Y/W. We discuss future research directions to test these ideas.

Morph-Specific Patterns of Reproductive Senescence: Connections to Discrete Reproductive Strategies

How reproductive strategies contribute to patterns of senescence in natural populations remains contentious. We studied reproductive senescence in the dimorphic white-throated sparrow, an excellent species for exploring this issue. Within both sexes the morphs use distinct reproductive strategies, and disassortative pairing by morph results in pair types with distinct parental systems. White morph birds are more colorful and aggressive than tan counterparts, and white males compete for extrapair matings, whereas tan males are more parental. Tan males and white females share parental care equally, whereas white males provide little parental support to tan females. We found morph-specific patterns of reproductive senescence in both sexes. White males exhibited greater reproductive senescence than tan males. This result likely reflects the difficulty of sustaining a highly competitive reproductive strategy as aging progresses rather than high physiological costs of competitiveness, since white males were also long-lived. Moreover, morph was not consistently related to reproductive senescence across the sexes, arguing against especially high costs of the traits associated with white morph identity. Rather, tan females exhibited earlier reproductive senescence than white females and were short-lived, perhaps reflecting the challenges of unsupported motherhood. Results underscore the importance of social dynamics in determining patterns of reproductive senescence.

Immunosenescence patterns differ between populations but not between sexes in a long-lived mammal

In animals, physiological mechanisms underlying reproductive and actuarial senescence remain poorly understood. Immunosenescence, the decline in the ability to display an efficient immune response with increasing age, is likely to influence both reproductive and actuarial senescence through increased risk of disease. Evidence for such a link has been reported from laboratory animal models but has been poorly investigated in the wild, where variation in resource acquisitions usually drives life-history trade-offs. We investigated immunosenescence patterns over 7 years in both sexes of two contrasting roe deer populations (Capreolus capreolus). We first measured twelve immune markers to obtain a thorough identification of innate and adaptive components of immunity and assessed, from the same individuals, the age-dependent variation observed in parasitic infections. Although the level of innate traits was maintained at old age, the functional innate immune traits declined with increasing age in one of two populations. In both populations, the production of inflammatory markers increased with advancing age. Finally, the adaptive response declined in late adulthood. The increasing parasite burden with age we reported suggests the effective existence of immunosenescence. Age-specific patterns differed between populations but not between sexes, which indicate that habitat quality could shape age-dependent immune phenotype in the wild.

The 'Evo-Demo' Implications of Condition-Dependent Mortality

Animals in the wild die from a variety of different mortality sources, including predation, disease, and starvation. Different mortality sources selectively remove individuals with different body condition in different ways, and this variation in the condition dependence of mortality has evolutionary and demographic implications. Subsequent population dynamics are influenced by the strength of condition-dependent mortality during specific periods, with population growth impacted in different ways in short- versus long-lived species. The evolution of lifespan is strongly influenced by condition-dependent mortality, with strikingly different outcomes expected in senescence rates when the relationship between condition and mortality is altered. A coupling of field and laboratory studies is now required to further reveal the evolutionary implications of condition-dependent mortality.

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.