Reproductive senescence: new perspectives in the wild

Variation in the vital rates of an Antarctic marine predator: the role of individual heterogeneity

Variation in life-history traits such as lifespan and lifetime reproductive output is thought to arise, in part, due to among-individual differences in the underlying probabilities of survival and reproduction. However, the stochastic nature of demographic processes can also generate considerable variation in fitness-related traits among otherwise-identical individuals. An improved understanding of life-history evolution and population dynamics therefore depends on evaluating the relative role of each of these processes. Here, we used a 33-yr data set with reproductive histories for 1,274 female Weddell seals from Erebus Bay, Antarctica, to assess the strength of evidence for among-individual heterogeneity in the probabilities of survival and reproduction, while accounting for multiple other sources of variation in vital rates. Our analysis used recent advances in Bayesian model selection techniques and diagnostics to directly compare model fit and predictive power between models that included individual effects on survival and reproduction to those that did not. We found strong evidence for costs of reproduction to both survival and future reproduction, with breeders having rates of survival and subsequent reproduction that were 3% and 6% lower than rates for non-breeders. We detected age-related changes in the rates of survival and reproduction, but the patterns differed for the two rates. Survival rates steadily declined from 0.92 at age 7 to 0.56 at the maximal age of 31 yr. In contrast, reproductive rates increased from 0.68 at age 7 to 0.79 at age 16 and then steadily declined to 0.37 for the oldest females. Models that included individual effects explained more variation in observed life histories and had better estimated predictive power than those that did not, indicating their importance in understanding sources of variation among individuals in life-history traits. We found that among-individual heterogeneity in survival was small relative to that for reproduction. Our study, which found patterns of variation in vital rates that are consistent with a series of predictions from life-history theory, is the first to provide a thorough assessment of variation in important vital rates for a long-lived, high-latitude marine mammal while taking full advantage of recent developments in model evaluation.

Disentangling Pre- and Postnatal Maternal Age Effects on Offspring Performance in an Insect with Elaborate Maternal Care

Maternal effect senescence has attracted much recent scientific interest. However, the age-related effects of pre- and postnatal maternal age are often conflated, as these naturally originate from the same individual. Additionally, many maternal effect senescence studies fail to account for potential biases associated with selective disappearance. Here we use a cross-fostered laboratory population of a burying beetle, Nicrophorus vespilloides, to examine both the effects of female pre- and postnatal maternal age on offspring life-history traits and the postcare outcomes of mothers while accounting for selective disappearance of postnatal caregivers. Neither pre- nor postnatal maternal age affected offspring longevity or larval weight at hatching, and postnatal age had no effect on postcare maternal outcomes except to confirm the presence of actuarial senescence. There was weak evidence for concave relationships between two larval traits (dispersal weight and survival) and the age of egg producers. Selective disappearance of caregivers had no clear effect on any of the measured offspring traits. Contrary to predictions from evolutionary theory, maternal effect senescence and reproductive effort increases do not always manifest, and current theory may be insufficient to account for the true diversity of aging patterns relating to maternal care.

Horizons in the evolution of aging

Between the 1930s and 50s, evolutionary biologists developed a successful theory of why organisms age, firmly rooted in population genetic principles. By the 1980s the evolution of aging had a secure experimental basis. Since the force of selection declines with age, aging evolves due to mutation accumulation or a benefit to fitness early in life. Here we review major insights and challenges that have emerged over the last 35 years: selection does not always necessarily decline with age; higher extrinsic (i.e., environmentally caused) mortality does not always accelerate aging; conserved pathways control aging rate; senescence patterns are more diverse than previously thought; aging is not universal; trade-offs involving lifespan can be 'broken'; aging might be 'druggable'; and human life expectancy continues to rise but compressing late-life morbidity remains a pressing challenge.

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.

Senescence impacts reproduction and maternal investment in bottlenose dolphins

Reproductive senescence is evident across many mammalian species. An emerging perspective considers components of reproductive senescence as evolutionarily distinct phenomena: fertility senescence and maternal-effect senescence. While fertility senescence is regarded as the ageing of reproductive physiology, maternal-effect senescence pertains to the declining capacity to provision and rear surviving offspring due to age. Both contribute to reproductive failure in utero making it difficult to differentiate between the two prenatally in the wild. We investigated both components in a long-lived mammal with prolonged maternal care through three parameters: calf survival, interbirth interval (IBI) and lactation period. We provide clear evidence for reproductive senescence in a wild population of bottlenose dolphins (Tursiops aduncus) using 34+ years of longitudinal data on 229 adult females and 562 calves. Calf survival decreased with maternal age, and calves with older mothers had lower survival than predicted by birth order, suggesting maternal-effect senescence. Both lactation period and IBIs increased with maternal age, and IBIs increased regardless of calf mortality, indicating interactions between fertility and maternal-effect senescence. Of calves that survived to weaning, last-born calves weaned later than earlier-born calves, evidence of terminal investment, a mitigating strategy given reduced reproductive value caused by either components of reproductive senescence.

Sex-specific effects of inbreeding on reproductive senescence

Inbreeding depression plays a significant role in evolutionary biology and ecology. However, we lack a clear understanding of the fitness consequences of inbreeding depression. Studies often focus on short-term effects of inbreeding in juvenile offspring, whereas inbreeding depression in adult traits and the interplay between inbreeding depression and age are rarely addressed. Inbreeding depression may increase with age and accelerate the decline in reproductive output in ageing individuals (reproductive senescence), which could be subject to sex-specific dynamics. We test this hypothesis with a longitudinal experimental study in a short-lived songbird. Adult inbred and outbred male and female canaries were paired in a 2 × 2 factorial design, and survival and annual reproductive performance were studied for 3 years. We found inbreeding depression in female egg-laying ability, male fertilization success and survival of both sexes. Annual reproductive success of both males and females declined when paired with an inbred partner independent of their own inbreeding status. This shows that inbreeding can have fitness costs in outbred individuals when they mate with an inbred individual. Further, inbred females showed faster reproductive senescence than outbred females, confirming that inbreeding depression and age can interact to affect fitness. By contrast, there was no evidence for an interaction between inbreeding depression and reproductive senescence in male fertilization success. Our findings highlight the importance of considering sex-specific effects and age to determine the full range of fitness consequences of inbreeding and demonstrate that inbreeding depression can accelerate reproductive senescence.

Declining home range area predicts reduced late-life survival in two wild ungulate populations

Demographic senescence is increasingly recognised as an important force shaping the dynamics of wild vertebrate populations. However, our understanding of the processes that underpin these declines in survival and fertility in old age remains limited. Evidence for age-related changes in foraging behaviour and habitat use is emerging from wild vertebrate studies, but the extent to which these are driven by within-individual changes, and the consequences for fitness, remain unclear. Using longitudinal census observations collected over four decades from two long-term individual-based studies of unmanaged ungulates, we demonstrate consistent within-individual declines in home range area with age in adult females. In both systems, we found that within-individual decreases in home range area were associated with increased risk of mortality the following year. Our results provide the first evidence from the wild that age-related changes in space use are predictive of adult mortality.

Longitudinal demographic study of wild populations of African annual killifish

The natural history of model organisms is often overlooked despite its importance to correctly interpret the outcome of laboratory studies. Ageing is particularly understudied in natural populations. To address this gap, we present lifetime demographic data from wild populations of an annual species, the turquoise killifish, Nothobranchius furzeri, a model species in ageing research, and two other species of coexisting annual killifishes. Annual killifish hatch synchronously, have non-overlapping generations, and reproduce daily after reaching sexual maturity. Data from 13 isolated savanna pools in southern Mozambique demonstrate that the pools supporting killifish populations desiccated 1–4 months after their filling, though some pools persisted longer. Declines in population size over the season were stronger than predicted, because they exceeded the effect of steady habitat shrinking on population density that, contrary to the prediction, decreased. Populations of N. furzeri also became more female-biased with progressing season suggesting that males had lower survival. Nothobranchius community composition did not significantly vary across the season. Our data clearly demonstrate that natural populations of N. furzeri and its congeners suffer strong mortality throughout their lives, with apparent selective disappearance (condition-dependent mortality) at the individual level. This represents selective force that can shape the evolution of lifespan, and its variation across populations, beyond the effects of the gradient in habitat persistence.

Postreproductive lifespans are rare in mammals

A species has a post‐reproductive stage if, like humans, a female entering the adult population can expect to live a substantial proportion of their life after their last reproductive event. However, it is conceptually and statistically challenging to distinguish these true post‐reproductive stages from the usual processes of senescence, which can result in females occasionally surviving past their last reproductive event. Hence, despite considerable interest, the taxonomic prevalence of post‐reproductive stages remains unclear and debated. In this study we use life tables constructed from published data on wild populations of mammals, and statistical measures of post‐reproductive lifespans, to distinguish true post‐reproductive stages from artefacts of senescence and demography in 52 species. We find post‐reproductive stages are rare in mammals and are limited to humans and a few species of toothed whales. By resolving this long‐standing debate, we hope to provide clarity for researchers in the field of evolutionary biology and a solid foundation for further studies investigating the evolution and adaptive significance of this unusual life history trait.