Age-related patterns of reproductive success among female mountain gorillas

Multiple mechanisms for inbreeding avoidance used simultaneously in a wild ape

Mating with close kin can have considerable negative fitness consequences, which are expected to result in selective pressure for inbreeding avoidance mechanisms, such as dispersal, mate choice and post-copulatory biases. Captive studies have suggested that inbreeding avoidance through mate choice is far less widespread than expected and may be absent where other mechanisms already limit inbreeding. However, few studies have examined multiple mechanisms of inbreeding avoidance simultaneously, particularly in the wild. We use 13 years of detailed dispersal, copulation and paternity data from mountain gorillas to examine inbreeding avoidance. We find that partial dispersal of both sexes results in high kinship in multimale groups, but that copulations between close kin occur 40% less than expected. We find strong kin discrimination in mate choice, with significant avoidance of maternal kin but more limited avoidance of paternal kin. We find no evidence for post-copulatory inbreeding avoidance. Our analyses support familiarity-based mechanisms of kin identification and age-based avoidance that limits mating between fathers and daughters in their natal group. Our findings demonstrate that multiple complementary mechanisms for inbreeding avoidance can evolve in a single species and suggest that inbreeding avoidance through mate choice may enable more flexible dispersal systems to evolve.

The promise of great apes as model organisms for understanding the downstream consequences of early life experiences

Early life experiences have a significant influence on adult health and aging processes in humans. Despite widespread interest in the evolutionary roots of this phenomenon, very little research on this topic has been conducted in humans' closest living relatives, the great apes. The longitudinal data sets that are now available on wild and captive great ape populations hold great promise to clarify the nature, evolutionary function, and mechanisms underlying these connections in species which share key human life history characteristics. Here, we explain features of great ape life history and socioecologies that make them of particular interest for this topic, as well as those that may limit their utility as comparative models; outline the ways in which available data are complementary to and extend the kinds of data that are available for humans; and review what is currently known about the connections among early life experiences, social behavior, and adult physiology and biological fitness in our closest living relatives. We conclude by highlighting key next steps for this emerging area of research.

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.

Population dynamics of western gorillas at Mbeli Bai

Long-term studies of population dynamics can provide insights into life history theory, population ecology, socioecology, conservation biology and wildlife management. Here we examine 25 years of population dynamics of western gorillas at Mbeli Bai, a swampy forest clearing in Nouabalé-Ndoki National Park, the Republic of Congo. The Mbeli population more than doubled from 101 to 226 gorillas during the study. After adjusting for a net influx of gorillas into the study population, the increase represents an inherent growth rate of 0.7% per year, with 95% confidence limits between -0.7% and 2.6%. The influx of gorillas mainly involved immigration of individuals into existing study groups (social dispersal), but it also included the appearance of a few previously unknown groups (locational dispersal). The average group size did not change significantly during the study, which is consistent with the possibility that western gorillas face socioecological constraints on group size, even when the population is increasing. We found no significant evidence of density dependence on female reproductive success or male mating competition. The distribution of gorillas among age/sex categories also did not change significantly, which suggests that the population had a stable age structure. Our results provide evidence of population stability or growth for some western gorillas (albeit within a small area). The results highlight the value of law enforcement, long-term monitoring, and protected areas; but they do not diminish the importance of improving conservation for this critically endangered species.

Effects of maternal age and offspring sex on milk yield, composition and calf growth of red deer (Cervus elaphus)

Differential maternal allocation theory states that mothers will invest more heavily in the offspring sex that will secure higher reproductive output. Senescence theory is concerned with the gradual deterioration of physiological function with age. We analysed the offspring sex-dependent response of calf growth and milk traits to mother age in an Iberian population of captive red deer (Cervus elaphus) using a 22 year time series longitudinal data set. Previous studies revealed that there was little evidence for the differential allocation theory on milk traits and that most studies lacked proper control for confounding factors. Our results indicated that (i) calf growth was offspring male-biased, negatively affected by mother age and positively influenced by mother weight and parity, and (ii) there was no support for differential allocation offspring sex-dependence in milk traits (yield, energy density, fat, protein and lactose content). Our findings suggest that maternal allocation responds to offspring energy requirements, which are mainly driven by offspring body weight, and contingent on mother age and weight and previous maternal reproductive effort.

Effects of infant age and sex, and maternal parity on the interaction of lactation with infant feeding development in chimpanzees

The interaction between infant feeding and maternal lactational physiology influences female inter-birth intervals and mediates maternal reproductive trade-offs. We investigated variation in feeding development in 72 immature wild chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda, and made inferences about maternal lactation over the course of infancy. We compared the percentage (%) of time that mothers nursed infants as a function of infant age and assessed how hourly rates and bout durations of nursing and foraging varied in association with differences in offspring age, sex, and maternal parity. Nursing % times, rates and durations were highest for infants ≤ 6 months old but did not change significantly from 6 months to 5 years old. Nursing continued at a decreasing rate for some 5- to 7-year-olds. Infants ≤ 6 months old foraged little. Foraging rates did not change after 1 year old, but foraging durations and the % time devoted to foraging increased with age. Independent foraging probably became a dietary requirement for infants at 1 year old, when their energy needs may have surpassed the available milk energy. Infants spent as much time foraging by the time they were 4 to 5 years old as adults did. No sex effect on infant nursing or foraging was apparent, but infants of primiparous females had higher foraging rates and spent more time foraging than the infants of multiparous females did. Although no data on milk composition were collected, these findings are consistent with a working hypothesis that like other hominoids, chimpanzee mothers maintained a fixed level of lactation effort over several years as infants increasingly supplemented their growing energy, micronutrient and hydration needs via independent foraging. Plateauing lactation may be a more widespread adaptation that allows hominoid infants time to attain the physiology and skills necessary for independent feeding, while also providing them with a steady dietary base on which they could rely consistently through infancy, and enabling mothers to maintain a fixed, predictable level of lactation effort.

Female reproductive aging in seven primate species: Patterns and consequences

Age-related changes in fertility have increasingly been documented in wild animal populations: In many species the youngest and oldest reproducers are disadvantaged relative to prime adults. How do these effects evolve, and what explains their diversity across species? Tackling this question requires detailed data on patterns of age-related reproductive performance in multiple animal species. Here, we compare patterns and consequences of age-related changes in female reproductive performance in seven primate populations that have been subjects of long-term continuous study for 29 to 57 y. We document evidence of age effects on fertility and on offspring performance in most, but not all, of these primate species. Specifically, females of six species showed longer interbirth intervals in the oldest age classes, youngest age classes, or both, and the oldest females also showed relatively fewer completed interbirth intervals. In addition, five species showed markedly lower survival among offspring born to the oldest mothers, and two species showed reduced survival for offspring born to both the youngest and the oldest mothers. In contrast, we found mixed evidence that maternal age affects the age at which daughters first reproduce: Only in muriquis and to some extent in chimpanzees, the only two species with female-biased dispersal, did relatively young mothers produce daughters that tended to have earlier first reproduction. Our findings demonstrate shared patterns as well as contrasts in age-related changes in female fertility across species of nonhuman primates and highlight species-specific behavior and life-history patterns as possible explanations for species-level differences.

The Contributions of Maternal Age Heterogeneity to Variance in Lifetime Reproductive Output

AbstractVariance among individuals in fitness components reflects both genuine heterogeneity between individuals and stochasticity in events experienced along the life cycle. Maternal age represents a form of heterogeneity that affects both the mean and the variance of lifetime reproductive output (LRO). Here, we quantify the relative contribution of maternal age heterogeneity to the variance in LRO using individual-level laboratory data on the rotifer Brachionus manjavacas to parameterize a multistate age × maternal age matrix model. In B. manjavacas, advanced maternal age has large negative effects on offspring survival and fertility. We used multistate Markov chains with rewards to quantify the contributions to variance in LRO of heterogeneity and of the stochasticity inherent in the outcomes of probabilistic transitions and reproductive events. Under laboratory conditions, maternal age heterogeneity contributes 26% of the variance in LRO. The contribution changes when mortality and fertility are reduced to mimic more ecologically relevant environments. Over the parameter space where populations are near stationarity, maternal age heterogeneity contributes an average of 3% of the variance. Thus, the contributions of maternal age heterogeneity and individual stochasticity can be expected to depend strongly on environmental conditions; over most of the parameter space, the variance in LRO is dominated by stochasticity.

Variability of weaning age in mountain gorillas (Gorilla beringei beringei)

OBJECTIVES

Weaning is a key life history milestone for mammals that represents both the end of nutritional investment from the perspective of mothers and the start of complete nutritional independence for the infants. The age at weaning may vary depending on ecological, social, and demographic factors experienced by the mother and infant. Bwindi mountain gorillas live in different environmental conditions and have longer interbirth intervals than their counterparts in the Virunga Volcanoes, yet other life history characteristics of this population remain less well known. We use long-term data from Bwindi Impenetrable National Park, Uganda to examine factors related to weaning age.

MATERIALS AND METHODS

We analyzed data on infants born in four mountain gorilla groups in Bwindi to quantify their age of weaning (defined as last nipple contact) and to test if the sex of offspring, parity, and dominance rank of mother influences age of weaning. We also compared the age at weaning and time to conception after resumption of mating in Bwindi and Virunga gorillas.

RESULTS

Bwindi gorillas were weaned at an average age of 57.5 months. No difference was found between age of weaning for primiparous and multiparous mothers, nor did maternal dominance rank influence age of weaning, but sons were weaned at a later age than daughters. The majority of Bwindi mothers were still suckling when they resumed mating and mothers generally conceived before they weaned their previous offspring. The age of weaning was significantly later in Bwindi than in Virunga gorillas. After mothers resumed mating, the time to conceiving the next offspring was not significantly longer for Bwindi females than Virungas females (6 vs. 4 months).

DISCUSSION

Later weaning age for sons than daughters is similar to findings of other studies of great apes. Bwindi mountain gorillas are weaned at approximately the same age as western gorillas and chimpanzees, which is more than a year later than Virunga mountain gorillas. The results of this study suggest that variation in ecological conditions of populations living in close geographic proximity can result in variation in life history patterns, which has implications for understanding the evolution of the unique life history patterns of humans.

Skeletal ageing in Virunga mountain gorillas

Bone loss and heightened fracture risk are common conditions associated with ageing in modern human populations and have been attributed to both hormonal and other metabolic and behavioural changes. To what extent these age-related trends are specific to modern humans or generally characteristic of natural populations of other taxa is not clear. In this study, we use computed tomography to examine age changes in long bone and vertebral structural properties of 34 wild-adult Virunga mountain gorillas (Gorilla beringei beringei) whose skeletons were recovered from natural accumulations. Chronological ages were known or estimated from sample-specific dental wear formulae and ranged between 11 and 43 years. Gorillas show some of the same characteristics of skeletal ageing as modern humans, including endosteal and some periosteal expansion. However, unlike in humans, there is no decline in cortical or trabecular bone density, or in combined geometric-density measures of strength, nor do females show accelerated bone loss later in life. We attribute these differences to the lack of an extended post-reproductive period in gorillas, which provides protection against bone resorption. Increases in age-related fractures (osteoporosis) in modern humans may be a combined effect of an extended lifespan and lower activity levels earlier in life. This article is part of the theme issue 'Evolution of the primate ageing process'.

Why Males Compete Rather Than Care, with an Application to Supplying Collective Goods

The question of why males invest more into competition than offspring care is an age-old problem in evolutionary biology. On the one hand, paternal care could increase the fraction of offspring surviving to maturity. On the other hand, competition could increase the likelihood of more paternities and thus the relative number of offspring produced. While drivers of these behaviours are often intertwined with a wide range of other constraints, here we present a simple dynamic model to investigate the benefits of these two alternative fitness-enhancing pathways. Using this framework, we evaluate the sensitivity of equilibrium dynamics to changes in payoffs for male allocation to mating versus parenting. Even with strong effects of care on offspring survivorship, small competitive benefits can outweigh benefits from care. We consider an application of the model that includes men's competition for hunting reputations where big game supplies a benefit to all and find a frequency-dependent parameter region within which, depending on initial population proportions, either strategy may outperform the other. Results demonstrate that allocation to competition gives males greater fitness than offspring care for a range of circumstances that are dependent on life-history parameters and, for the large-game hunting application, frequency dependent. The greater the collective benefit, the more individuals can be selected to supply it.

Comparing measures of social complexity: larger mountain gorilla groups do not have a greater diversity of relationships

Social complexity reflects the intricate patterns of social interactions in societies. Understanding social complexity is fundamental for studying the evolution of diverse social systems and the cognitive innovations used to cope with the demands of social life. Social complexity has been predominantly quantified by social unit size, but newer measures of social complexity reflect the diversity of relationships. However, the association between these two sets of measures remains unclear. We used 12 years of data on 13 gorilla groups to investigate how measures of social complexity relate to each other. We found that group size was a poor proxy for relationship diversity and that the social complexity individuals experienced within the same group varied greatly. Our findings demonstrate two fundamental takeaways: first, that the number of relationships and the diversity of those relationships represent separate components of social complexity, both of which should be accounted for; and second, that social complexity measured at the group level may not represent the social complexity experienced by individuals in those groups. These findings suggest that comprehensive studies of social complexity, particularly those relating to the social demands faced by individuals, may require fine-scale social data to allow accurate comparisons across populations and species.

Dominance rank but not body size influences female reproductive success in mountain gorillas

According to life history theory, natural selection has shaped trade-offs for allocating energy among growth, reproduction and maintenance to maximize individual fitness. In social mammals body size and dominance rank are two key variables believed to influence female reproductive success. However, few studies have examined these variables together, particularly in long-lived species. Previous studies found that female dominance rank correlates with reproductive success in mountain gorillas (Gorilla beringei beringei), which is surprising given they have weak dominance relationships and experience seemingly low levels of feeding competition. It is not currently known whether this relationship is primarily driven by a positive correlation between rank and body size. We used the non-invasive parallel laser method to measure two body size variables (back breadth and body length) of 34 wild adult female mountain gorillas, together with long-term dominance and demography data to investigate the interrelationships among body size, dominance rank and two measures of female reproductive success (inter-birth interval N = 29 and infant mortality N = 64). Using linear mixed models, we found no support for body size to be significantly correlated with dominance rank or female reproductive success. Higher-ranking females had significantly shorter inter-birth intervals than lower-ranking ones, but dominance rank was not significantly correlated with infant mortality. Our results suggest that female dominance rank is primarily determined by factors other than linear body dimensions and that high rank provides benefits even in species with weak dominance relationships and abundant year-round food resources. Future studies should focus on the mechanisms behind heterogeneity in female body size in relation to trade-offs in allocating energy to growth, maintenance and lifetime reproductive success.

On age-specific selection and extensive lifespan beyond menopause

Standard evolutionary theory of ageing predicts weaker purifying selection on genes critical to later life stages. Prolonged post-reproductive lifespan (PPRLS), observed only in a few species like humans, is likely a result of disparate relaxation of purifying selection on survival and reproduction in late life stages. While the exact origin of PPRLS is under debate, many researchers agree on hypotheses like mother-care and grandmother-care, which ascribe PPRLS to investment into future generations-provision to one's descendants to enhance their overall reproductive success. Here, we simulate an agent-based model, which properly accounts for age-specific selection, to examine how different investment strategies affect the strength of purifying selection on survival and reproduction. We observed in the simulations that investment strategies that allow a female individual to remain contributive to its own descendants (infants and adults) at late life stages may lead to differential relaxation of selection on survival and reproduction, and incur the adaptive evolution of PPRLS.

Maternal age at birth shapes offspring life-history trajectory across generations in long-lived Asian elephants

Advanced maternal age at birth can have pronounced consequences for offspring health, survival and reproduction. If carried over to the next generation, such fitness effects could have important implications for population dynamics and the evolution of ageing, but these remain poorly understood. While many laboratory studies have investigated maternal age effects, relatively few studies have been conducted in natural populations, and they usually only present a "snapshot" of an offspring's lifetime. In the present study, we focus on how maternal age influences offspring life-history trajectories and performance in a long-lived mammal. We use a multigenerational demographic dataset of semi-captive Asian elephants to investigate maternal age effects on several offspring life-history traits: condition, reproductive success and overall survival. We show that offspring born to older mothers display reduced overall survival but higher reproductive success, and reduced survival of their own progeny. Our results show evidence of a persistent effect of maternal age on fitness across generations in a long-lived mammal. By highlighting transgenerational effects on the fitness of the next generation associated with maternal age, the present study helps increase our understanding of factors contributing to individual variation in ageing rates and fitness.

Sources of variation in weaned age among wild chimpanzees in Gombe National Park, Tanzania

OBJECTIVES

A key feature of human life history evolution is that modern humans wean their infants 2-4 years earlier on average than African apes. However, our understanding of weaning variation in apes remains limited. Here we provide the first such report in chimpanzees by examining weaned age variation using long-term data from Gombe National Park, Tanzania.

MATERIAL AND METHODS

We analyzed 41 years of observational behavioral data from 65 offspring of 29 mothers to examine the relationships between weaned age (defined as cessation of suckling) in wild chimpanzees and maternal age, dominance rank and parity, and offspring sex. We used Cox proportional hazards regression with mixed effects to model time to weaning and to examine potential sources of variation in offspring weaned age.

RESULTS

We found that male offspring were less likely than female offspring to wean by a given age and that weaned age of males varied more than weaned age of females. In addition, maternal dominance rank interacted with offspring age, such that low-ranking mothers were less likely to wean offspring early, but this effect decreased with offspring age.

DISCUSSION

We found that male offspring and offspring of low-ranking females were less likely to wean early, but did not find evidence for variable weaning according to maternal age or parity. As more data accumulate, we will be better able to disentangle the effects of maternal dominance rank, age and parity. Such studies will not only provide a richer understanding of living ape life history characteristics, but will also provide an important framework for understanding the evolution of early weaning in humans.

Fertility as a constraint on group size in African great Apes

Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).

Chemical cues of female fertility states in a non-human primate

An increasing number of studies suggest that olfaction is important for communication throughout the order of primates. Callitrichids, in particular, have well-developed olfactory systems and use anogenital glands to produce scent marks. Behavioural studies have shown that male common marmosets (Callithrix jacchus) distinguish between odours from the peri-ovulatory and luteal phase of females. However, large gaps remain in understanding the chemical underpinnings of olfactory cues. To investigate whether chemical cues vary with female fertility and reproductive quality, our study combined behavioural bioassays with chemical analyses of the anogenital odours of female common marmosets using gas chromatography-mass spectrometry. We found that cycle states, age and parity have an impact on chemical profiles and further identified affected chemical substances. Our results confirm and expand on previous behavioural evidence for cues of fertility. Our results indicate that cycle-related substances likely act as chemical cues. Males could use such olfactory fertility cues to optimize their mating effort and thereby increase their paternity certainty. This certainty could enhance paternal care for their infants. The results of our study open a promising avenue to find the metabolic pathways from which chemical cues of fertility arise and to unravel their importance during primate evolution in future comparative studies.

Intersexual conflict influences female reproductive success in a female-dispersing primate

Abstract

In group-living mammals, individual efforts to maximize reproductive success result in conflicts and compromises between the sexes. Females utilize counterstrategies to minimize the costs of sexual coercion by males, but few studies have examined the effect of such behaviors on female reproductive success. Secondary dispersal by females is rare among group-living mammals, but in western gorillas, it is believed to be a mate choice strategy to minimize infanticide risk and infant mortality. Previous research suggested that females choose males that are good protectors. However, how much female reproductive success varies depending on male competitive ability and whether female secondary dispersal leads to reproductive costs or benefits has not been examined. We used data on 100 females and 229 infants in 36 breeding groups from a 20-year long-term study of wild western lowland gorillas to investigate whether male tenure duration and female transfer rate had an effect on interbirth interval, female birth rates, and offspring mortality. We found that offspring mortality was higher near the end of males’ tenures, even after excluding potential infanticide when those males died, suggesting that females suffer a reproductive cost by being with males nearing the end of their tenures. Females experience a delay in breeding when they dispersed, having a notable effect on birth rates of surviving offspring per female if females transfer multiple times in their lives. This study exemplifies that female counterstrategies to mitigate the effects of male-male competition and sexual coercion may not be sufficient to overcome the negative consequences of male behavior.

Significance

Individual reproductive success is maximized through conflicts and compromises between the sexes in social mammals. In species with high dimorphism, females can experience sexual coercion by males and develop counterstrategies to reduce these costs. We studied this link in western lowland gorillas, which exhibit female secondary dispersal, a strategy that is believed to reflect female choice for the protective abilities of males. We found that females are faced with the dilemma of staying with a silverback at the end of his tenure and risk higher infant mortality versus dispersing and suffering reproductive delays and lower birth rates. This study shows that female reproductive strategies, namely dispersal, used to counter the effects of sexual coercion by males are not sufficient to overcome the negative consequences of male behavior.

Aggression by male bonobos against immature individuals does not fit with predictions of infanticide

The selective advantage of male infanticide is enhancement of reproductive success of the aggressor. This implies that aggression is directed at individuals sired by others, infant loss shortens the mother's inter-birth interval, and the aggressor has a greater likelihood of siring the next offspring of the victims' mother. As these conditions are not always met, the occurrence of male infanticide is expected to vary, and hominoid primates offer an interesting example of variation in male infanticide. Infanticide has been reported in gorillas and chimpanzees but appears to be absent in orangutans and bonobos. One argument for the absence of infanticide in bonobos is reduction of male aggression. However, given that male aggression against immature individuals occurs and that females engage in behavior that is considered to be counterstrategy against male infanticide, the risk of male infanticide may pose a potential threat. Here, we explored whether aggression by male bonobos fits predictions of male infanticide. Male aggression toward immature individuals was rare and did not have lethal consequences, but the majority of observed cases exposed targets to risks of injury. Males did not target their own offspring less frequently than unrelated immatures, and the risk of being the target of male aggression increased with the targets' age. Overall, these results do not match the predictions of the adaptive male infanticide hypothesis. Instead, aggression by males may promote the emigration of the targets and older males may reinforce their superior status toward individuals that will soon compete for the same resources.

Dispersal and reproductive careers of male mountain gorillas in Bwindi Impenetrable National Park, Uganda

Dispersal is a key event in the life of an animal and it influences individual reproductive success. Male mountain gorillas exhibit both philopatry and dispersal, resulting in a mixed one-male and multimale social organization. However, little is known about the relationship between male dispersal or philopatry and reproductive careers in Bwindi mountain gorillas. Here we analyze data spanning from 1993 to 2017 on social groups in Bwindi Impenetrable National Park, Uganda to examine the proportion of males that disperse, age of dispersal, pathways to attaining alpha status, fate of dispersing males and philopatric males, and male tenure length as well as make comparisons of these variables to the Virunga mountain gorilla population. We report previously undocumented cases of dispersal by immature males and old males and we also observed the only known case of a fully mature male immigrating into a breeding group. We used genetic tracking of known individuals to estimate that a minimum of 25% of males that disperse to become solitary males eventually form new groups. No differences were found between the Bwindi and Virunga population in the age of male dispersal, the proportion of males that disperse, the age of alpha male acquisition, and dominance tenure length. The lack of differences may be due to small sample sizes or because the observed ecological variability does not lead to life history differences between the populations. Males in both populations follow variable strategies to attain alpha status leading to the variable one-male and multimale social organization, including dispersal to become solitary and eventually form a group, via group fissioning, usurping another alpha male, or inheriting the alpha position when a previous group leader dies.

Cumulative reproductive costs on current reproduction in a wild polytocous mammal

The cumulative cost of reproduction hypothesis predicts that reproductive costs accumulate over an individual's reproductive life span. While short-term costs have been extensively explored, the prevalence of cumulative long-term costs and the circumstances under which such costs occur alongside or instead of short-term costs, are far from clear. Indeed, few studies have simultaneously tested for both short-term and cumulative long-term reproductive costs in natural populations. Even in mammals, comparatively little is known about cumulative effects of previous reproduction, especially in species with high variation in offspring numbers, where costs could vary among successful reproductive events. Here, we quantify effects of previous short-term and cumulative long-term reproduction on current reproduction probability and litter size in wild female yellow-bellied marmots (Marmota flaviventer) and test how these effects vary with age and between two contrasting environments. We provide evidence for cumulative long-term effects: females that had both reproduced frequently and weaned large litters on average in previous years had decreased current reproduction probability. We found no evidence for short-term reproductive costs between reproductive bouts. However, females weaned larger litters when they had weaned larger litters on average in previous years and had lower current reproduction probability when their previous reproductive success was low. Together these results suggest that, alongside persistent among-individual variation, long-term reproductive history affects current reproductive success.

Variation in the social organization of gorillas: Life history and socioecological perspectives

A focus of socioecological research is to understand how ecological, social, and life history factors influence the variability of social organization within and between species. The genus Gorilla exhibits variability in social organization with western gorilla groups being almost exclusively one-male, yet approximately 40% of mountain gorilla groups are multimale. We review five ultimate causes for the variability in social organization within and among gorilla populations: human disturbance, ecological constraints on group size, risk of infanticide, life history patterns, and population density. We find the most evidence for the ecological constraints and life history hypotheses, but an over-riding explanation remains elusive. The variability may hinge on variation in female dispersal patterns, as females seek a group of optimal size and with a good protector male. Our review illustrates the challenges of understanding why the social organization of closely related species may deviate from predictions based on socioecological and life history theory.

Why does women's fertility end in mid-life? Grandmothering and age at last birth

Great apes, the other living members of our hominid family, become decrepit before the age of forty and rarely outlive their fertile years. In contrast, women - even in high mortality hunter-gatherer populations - usually remain healthy and productive well beyond menopause. The grandmother hypothesis aims to account for the evolution of this distinctive feature of human life history. Our previous mathematical simulations of that hypothesis fixed the end of female fertility at the age of 45, based on the similarities among living hominids, and then modeled the evolution of human-like longevity from an ancestral state, like that of the great apes, due only to grandmother effects. A major modification here allows the age female fertility ends to vary as well, directly addressing a version of the question, influentially posed by GC Williams six decades ago: Why isn't menopause later in humans? Our model is an agent-based model (ABM) that accounts for the coevolution of both expected adult lifespan and end of female fertility as selection maximizes reproductive value. We find that grandmother effects not only drive the population from an equilibrium representing a great ape-like longevity to a new human-like longevity, they also maintain the observed termination of women's fertility before the age of 50.

Interbirth intervals in wild baboons: Environmental predictors and hormonal correlates

OBJECTIVES

Interbirth intervals (IBIs) are a key metric of female reproductive success; understanding how they are regulated by environmental, social, and demographic factors can provide insight into sources of variance in female fitness.

MATERIALS AND METHODS

Using 36 years of reproductive data on 490 IBIs for 160 wild female baboons, we identified sources of variance in the duration of IBIs and of their component phases: postpartum amenorrhea (PPA), sexual cycling, and pregnancy. We also examined how body fat and fecal hormone concentrations varied during female IBIs.

RESULTS

We found that IBIs tended to be shorter (reproduction was accelerated) when female traits and environmental variables promoted energy acquisition, but with different specific effects for different component phases of the IBI. We also found that females lost a substantial amount of body fat during PPA, indicating that PPA imposes accumulating energetic costs as it progresses. Prior to cycle resumption females began to regain body fat; body fat was stable across the cycling phase and increased throughout most of pregnancy. However, body fat scores per se were not associated with the duration of any of the component phases. Finally, we found that fecal glucocorticoid concentrations decreased as PPA progressed, suggesting a decline in energetic stress over this phase. Fecal progestogen and estrogen concentrations changed over time during sexual cycling; the direction of these changes depended on the phase of the sexual cycle (luteal versus early or late follicular phases).

DISCUSSION

Our study lends insight into the energetic constraints on female primate reproduction, revealing how female environments, changes in body fat, and steroid hormone concentrations relate to IBI duration and to reproductive readiness.

Aged Black-and-Gold Howler Monkey Female (Alouatta caraya): A Sign of Reproductive Senescence?

Reproductive senescence patterns have been scarcely studied in Neotropical primates. The few studies available on the hormonal profiles of aging female monkeys indicate that the decline of ovarian function in nonhuman primates may resemble the hormonal events associated with the perimenopause in women. In this study, we explore a reproductive hormone profile of an aged black-and-gold howler monkey female (Alouatta caraya) from a wild population in northeastern Argentina and compare this profile with that of a cycling female in the same population. As part of a larger study, we recorded sociosexual behaviors in adult and subadult females belonging to two groups, and we collected urine (n = 877) to determine the sex hormone profile of each female. These samples were analyzed using enzyme immunoassays for estrone conjugates and pregnanediol-3-glucuronide (PdG). We found differences in mean values of PdG between the younger (cycling) and the older female. These hormone values were lower in the older female, and she did not show any signs of cyclicity for either reproductive hormone. Our results show that the aging female in this wild population shows signs of ovarian senescence, indicated by low, acyclic levels of progesterone metabolites.

Age at reproductive debut: Developmental predictors and consequences for lactation, infant mass, and subsequent reproduction in rhesus macaques (Macaca mulatta)

OBJECTIVES

The age at which females initiate their reproductive career is a critical life-history parameter with potential consequences on their residual reproductive value and lifetime fitness. The age at reproductive debut may be intimately tied to the somatic capacity of the mother to rear her young, but relatively little is known about the influence of age of first birth on milk synthesis within a broader framework of reproductive scheduling, infant outcomes, and other life-history tradeoffs.

MATERIAL AND METHODS

Our study investigated the predictors of age at first reproduction among 108 captive rhesus macaque (Macaca mulatta) females, and associations with their milk synthesis at peak lactation, infant mass, and ability to subsequently conceive and reproduce.

RESULTS

The majority of females reproduced in their fourth year (typical breeders); far fewer initiated their reproductive career one year earlier or one year later (respectively early and late breeders). Early breeders (3-year-old) benefited from highly favorable early life development (better juvenile growth, high dominance rank) to accelerate reproduction, but were impaired in milk synthesis due to lower somatic resources and their own continued growth. Comparatively, late breeders suffered from poor developmental conditions, only partially compensated by their delayed reproduction, and evinced compromised milk synthesis. Typical breeders not only produced higher available milk energy but also had best reproductive performance during the breeding and birth seasons following primiparity.

DISCUSSION

Here, we refine and extend our understanding of how life-history tradeoffs manifest in the magnitude, sources, and consequences of variation in age of reproductive debut. These findings provide insight into primate reproductive flexibility in the context of constraints and opportunities.

Indirect effects of prey fluctuation on survival of juvenile arctic fox (Vulpes lagopus): a matter of maternal experience and litter attendance

Reproductive experience affects juvenile survival in a wide range of species with possible links to differences in foraging capacity and predation. Using supplementary feeding, we aimed to limit direct effect of prey abundance to investigate indirect effects of small-rodent availability and maternal experience on juvenile summer survival rates in an endangered population of arctic fox (Vulpes lagopus (L., 1758)). We used data spanning 7 years, included a complete small-rodent cycle, comprising 49 litters and 394 cubs. The effect of small-rodent abundance on juvenile survival depended on maternal breeding experience. Cubs born by first-time-breeding females had lower survival rate when small-rodent abundance was low compared with juveniles born to experienced mothers who remained unaffected. It was unlikely due to starvation, as physical condition was unrelated to survival. Instead, we favour the explanation that intraguild predation was an important cause of mortality. There was a negative relationship between survival and amount of time cubs were left unattended, suggesting that parental behaviour affected predation. We propose that a prey switch related to small-rodent abundance caused fluctuations in intraguild predation pressure and that inexperienced females were less able to cope with predation when small rodents were scarce.

Development and behavior of wild infant-juvenile East Bornean orangutans (Pongo pygmaeus morio) in Danum Valley

Orangutans have a long period of immaturity and the longest inter-birth interval (IBI) of all mammals, which can be explained by their solitary life style, preventing the mother from rearing two offspring simultaneously (solitary life hypothesis) [corrected]. We collected data on mother-offspring dyads living in a primary lowland forest in Danum Valley, East Borneo in an effort to examine the developmental and behavioral patterns of the subspecies Pongo pygmaeus morio. We analyzed developmental changes in mother-offspring distance, contact, and activity budgets in orangutans ranging from 1 to 7 years of age. The results indicated decreased resting and playing with increasing age, whereas feeding, traveling and social play all increased significantly. Mothers' feeding and traveling time were good predictors of their offspring's feeding and traveling activities. Mother-offspring contact lasted longer in resting contexts; contact during traveling was almost non-existent after 4 years of age. Comparisons with previously published data on the Sumatran species Pongo abelli revealed no fundamental differences in these behavioral measures. However, a shorter association time with the mother after behavioral independence is documented for this East Bornean population in comparison to Sumatran populations. These results are best explained by the solitary life hypothesis, in agreement with previous studies. We suggest that environmental constraints in Bornean forests, as well as a lower population density, should be considered when interpreting the differences between Sumatran and Bornean orangutans in both the period of association with mother and the IBI.

Endocrine assessment of ovarian cycle activity in wild female mountain gorillas (Gorilla beringei beringei)

Variability of fertility (i.e. number of births per female per year) has been reported in females of many primate species but only a few studies have explored the associated physiological and behavioral patterns. To investigate the proximate mechanisms of variability in fertility of wild female mountain gorillas (Gorilla beringei beringei), we quantified the occurrence of ovulation, matings, and successful pregnancies among females. We examined the profiles of immunoreactive pregnanediol-3-glucuronide (iPdG) for sixteen females (seven nulliparous and nine parous females, including one geriatric female; average sampling period for fecal sample collection and behavioral observations per female=175 days; SD=94 days, range=66-358 days) monitored by the staff of the Dian Fossey Gorilla Fund's Karisoke Research Center in Parc National des Volcans, Rwanda. We quantified ovarian cycles from iPdG profiles using an algorithm that we developed by adjusting the method of Kassam et al. (1996) to the characteristics of ovarian cycle profiles based on fecal hormone measurements. The mean length of ovarian cycles was 29±4 days (median: 28 days, N=13 cycles), similar to ovarian cycle lengths of other great apes and humans. As expected, we found that female mountain gorillas exhibit longer follicular phases (mean±SD: 21±3 days, N=13 cycles) than luteal phases (mean±SD: 8±3 days, N=13 cycles). We also found that the frequency of ovarian cycles was greater in parous females (i.e. 20 ovarian cycles across 44 periods of 28 days; 45.5%) than in nulliparous females (i.e. two ovarian cycles across 34 periods of 28 days; 6%). However, the frequency of days on which matings were observed did not differ significantly between parous and nulliparous females, nor between pregnant and non-pregnant females. Five pregnancies were detected with iPdG levels, but only three resulted in live births, indicating miscarriages of the other two. In sum, this study provides information on the underlying endocrine patterns of variation in fertility depending on parity, mating behavior, and pregnancy success in a critically endangered great ape.

Age-related changes in molar topography and shearing crest length in a wild population of mountain Gorillas from Volcanoes National Park, Rwanda

OBJECTIVES

Great ape teeth must remain functional over long lifespans. The molars of the most folivorous apes, the mountain gorillas, must maintain shearing function for 40+ years while the animals consume large quantities of mechanically challenging foods. While other folivorous primates experience dental senescence, which compromises their occlusal surfaces and affects their reproductive success as they age, it is unknown whether dental senescence also occurs in mountain gorillas. In this article, we quantified and evaluated how mountain gorilla molars change throughout their long lifespans.

MATERIALS AND METHODS

We collected high-resolution replicas of M(1)s (n = 15), M(2)s (n = 13), and M(3)s (n = 11) from a cross-sectional sample of wild mountain gorilla skeletons from the Virunga Volcanoes, ranging in age from 4 to 43 years. We employed dental topographic analyses to track how aspects of occlusal slope, angularity, relief index, and orientation patch count rotated change with age. In addition, we measured the relative length of shearing crests in two- and three-dimensions.

RESULTS

Occlusal topography was found to decrease, while 2D relative shearing crest length increased, and 3D relative crest lengths were maintained with age.

DISCUSSION

Our findings indicate that shearing function is maintained throughout the long lifetimes of mountain gorillas. Unlike the dental senescence experienced by other folivorous primates, mountain gorillas do not appear to possess senesced molars despite their long lifetimes, mechanically challenging diets, and decreases in occlusal topography with age.

Group structure predicts variation in proximity relationships between male-female and male-infant pairs of mountain gorillas (Gorilla beringei beringei)

Relationships between conspecifics are influenced by both ecological factors and the social organization they live in. Systematic variation of both--consistent with predictions derived from socioecology models--is well documented, but there is considerable variation within species and populations that is poorly understood. The mountain gorilla (Gorilla beringei) is unusual because, despite possessing morphology associated with male contest competition (e.g., extreme sexual dimorphism), they are regularly observed in both single-male and multimale groups. Both male-female and male-infant bonds are strong because males provide protection against infanticide and/or predation. Risk of these threats varies with social structure, which may influence the strength of social relationships among group members (including females and offspring, if females with lower infant mortality risk are less protective of infants). Here, we investigate the relationship between group structure and the strength of proximity relationships between males and females, males and infants, and females and offspring. Data come from 10 social groups containing 1-7 adult males, monitored by the Dian Fossey Gorilla Fund's Karisoke Research Center in Volcanoes National Park, Rwanda. After controlling for group size and infant age, association strength was similar for male-female pairs across group types with both dominant and nondominant males, but male-infant relationships were strongest in single-male groups where paternity certainty was high and animals had fewer social partners to choose from. The male:female and male:infant ratios better predicted both male-female and male-infant associations than the absolute number of males, females, or infants did. The fewer the number of males per female or infant, the more both pair types associated. Dominant males in groups containing fewer males had higher eigenvector centrality (a measure of importance in a social network) than dominant males in groups with more males. Results indicate that nondominant males are an important influence on relationships between dominant males and females/infants despite their peripheral social positions, and that relationships between males and infants must be considered an important foundation of gorilla social structure.

Grandmothering life histories and human pair bonding

The evolution of distinctively human life history and social organization is generally attributed to paternal provisioning based on pair bonds. Here we develop an alternative argument that connects the evolution of human pair bonds to the male-biased mating sex ratios that accompanied the evolution of human life history. We simulate an agent-based model of the grandmother hypothesis, compare simulated sex ratios to data on great apes and human hunter-gatherers, and note associations between a preponderance of males and mate guarding across taxa. Then we explore a recent model that highlights the importance of mating sex ratios for differences between birds and mammals and conclude that lessons for human evolution cannot ignore mammalian reproductive constraints. In contradiction to our claim that male-biased sex ratios are characteristically human, female-biased ratios are reported in some populations. We consider the likelihood that fertile men are undercounted and conclude that the mate-guarding hypothesis for human pair bonds gains strength from explicit links with our grandmothering life history.

Modeling variation in early life mortality in the western lowland gorilla: Genetic, maternal and other effects

Uncovering sources of variation in gorilla infant mortality informs conservation and life history research efforts. The international studbook for the western lowland gorilla provides information on a sample of captive gorillas large enough for which to analyze genetic, maternal, and various other effects on early life mortality in this critically endangered species. We assess the importance of variables such as sex, maternal parity, paternal age, and hand rearing with regard to infant survival. We also quantify the proportions of variation in mortality influenced by heritable variation and maternal effects from these pedigree and survival data using variance component estimation. Markov chain Monte Carlo simulations of generalized linear mixed models produce variance component distributions in an animal model framework that employs all pedigree information. Two models, one with a maternal identity component and one with both additive genetic and maternal identity components, estimate variance components for different age classes during the first 2 years of life. This is informative of the extent to which mortality risk factors change over time during gorilla infancy. Our results indicate that gorilla mortality is moderately heritable with the strongest genetic influence just after birth. Maternal effects are most important during the first 6 months of life. Interestingly, hand-reared infants have lower mortality for the first 6 months of life. Aside from hand rearing, we found other predictors commonly used in studies of primate infant mortality to have little influence in these gorilla data.

Maternal Behavior by Birth Order in Wild Chimpanzees (Pan troglodytes): Increased Investment by First-Time Mothers

Parental investment theory predicts that maternal resources are finite and allocated among offspring based on factors including maternal age and condition, and offspring sex and parity. Among humans, firstborn children are often considered to have an advantage and receive greater investment than their younger siblings. However, conflicting evidence for this "firstborn advantage" between modern and hunter-gatherer societies raises questions about the evolutionary history of differential parental investment and birth order. In contrast to humans, most non-human primate firstborns belong to young, inexperienced mothers and exhibit higher mortality than laterborns. In this study, we investigated differences in maternal investment and offspring outcomes based on birth order (firstborn vs. later-born) among wild chimpanzees (Pan troglodyte schweinfurthii). During the critical first year of life, primiparous mothers nursed, groomed, and played with their infants more than did multiparous mothers. Furthermore, this pattern of increased investment in firstborns appeared to be compensatory, as probability of survival did not differ by birth order. Our study did not find evidence for a firstborn advantage as observed in modern humans but does suggest that unlike many other primates, differences in maternal behavior help afford chimpanzee first-borns an equal chance of survival.

Costs of and Investment in Mate-Guarding in Wild Long-Tailed Macaques (Macaca fascicularis): Influences of Female Characteristics and Male-Female Social Bonds

Male primates living in multimale groups tend to direct mate and mate-guarding choices toward females of high reproductive value, i.e., high-ranking, parous females, or females with which they share strong bonds. Little is known, however, about the constraints that may limit male mate-guarding choices (the costs of this behavior) and the influence of the females’ quality on male investment in mate-guarding. We aimed to study the effects of female rank, parity status, and male–female social bond strength on the costs of and investment in mate-guarding by males. We carried out our study during two reproductive seasons on three groups of wild long-tailed macaques in Indonesia. We combined behavioral observations on male locomotion and activity with noninvasive measurements of fecal glucocorticoids (fGC). Males spent less time feeding when mate-guarding nulliparous females than when mate-guarding parous females and tended to have higher fGC levels when mate-guarding low-ranking nulliparous females than when mate-guarding high-ranking nulliparous ones. Evolution should thus favor male choice for high-ranking parous females because such a decision brings benefits at proximate (reduced costs of mate-guarding) and ultimate (higher reproductive value) levels. Further, male investment in mate-guarding was flexible and contingent on female reproductive and social value. Males were more vigilant and more aggressive toward other males when mate-guarding females to which they were strongly bonded and/or high-ranking ones than when mate-guarding other females. Our findings bring a new dimension to the study of mate choice by showing that males not only mate preferentially with high-quality females but may also aim to secure paternity with these females through optimized monopolization.

Grandmothering drives the evolution of longevity in a probabilistic model

We present a mathematical model based on the Grandmother Hypothesis to simulate how human post-menopausal longevity could have evolved as ancestral grandmothers began to assist the reproductive success of younger females by provisioning grandchildren. Grandmothers׳ help would allow mothers to give birth to subsequent offspring sooner without risking the survival of existing offspring. Our model is an agent-based model (ABM), in which the population evolves according to probabilistic rules governing interactions among individuals. The model is formulated according to the Gillespie algorithm of determining the times to next events. Grandmother effects drive the population from an equilibrium representing a great-ape-like average adult lifespan in the lower twenties to a new equilibrium with a human-like average adult lifespan in the lower forties. The stochasticity of the ABM allows the possible coexistence of two locally-stable equilibria, corresponding to great-ape-like and human-like lifespans. Populations with grandmothering that escape the ancestral condition then shift to human-like lifespan, but the transition takes longer than previous models (Kim et al., 2012). Our simulations are consistent with the possibility that distinctive longevity is a feature of genus Homo that long antedated the appearance of our species.

Impact of male infanticide on the social structure of mountain gorillas

Infanticide can be a major influence upon the social structure of species in which females maintain long-term associations with males. Previous studies have suggested that female mountain gorillas benefit from residing in multimale groups because infanticide occurs when one-male groups disintegrate after the dominant male dies. Here we measure the impact of infanticide on the reproductive success of female mountain gorillas, and we examine whether their dispersal patterns reflect a strategy to avoid infanticide. Using more than 40 years of data from up to 70% of the entire population, we found that only 1.7% of the infants that were born in the study had died from infanticide during group disintegrations. The rarity of such infanticide mainly reflects a low mortality rate of dominant males in one-male groups, and it does not dispel previous observations that infanticide occurs during group disintegrations. After including infanticide from causes other than group disintegrations, infanticide victims represented up to 5.5% of the offspring born during the study, and they accounted for up to 21% of infant mortality. The overall rates of infanticide were 2-3 times higher in one-male groups than multimale groups, but those differences were not statistically significant. Infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups, so we found no significant fitness benefits for females to prefer multimale groups. In addition, we found limited evidence that female dispersal patterns reflect a preference for multimale groups. If the strength of selection is modest for females to avoid group disintegrations, than any preference for multimale groups may be slow to evolve. Alternatively, variability in male strength might give some one-male groups a lower infanticide risk than some multimale groups, which could explain why both types of groups remain common.