Female reproductive aging in seven primate species: Patterns and consequences

Female reproductive ageing persists despite high infanticide risk in chacma baboons and geladas

Across mammals, fertility and offspring survival are often lowest at the beginning and end of females' reproductive careers. However, extrinsic drivers of reproductive success-including infanticide by males-could stochastically obscure these expected age-related trends. Here, we modelled reproductive ageing trajectories in two cercopithecine primates that experience high rates of male infanticide: the chacma baboon (Papio ursinus) and the gelada (Theropithecus gelada). We found that middle-aged mothers generally achieved the shortest interbirth intervals in chacma baboons. By contrast, old gelada females often showed shorter interbirth intervals than their younger group-mates with one exception: the oldest females typically failed to produce additional offspring before their deaths. Infant survival peaked in middle-aged mothers in chacma baboons but in young mothers in geladas. While infant mortality linked with maternal death increased as mothers aged in both species, infanticide risk did not predictably shift with maternal age. Thus, infanticide patterns cannot explain the surprising young mother advantage observed in geladas. Instead, we argue that this could be a product of their graminivorous diets, which might remove some energetic constraints on early reproduction. In sum, our data suggest that reproductive ageing is widespread but may be differentially shaped by ecological pressures.

Wild capuchin monkeys as a model system for investigating the social and ecological determinants of ageing

Studying biological ageing in animal models can circumvent some of the confounds exhibited by studies of human ageing. Ageing research in non-human primates has provided invaluable insights into human lifespan and healthspan. Yet data on patterns of ageing from wild primates remain relatively scarce, centred around a few populations of catarrhine species. Here, we introduce the white-faced capuchin, a long-lived platyrrhine primate, as a promising new model system for ageing research. Like humans, capuchins are highly social, omnivorous generalists, whose healthspan and lifespan relative to body size exceed that of other non-human primate model species. We review recent insights from capuchin ageing biology and outline our expanding, integrative research programme that combines metrics of the social and physical environments with physical, physiological and molecular hallmarks of ageing across the natural life courses of multiple longitudinally tracked individuals. By increasing the taxonomic breadth of well-studied primate ageing models, we generate new insights, increase the comparative value of existing datasets to geroscience and work towards the collective goal of developing accurate, non-invasive and reliable biomarkers with high potential for standardization across field sites and species, enhancing the translatability of primate studies.This article is part of the discussion meeting issue 'Understanding age and society using natural populations'.

Diagnosis of Menopause in a Captive Sumatran Orangutan (Pongo abelii)

Humans were long thought to be the only mammal to experience menopause, the permanent cessation of reproduction followed by a long post-reproductive lifespan. More recently, evidence has been found for the existence of menopause in other long-lived mammals, including chimpanzees and gorillas. However, orangutans, which have the longest interbirth interval of any primate, have rarely been studied in this period of their lives. In this paper, we describe clinical, ultrasound, endocrine, and histological evidence consistent with a natural menopause in a captive, previously fertile, Sumatran orangutan (Pongo abelii), aged approximately 50. Consecutive serum samples showed low levels of estradiol and high levels of follicle-stimulating hormone. Transvaginal ultrasound revealed an atrophic uterus with an antero-posterior diameter of 2.36 cm, an endometrial thickness of 2 mm, and inactive ovaries. Following this female's death from a subdural hematoma, histological examination of the ovaries showed a dense stroma with corpora albicantia, in comparison to the numerous primordial follicles seen in the ovaries of a stillborn infant female orangutan. These multiple lines of evidence suggest that Sumatran orangutans can now be added to the list of mammals which undergo a true menopause, which may ensure that females' final offspring can be reared to independence.

Female Mate Choice in Wild Kenyan Blue Monkeys (Cercopithecus mitis)

Female mate choice may drive sexual selection, but discerning whether female behaviors reflect free expression of choice or responses to constraints can be difficult. We investigated the efficacy of female choice in wild blue monkeys using 10 years of behavior and paternity data (N = 178 male-female dyads). Although blue monkeys live modally in one-male polygynous groups, where male-biased intersexual power is expected, females can access multiple potential mates during seasonal male influxes and occasional intergroup encounters. Additionally, extra-group males sire offspring. We examined female resistance rates to male-initiated sexual interactions, and unsolicited proceptive behavior that females directed to males (corrected for male availability). Females seldom resisted male solicitation, but initiated sexual interactions more than males. Females generally preferred residents. Those who preferred non-residents tended to have residents with longer tenures, but neither female parity nor rank influenced the tendency to prefer non-residents vs. residents. The male most solicited by a particular female fathered that female's infant 82% of the time; odds of siring were 26 times higher for most vs. nonpreferred males. Female preference predicted paternity even more strongly among non-resident males, with odds of siring 33 times higher for most vs. nonpreferred non-residents. Neither female rank nor parity influenced her likelihood of having her preferred partner as sire. Paternity by preferred males did not affect infant survival. While we cannot fully discount the effect of male-male competition on paternity, these results suggest that blue monkey females can exercise choice successfully, even in a polygynous mating system.

Infant survival is significantly impacted by dam- and management-related factors in zoo-managed Eulemur populations

Due to their potential impact on population growth, many studies have investigated factors affecting infant survival in mammal populations under human care. Here we used more than 30 years of Association of Zoos and Aquariums (AZA) studbook data and contraception data from the AZA Reproductive Management Center, along with logistic regression models, to investigate which factors affect infant survival in four Eulemur species managed as Species Survival Plans® in AZA. Across species, infant survival to 1 month ranged from 65% to 78%. Previous experience producing surviving offspring was positively correlated to infant survival in collared (Eulemur collaris), crowned (Eulemur coronatus), and mongoose (Eulemur mongoz) lemurs. Both dam age and previous use of contraception were negatively correlated to infant survival for collared lemurs, though our results suggest the latter may be confounded with other factors. Blue-eyed black lemurs (Eulemur flavifrons) were affected by birth location, suggesting differences in husbandry that may affect infant survival. These results can be used to assist in reproductive planning or to anticipate the likelihood of breeding success. Population managers may also be able to focus their reproductive planning on younger dams or those with previous experience to predict successful births. Future studies should seek to determine what aspects of previous dam success are most important to infant survival, investigate sire-related factors, and examine factors related to cause of death in infants that may lead to differential survival. Our hope is to present a framework that may be useful for investigating infant survival in other mammal species' breeding programs.

Sniffing behavior of semi free-ranging Barbary macaques (Macaca sylvanus)

Olfaction is one of the evolutionarily oldest senses and plays a fundamental role in foraging and social interactions across mammals. In primates, the role of olfaction is now well recognized, but better investigated in strepsirrhine and platyrrhine primates than in catarrhines. We observed the sniffing behavior of semi-free ranging Barbary macaques, Macaca sylvanus, at Affenberg Salem, Germany, to assess how frequently macaques sniff and in which contexts, and how sniffing is affected by sex and age. Focal observations of 24 males and 24 females aged 1-25 years showed that Barbary macaques sniffed, on average, 5.24 times per hour, with more than 80% of sniffs directed at food. Irrespective of the context, younger individuals sniffed more often than older ones. Females' sniffs were more often directed at food than male sniffs, while males sniffed more often than females in a social context. Sniffs at conspecifics occurred primarily in a sexual context, with 70% of social sniffs directed at female anogenital swellings performed by males. Of the observed 176 anogenital inspections, 51 involved sniffing of the swelling. Olfactory inspections were followed by copulation significantly less often than merely visual inspections, suggesting that anogenital odors may play a role in male mating decisions, but the role of olfaction in sexual interactions warrants further investigations. In sum, results show that Barbary macaques routinely use olfaction during feeding, but also in a socio-sexual context, corroborating the relevance of the olfactory sense in the lives of catarrhine primates.

Transcriptomic Profiling of Reproductive Age Marmoset Monkey Ovaries

The decline in ovarian reserve and the aging of the ovaries is a significant concern for women, particularly in the context of delayed reproduction. However, there are ethical limitations and challenges associated with conducting long-term studies to understand and manipulate the mechanisms that regulate ovarian aging in human. The marmoset monkey offers several advantages as a reproductive model, including a shorter gestation period and similar reproductive physiology to that of human. Additionally, they have a relatively long lifespan compared to other mammals, making them suitable for long-term studies. In this study, we focused on analyzing the structural characteristics of the marmoset ovary and studying the mRNA expression of 244 genes associated with ovarian aging. We obtained ovaries from marmosets at three different reproductive stages: pre-pubertal (1.5 months), reproductive (82 months), and menopausal (106 months) ovaries. The structural analyses revealed the presence of numerous mitochondria and lipid droplets in the marmoset ovaries. Many of the genes expressed in the ovaries were involved in multicellular organism development and transcriptional regulation. Additionally, we identified the expression of protein-binding genes. Within the expressed genes, VEGFA and MMP9 were found to be critical for regulating ovarian reserve. An intriguing finding of the study was the strong correlation between genes associated with female infertility and genes related to fibrosis and wound healing. The authors suggest that this correlation might be a result of the repeated rupture and subsequent healing processes occurring in the ovary due to the menstrual cycle, potentially leading to the indirect onset of fibrosis. The expression profile of ovarian aging-related gene set in the marmoset monkey ovaries highlight the need for further studies to explore the relationship between fibrosis, wound healing, and ovarian aging.

Early life drought predicts components of adult body size in wild female baboons

OBJECTIVES

In many taxa, adverse early-life environments are associated with reduced growth and smaller body size in adulthood. However, in wild primates, we know very little about whether, where, and to what degree trajectories are influenced by early adversity, or which types of early adversity matter most. Here, we use parallel-laser photogrammetry to assess inter-individual predictors of three measures of body size (leg length, forearm length, and shoulder-rump length) in a population of wild female baboons studied since birth.

MATERIALS AND METHODS

Using >2000 photogrammetric measurements of 127 females, we present a cross-sectional growth curve of wild female baboons (Papio cynocephalus) from juvenescence through adulthood. We then test whether females exposed to several important sources of early-life adversity-drought, maternal loss, low maternal rank, or a cumulative measure of adversity-were smaller for their age than females who experienced less adversity. Using the "animal model," we also test whether body size is heritable in this study population.

RESULTS

Prolonged early-life drought predicted shorter limbs but not shorter torsos (i.e., shoulder-rump lengths). Our other measures of early-life adversity did not predict variation in body size. Heritability estimates for body size measures were 36%-67%. Maternal effects accounted for 13%-17% of the variance in leg and forearm length, but no variance in torso length.

DISCUSSION

Our results suggest that baboon limbs, but not torsos, grow plastically in response to maternal effects and energetic early-life stress. Our results also reveal considerable heritability for all three body size measures in this study population.

Socially bonded females face more sexual coercion in a female-philopatric primate

Sexual coercion is a manifestation of sexual conflict increasing male mating success while inflicting costs to females. Although previous work has examined inter-individual variation in male sexually coercive tactics, little is known about female counter-strategies. We investigated whether social bonding mitigates the extent of sexual coercion faced by female mandrills (Mandrillus sphinx), as a putative mechanism linking sociality to fitness. Surprisingly, females faced the most coercion from those males with whom they formed the strongest bonds, while the strength of a female-male bond was also positively correlated with coercion from all other males. Finally, greater social integration in the female network was positively correlated with coercion, through a direct 'public exposure' mechanism and not mediated by female reproductive success or retaliation potential. Altogether, this study shows that neither between- nor within-sex bonds are protective against sexual coercion and identifies, instead, a hidden cost of social bonding.

Correlation between the presence of a cecal appendix and reduced diarrhea severity in primates: new insights into the presumed function of the appendix

Increased severity or recurrence risk of some specific infectious diarrhea, such a salmonellosis or Clostridium difficile colitis, have been reported after an appendectomy in human patients. While several other mammals also possess an appendix, the suspected protective function against diarrhea conferred by this structure is known only in humans. From a retrospective collection of veterinary records of 1251 primates attributed to 45 species, including 13 species with an appendix and 32 without, we identified 2855 episodes of diarrhea, 13% of which were classified as severe diarrhea requiring a therapeutic medication or associated with a fatal issue. We identified a lower risk of severe diarrhea among primate species with an appendix, especially in the early part of life when the risk of diarrhea is maximal. Moreover, we observed a delayed onset of diarrhea and of severe diarrhea in species possessing an appendix. Interestingly, none of the primates with an appendix were diagnosed, treated or died of an acute appendicitis during the 20 years of veterinarian follow-up. These results clarify the function of the appendix among primates, as protection against diarrhea. This supports its presumed function in humans and is congruent with the existence of a selective advantage conferred by this structure.

Meager Milk: Lasting Consequences for Adult Daughters of Primiparous Mothers Among Rhesus Macaques (Macaca mulatta)

Among mammals, primipara who initiate reproduction before full maturity can be constrained in their maternal investment, both due to fewer somatic resources and tradeoffs between their own continued development and reproductive effort. Primipara are particularly limited in their capacity to synthesize milk during lactation, the costliest aspect of reproduction for most mammals, especially primates due to long periods of postnatal development. Due to reduced milk transfer, Firstborns may be at elevated risk for long-term consequences of deficits in early life endowment from their primiparous mothers. Here we investigated mass, growth, stature, and lactation performance among N = 273 adult daughters across N = 335 reproductions, who were their own mother's Firstborn or Laterborn progeny, among rhesus macaques (Macaca mulatta) at the California National Primate Research Center. We further explored mass during infancy of the offspring of Firstborn and Laterborn mothers. Firstborns had accelerated growth during infancy, but had slowed growth during juvenility, compared to Laterborns. Although both Firstborns and Laterborns were the same age at reproductive debut, Firstborns had lower body mass, an effect that persisted throughout the reproductive career. Available milk energy, the product of milk energetic density and milk yield, was on average 16% lower for Firstborns compared to Laterborns, a difference that was only partially mediated by their lower mass. Despite differences in their mothers' energy provision through milk, the mass of infants of Firstborn and Laterborn mothers did not differ at peak lactation, suggesting that infants of Firstborns devote a higher proportion of milk energy to growth than infants of Laterborns. To date few studies have explored how early life conditions shape capacities to synthesize milk and milk composition. Our findings contribute new information among primates on how early life maternal endowments are associated with persistent effects long after the period of maternal dependence well into reproductive maturity.

Cumulative early-life adversity does not predict reduced adult longevity in wild gorillas

Extensive research across fields has repeatedly confirmed that early-life adversity (ELA) is a major selective force for many taxa, in part via its ties to adult health and longevity.1,2,3 Negative effects of ELA on adult outcomes have been documented in a wide range of species, from fish to birds to humans.4 We used 55 years of long-term data collected on 253 wild mountain gorillas to examine the effects of six putative sources of ELA on survival, both individually and cumulatively. Although cumulative ELA was associated with high mortality in early life, we found no evidence that it had detrimental consequences for survival later in life. Experiencing three or more forms of ELA was associated with greater longevity, with a 70% reduction in the risk of death across adulthood, driven specifically by greater longevity in males. Although this higher survival in later life is likely a consequence of sex-specific viability selection5 during early life due to the immediate mortality consequences of adverse experiences, patterns in our data also suggest that gorillas have significant resilience to ELA. Our findings demonstrate that the detrimental consequences of ELA on later life survival are not universal, and indeed largely absent in one of humans' closest living relatives. This raises important questions about the biological roots of sensitivity to early experiences and the protective mechanisms that contribute to resiliency in gorillas, which could be critical for understanding how best to encourage similar resiliency to early-life shocks in humans.

Ecology and age, but not genetic ancestry, predict fetal loss in a wild baboon hybrid zone

OBJECTIVES

Pregnancy failure represents a major fitness cost for any mammal, particularly those with slow life histories such as primates. Here, we quantified the risk of fetal loss in wild hybrid baboons, including genetic, ecological, and demographic sources of variance. We were particularly interested in testing the hypothesis that hybridization increases fetal loss rates. Such an effect would help explain how baboons may maintain genetic and phenotypic integrity despite interspecific gene flow.

MATERIALS AND METHODS

We analyzed outcomes for 1020 pregnancies observed over 46 years in a natural yellow baboon-anubis baboon hybrid zone. Fetal losses and live births were scored based on records of female reproductive state and the appearance of live neonates. We modeled the probability of fetal loss as a function of a female's genetic ancestry (the proportion of her genome estimated to be descended from anubis [vs. yellow] ancestors), age, number of previous fetal losses, dominance rank, group size, climate, and habitat quality using binomial mixed effects models.

RESULTS

Female genetic ancestry did not predict fetal loss. Instead, the risk of fetal loss is elevated for very young and very old females. Fetal loss is most robustly predicted by ecological factors, including poor habitat quality prior to a home range shift and extreme heat during pregnancy.

DISCUSSION

Our results suggest that gene flow between yellow and anubis baboons is not impeded by an increased risk of fetal loss for hybrid females. Instead, ecological conditions and female age are key determinants of this component of female reproductive success.

Timeless or tainted? The effects of male ageing on seminal fluid

Reproductive ageing can occur due to the deterioration of both the soma and germline. In males, it has mostly been studied with respect to age-related changes in sperm. However, the somatic component of the ejaculate, seminal fluid, is also essential for maintaining reproductive function. Whilst we know that seminal fluid proteins (SFPs) are required for male reproductive success across diverse taxa, age-related changes in SFP quantity and composition are little understood. Additionally, only few studies have explored the reproductive ageing of the tissues that produce SFPs, and the resulting reproductive outcomes. Here we provide a systematic review of studies addressing how advancing male age affects the production and properties of seminal fluid, in particular SFPs and oxidative stress, highlighting many open questions and generating new hypotheses for further research. We additionally discuss how declines in function of different components of seminal fluid, such as SFPs and antioxidants, could contribute to age-related loss of reproductive ability. Overall, we find evidence that ageing results in increased oxidative stress in seminal fluid and a decrease in the abundance of various SFPs. These results suggest that seminal fluid contributes towards important age-related changes influencing male reproduction. Thus, it is essential to study this mostly ignored component of the ejaculate to understand male reproductive ageing, and its consequences for sexual selection and paternal age effects on offspring.

Reproductive Senescence in Two Lemur Lineages

The relationship between age and reproductive performance is highly variable across species. Humans and some cetaceans exhibit an extreme form of reproductive senescence in that female reproduction ceases years or even decades before average life expectancy is reached. However, neither the existence of reproductive senescence in some taxa nor its absence in others is fully understood. Comparative data from other long-lived mammals may contribute to a more comprehensive understanding of the evolution of menopause, but data from wild primates, in particular, are scarce. We therefore investigated age-related female reproductive performance in two wild sympatric populations of Malagasy primates: Verreaux’s sifakas (Propithecus verreauxi) and redfronted lemurs (Eulemur rufifrons), which have a maximal longevity of more than 20 years. Based on 25 years of long-term demographic data, we extracted information on reproductive output of 38 female Verreaux’s sifakas and 42 female redfronted lemurs. We modeled variation in female reproductive performance and interbirth intervals as a function of age, the number of adult females within a group to account for female competition, and rainfall as a proxy for annual variation in food availability. We also compared our results for these two species with data on captive populations of the same two genera that are buffered from fluctuations in environmental variables. Our analyses disclosed statistical evidence for reproductive senescence in three out of four populations (captive Coquerel’s sifakas, wild redfronted lemurs, and captive red lemurs) but not for wild Verreaux’s sifakas. Compared to wild populations, reproductive senescence was therefore not less pronounced in captive animals, even though the latter are buffered from environmental adversities. In wild redfronted lemurs, mothers were more likely to give birth in years with more rainfall, but neither the number of co-resident females, nor annual rainfall did predict variation in the probability of giving birth in wild Verreaux’s sifakas. Thus, our study contributes valuable comparative information on reproductive senescence in a basal group of primates, and offers insights into the modulating effects of environmental, social and phylogenetic factors on patterns and dynamics of age-specific female reproduction.

My path to primatology: some stories from the field

In this paper, I summarize the major facets of my 50-year career as a primatologist. I briefly describe the aspects of my upbringing and early education that led me to the study of primate behavior, first in captive settings and, later, in the wild. My research on the Arashiyama West Japanese macaques and my interactions with Japanese primatologists was a formative stage in my career, and I present the background of this international project and how it led to my growing focus on female life history studies. After a couple of failed attempts to establish a long-term study of primates in their native habitats, I began the Santa Rosa Primate Project in Costa Rica in 1983, which focuses mainly on white-faced capuchins, and to some extent on howlers and spider monkeys. The Santa Rosa project has expanded over the past four decades and continues to this day, with the participation of a large team of colleagues, local field assistants and students. I present some of the major findings of our Santa Rosa monkey research in the areas of female reproduction, sexual conflict and conservation of primates in a regenerating tropical dry forest. I also briefly describe how and why I came to develop a sideline of research on gender and science.