Social bonds do not mediate the relationship between early adversity and adult glucocorticoids in wild baboons

Effects of social environments on male primate HPG and HPA axis developmental programming

Developmental plasticity is particularly important for humans and other primates because of our extended period of growth and maturation, during which our phenotypes adaptively respond to environmental cues. The hypothalamus-pituitary-gonadal (HPG) and hypothalamus-pituitary-adrenal (HPA) axes are likely to be principal targets of developmental "programming" given their roles in coordinating fitness-relevant aspects of the phenotype, including sexual development, adult reproductive and social strategies, and internal responses to the external environment. In social animals, including humans, the social environment is believed to be an important source of cues to which these axes may adaptively respond. The effects of early social environments on the HPA axis have been widely studied in humans, and to some extent, in other primates, but there are still major gaps in knowledge specifically relating to males. There has also been relatively little research examining the role that social environments play in developmental programming of the HPG axis or the HPA/HPG interface, and what does exist disproportionately focuses on females. These topics are likely understudied in males in part due to the difficulty of identifying developmental milestones in males relative to females and the general quiescence of the HPG axis prior to maturation. However, there are clear indicators that early life social environments matter for both sexes. In this review, we examine what is known about the impact of social environments on HPG and HPA axis programming during male development in humans and nonhuman primates, including the role that epigenetic mechanisms may play in this programming. We conclude by highlighting important next steps in this research area.

DNA methylation signatures of early-life adversity are exposure-dependent in wild baboons

The early-life environment can profoundly shape the trajectory of an animal's life, even years or decades later. One mechanism proposed to contribute to these early-life effects is DNA methylation. However, the frequency and functional importance of DNA methylation in shaping early-life effects on adult outcomes is poorly understood, especially in natural populations. Here, we integrate prospectively collected data on fitness-associated variation in the early environment with DNA methylation estimates at 477,270 CpG sites in 256 wild baboons. We find highly heterogeneous relationships between the early-life environment and DNA methylation in adulthood: aspects of the environment linked to resource limitation (e.g., low-quality habitat, early-life drought) are associated with many more CpG sites than other types of environmental stressors (e.g., low maternal social status). Sites associated with early resource limitation are enriched in gene bodies and putative enhancers, suggesting they are functionally relevant. Indeed, by deploying a baboon-specific, massively parallel reporter assay, we show that a subset of windows containing these sites are capable of regulatory activity, and that, for 88% of early drought-associated sites in these regulatory windows, enhancer activity is DNA methylation-dependent. Together, our results support the idea that DNA methylation patterns contain a persistent signature of the early-life environment. However, they also indicate that not all environmental exposures leave an equivalent mark and suggest that socioenvironmental variation at the time of sampling is more likely to be functionally important. Thus, multiple mechanisms must converge to explain early-life effects on fitness-related traits.

Environmental, sex-specific and genetic determinants of infant social behaviour in a wild primate

Affiliative social bonds are linked to fitness components in many social mammals. However, despite their importance, little is known about how the tendency to form social bonds develops in young animals, or if the timing of development is heritable and thus can evolve. Using four decades of longitudinal observational data from a wild baboon population, we assessed the environmental determinants of an important social developmental milestone in baboons-the age at which a young animal first grooms a conspecific-and we assessed how the rates at which offspring groom their mothers develops during the juvenile period. We found that grooming development differs between the sexes: female infants groom at an earlier age and reach equal rates of grooming with their mother earlier than males. We also found that age at first grooming for both sexes is weakly heritable (h2 = 0.043, 95% CI: 0.002-0.110). These results show that sex differences in grooming emerge at a young age; that strong, equitable social relationships between mothers and daughters begin very early in life; and that age at first grooming is heritable and therefore can be shaped by natural selection.

Differential effects of early life adversity on male and female rhesus macaque lifespan

Early life adversity predicts shorter adult lifespan in several animal taxa. Yet, work on long-lived primate populations suggests the evolution of mechanisms that contribute to resiliency and long lives despite early life insults. Here, we tested associations between individual and cumulative early life adversity and lifespan on rhesus macaques at the Cayo Santiago Biological Field Station using 50 years of demographic data. We performed sex-specific survival analyses at different life stages to contrast short-term effects of adversity (i.e., infant survival) with long-term effects (i.e., adult survival). Female infants showed vulnerability to multiple adversities at birth, but affected females who survived to adulthood experienced a reduced risk later in life. In contrast, male infants showed vulnerability to a lower number of adversities at birth, but those who survived to adulthood were negatively affected by both early life individual and cumulative adversity. Our study shows profound immediate effects of insults  on female infant cohorts and suggests that affected female adults are more robust. In contrast, adult males who experienced harsh conditions early in life showed an increased mortality risk at older ages as expected from hypotheses within the life course perspective. Our analysis suggests sex-specific selection pressures on life histories and highlights the need for studies addressing the effects of early life adversity across multiple life stages.

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.

Advancing methods for the biodemography of aging within social contexts

Several social dimensions including social integration, status, early-life adversity, and their interactions across the life course can predict health, reproduction, and mortality in humans. Accordingly, the social environment plays a fundamental role in the emergence of phenotypes driving the evolution of aging. Recent work placing human social gradients on a biological continuum with other species provides a useful evolutionary context for aging questions, but there is still a need for a unified evolutionary framework linking health and aging within social contexts. Here, we summarize current challenges to understand the role of the social environment in human life courses. Next, we review recent advances in comparative biodemography and propose a biodemographic perspective to address socially driven health phenotype distributions and their evolutionary consequences using a nonhuman primate population. This new comparative approach uses evolutionary demography to address the joint dynamics of populations, social dimensions, phenotypes, and life history parameters. The long-term goal is to advance our understanding of the link between individual social environments, population-level outcomes, and the evolution of aging.

Natural Animal Populations as Model Systems for Understanding Early Life Adversity Effects on Aging

Adverse experiences in early life are associated with aging-related disease risk and mortality across many species. In humans, confounding factors, as well as the difficulty of directly measuring experiences and outcomes from birth till death, make it challenging to identify how early life adversity impacts aging and health. These challenges can be mitigated, in part, through the study of non-human animals, which are exposed to parallel forms of adversity and can age similarly to humans. Furthermore, studying the links between early life adversity and aging in natural populations of non-human animals provides an excellent opportunity to better understand the social and ecological pressures that shaped the evolution of early life sensitivities. Here, we highlight ongoing and future research directions that we believe will most effectively contribute to our understanding of the evolution of early life sensitivities and their repercussions.

Re-wilding model organisms: Opportunities to test causal mechanisms in social determinants of health and aging

Social experiences are strongly associated with individuals' health, aging, and survival in many mammalian taxa, including humans. Despite their role as models of many other physiological and developmental bases of health and aging, biomedical model organisms (particularly lab mice) remain an underutilized tool in resolving outstanding questions regarding social determinants of health and aging, including causality, context-dependence, reversibility, and effective interventions. This status is largely due to the constraints of standard laboratory conditions on animals' social lives. Even when kept in social housing, lab animals rarely experience social and physical environments that approach the richness, variability, and complexity they have evolved to navigate and benefit from. Here we argue that studying biomedical model organisms outside under complex, semi-natural social environments ("re-wilding") allows researchers to capture the methodological benefits of both field studies of wild animals and laboratory studies of model organisms. We review recent efforts to re-wild mice and highlight discoveries that have only been made possible by researchers studying mice under complex, manipulable social environments.

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.

Early life adversities and lifelong health outcomes: A review of the literature on large, social, long-lived nonhuman mammals

Social nonhuman animals are powerful models for studying underlying factors related to lifelong health outcomes following early life adversities (ELAs). ELAs can be linked to lifelong health outcomes depending on the species, system, sensitive developmental periods, and biological pathways. This review focuses on the literature surrounding ELAs and lifelong health outcomes in large, social, relatively long-lived nonhuman mammals including nonhuman primates, canids, hyenas, elephants, ungulates, and cetaceans. These mammals, like humans but unlike the most-studied rodent models, have longer life histories, complex social structures, larger brains, and comparable stress and reproductive physiology. Collectively, these features make them compelling models for comparative aging research. We review studies of caregiver, social, and ecological ELAs, often in tandem, in these mammals. We consider experimental and observational studies and what each has contributed to our knowledge of health across the lifespan. We demonstrate the continued and expanded need for comparative research to inform about the social determinants of health and aging in both humans and nonhuman animals.

Social and early life determinants of survival from cradle to grave: A case study in wild baboons

Field studies of natural mammal populations present powerful opportunities to investigate the determinants of health and aging using fine-grained observations of known individuals across the life course. Here, we synthesize five decades of findings from one such study: the wild baboons of the Amboseli ecosystem in Kenya. First, we discuss the profound associations between early life adversity, adult social conditions, and key aging outcomes in this population, especially survival. Second, we review potential mediators of the relationship between early life adversity and survival in our population. Notably, our tests of two leading candidate mediators-social isolation and glucocorticoid levels-fail to identify a single, strong mediator of early life effects on adult survival. Instead, early adversity, social isolation, and glucocorticoids are independently linked to adult lifespans, suggesting considerable scope for mitigating the negative consequences of early life adversity. Third, we review our work on the evolutionary rationale for early life effects on mortality, which currently argues against clear predictive adaptive responses. Finally, we end by highlighting major themes emerging from the study of sociality, development, and aging in the Amboseli baboons, as well as important open questions for future work.

DNA methylation signatures of early life adversity are exposure-dependent in wild baboons

The early life environment can profoundly shape the trajectory of an animal's life, even years or decades later. One mechanism proposed to contribute to these early life effects is DNA methylation. However, the frequency and functional importance of DNA methylation in shaping early life effects on adult outcomes is poorly understood, especially in natural populations. Here, we integrate prospectively collected data on fitness-associated variation in the early environment with DNA methylation estimates at 477,270 CpG sites in 256 wild baboons. We find highly heterogeneous relationships between the early life environment and DNA methylation in adulthood: aspects of the environment linked to resource limitation (e.g., low-quality habitat, early life drought) are associated with many more CpG sites than other types of environmental stressors (e.g., low maternal social status). Sites associated with early resource limitation are enriched in gene bodies and putative enhancers, suggesting they are functionally relevant. Indeed, by deploying a baboon-specific, massively parallel reporter assay, we show that a subset of windows containing these sites are capable of regulatory activity, and that, for 88% of early drought-associated sites in these regulatory windows, enhancer activity is DNA methylation-dependent. Together, our results support the idea that DNA methylation patterns contain a persistent signature of the early life environment. However, they also indicate that not all environmental exposures leave an equivalent mark and suggest that socioenvironmental variation at the time of sampling is more likely to be functionally important. Thus, multiple mechanisms must converge to explain early life effects on fitness-related traits.

A Causal Mediation Model for Longitudinal Mediators and Survival Outcomes with an Application to Animal Behavior

In animal behavior studies, a common goal is to investigate the causal pathways between an exposure and outcome, and a mediator that lies in between. Causal mediation analysis provides a principled approach for such studies. Although many applications involve longitudinal data, the existing causal mediation models are not directly applicable to settings where the mediators are measured on irregular time grids. In this paper, we propose a causal mediation model that accommodates longitudinal mediators on arbitrary time grids and survival outcomes simultaneously. We take a functional data analysis perspective and view longitudinal mediators as realizations of underlying smooth stochastic processes. We define causal estimands of direct and indirect effects accordingly and provide corresponding identification assumptions. We employ a functional principal component analysis approach to estimate the mediator process and propose a Cox hazard model for the survival outcome that flexibly adjusts the mediator process. We then derive a g-computation formula to express the causal estimands using the model coefficients. The proposed method is applied to a longitudinal data set from the Amboseli Baboon Research Project to investigate the causal relationships between early adversity, adult physiological stress responses, and survival among wild female baboons. We find that adversity experienced in early life has a significant direct effect on females' life expectancy and survival probability, but find little evidence that these effects were mediated by markers of the stress response in adulthood. We further developed a sensitivity analysis method to assess the impact of potential violation to the key assumption of sequential ignorability. Supplementary materials accompanying this paper appear on-line.

Shared community effects and the non-genetic maternal environment shape cortisol levels in wild chimpanzees

Mechanisms of inheritance remain poorly defined for many fitness-mediating traits, especially in long-lived animals with protracted development. Using 6,123 urinary samples from 170 wild chimpanzees, we examined the contributions of genetics, non-genetic maternal effects, and shared community effects on variation in cortisol levels, an established predictor of survival in long-lived primates. Despite evidence for consistent individual variation in cortisol levels across years, between-group effects were more influential and made an overwhelming contribution to variation in this trait. Focusing on within-group variation, non-genetic maternal effects accounted for 8% of the individual differences in average cortisol levels, significantly more than that attributable to genetic factors, which was indistinguishable from zero. These maternal effects are consistent with a primary role of a shared environment in shaping physiology. For chimpanzees, and perhaps other species with long life histories, community and maternal effects appear more relevant than genetic inheritance in shaping key physiological traits.

Early life adversity and adult social relationships have independent effects on survival in a wild primate

Adverse conditions in early life can have negative consequences for adult health and survival in humans and other animals. What variables mediate the relationship between early adversity and adult survival? Adult social environments represent one candidate: Early life adversity is linked to social adversity in adulthood, and social adversity in adulthood predicts survival outcomes. However, no study has prospectively linked early life adversity, adult social behavior, and adult survival to measure the extent to which adult social behavior mediates this relationship. We do so in a wild baboon population in Amboseli, Kenya. We find weak mediation and largely independent effects of early adversity and adult sociality on survival. Furthermore, strong social bonds and high social status in adulthood can buffer some negative effects of early adversity. These results support the idea that affiliative social behavior is subject to natural selection through its positive relationship with survival, and they highlight possible targets for intervention to improve human health and well-being.

Transition to siblinghood causes a substantial and long-lasting increase in urinary cortisol levels in wild bonobos

In animals with slow ontogeny and long-term maternal investment, immatures are likely to experience the birth of a younger sibling before reaching maturity. In these species, the birth of a sibling marks a major event in an offspring’s early life as the older siblings experience a decrease in maternal support. The transition to siblinghood (TTS) is often considered to be stressful for the older offspring, but physiological evidence is lacking. To explore the TTS in wild bonobos, we investigated physiological changes in urinary cortisol (stress response), neopterin (cell-mediated immunity), and total triiodothyronine (T3, metabolic rate), as well as changes in behaviors that reflect the mother–offspring relationship. Following a sibling’s birth, urinary cortisol levels of the older offspring increased fivefold, independent of their age, and remained elevated for 7 months. The cortisol level increase was associated with declining neopterin levels; however, T3 levels and behavioral measures did not change. Our results indicate that the TTS is accompanied by elevated cortisol levels and that this change does not coincide with nutritional weaning and attainment of physical independence. Our results suggest that bonobos and humans experience TTS in similar ways and that this developmental event may have emerged in the last common ancestor.

Social support correlates with glucocorticoid concentrations in wild African elephant orphans

Social relationships have physiological impacts. Here, we investigate whether loss of the mother/offspring relationship has lasting effects on fecal glucocorticoid metabolite (fGCM) concentrations in wild African elephant orphans several years following their mothers’ deaths. We find no difference in fGCM concentrations between orphans and nonorphans, but find lower fGCM concentrations in elephants with more age mates in their family. We also unexpectedly identify lower concentrations in orphans without their natal family versus nonorphans and natal orphans, which we speculate may be due to the development of hypocortisolism following a prolonged period without familial support. An index of plant productivity (i.e. food) shows the largest correlation with fGCM concentrations. Our findings indicate no lasting differences in glucocorticoid concentrations of surviving orphan elephants who are with their family, suggest the presence of age mates may reduce glucocorticoid concentrations in elephants, and emphasize that basic survival needs are the primary regulators of the stress response.

Remaining with the family group may reduce long-term stress effects in orphaned African elephants.

Distinct gene regulatory signatures of dominance rank and social bond strength in wild baboons

The social environment is a major determinant of morbidity, mortality and Darwinian fitness in social animals. Recent studies have begun to uncover the molecular processes associated with these relationships, but the degree to which they vary across different dimensions of the social environment remains unclear. Here, we draw on a long-term field study of wild baboons to compare the signatures of affiliative and competitive aspects of the social environment in white blood cell gene regulation, under both immune-stimulated and non-stimulated conditions. We find that the effects of dominance rank on gene expression are directionally opposite in males versus females, such that high-ranking males resemble low-ranking females, and vice versa. Among females, rank and social bond strength are both reflected in the activity of cellular metabolism and proliferation genes. However, while we observe pronounced rank-related differences in baseline immune gene activity, only bond strength predicts the fold-change response to immune (lipopolysaccharide) stimulation. Together, our results indicate that the directionality and magnitude of social effects on gene regulation depend on the aspect of the social environment under study. This heterogeneity may help explain why social environmental effects on health and longevity can also vary between measures. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Early life adversity has long-term effects on sociality and interaction style in female baboons

Social bonds enhance fitness in many group-living animals, generating interest in the processes that create individual variation in sociality. Previous work on female baboons shows that early life adversity and temperament both influence social connectedness in adulthood. Early life adversity might shape sociality by reducing ability to invest in social relationships or through effects on attractiveness as a social partner. We examine how females' early life adversity predicts sociality and temperament in wild olive baboons, and evaluate whether temperament mediates the relationship between early life adversity and sociality. We use behavioural data on 31 females to quantify sociality. We measure interaction style as the tendency to produce grunts (signals of benign intent) in contexts in which the vocalization does not produce immediate benefits to the actor. Early life adversity was negatively correlated with overall sociality, but was a stronger predictor of social behaviours received than behaviours initiated. Females who experienced less early life adversity had more benign interaction styles and benign interaction styles were associated with receiving more social behaviours. Interaction style may partially mediate the association between early life adversity and sociality. These analyses add to our growing understanding of the processes connecting early life experiences to adult sociality.

Cohort consequences of drought and family disruption for male and female African elephants

Cohort effects, reflecting early adversity or advantage, have persisting consequences for growth, reproductive onset, longevity, and lifetime reproductive success. In species with prolonged life histories, cohort effects may establish variation in age-sex structures, while social structure may buffer individuals against early adversity. Using periods of significant ecological adversity, we examined cohort effects for male and female elephants (Loxodonta africana) over almost 50 years in Amboseli, Kenya. Mortality spiked during severe droughts with highest mortality among calves under 2 years and females over 40 years. Deaths of oldest females resulted in social disruption via matriarch turnover, with potential impacts on resource acquisition for survivors. We predicted that survivors of high mortality and social challenges would have altered life-history trajectories, with later age at first reproduction and reduced age-specific fertility for females and slow transitions to independence and late-onset of potential mating or musth among males. Contrary to expectations, there were no persisting early drought effects on female age at first conception while matriarch loss around puberty accelerated reproductive onset. Experience of an early life drought did not influence age-specific reproductive rates once females commenced reproduction. Males who survived an early drought exhibited complex consequences: male age at family independence was later with larger peer cohort size, but earlier with drought in year of independence (13.9 vs 14.6 years). Early drought had no effect on age at first musth, but male reproductive onset was weakly associated with the number of peers (negative) and age at independence (positive).

Dissecting genomic determinants of positive selection with an evolution-guided regression model

In evolutionary genomics, it is fundamentally important to understand how characteristics of genomic sequences, such as gene expression level, determine the rate of adaptive evolution. While numerous statistical methods, such as the McDonald–Kreitman (MK) test, are available to examine the association between genomic features and the rate of adaptation, we currently lack a statistical approach to disentangle the independent effect of a genomic feature from the effects of other correlated genomic features. To address this problem, I present a novel statistical model, the MK regression, which augments the MK test with a generalized linear model. Analogous to the classical multiple regression model, the MK regression can analyze multiple genomic features simultaneously to infer the independent effect of a genomic feature, holding constant all other genomic features. Using the MK regression, I identify numerous genomic features driving positive selection in chimpanzees. These features include well-known ones, such as local mutation rate, residue exposure level, tissue specificity, and immune genes, as well as new features not previously reported, such as gene expression level and metabolic genes. In particular, I show that highly expressed genes may have a higher adaptation rate than their weakly expressed counterparts, even though a higher expression level may impose stronger negative selection. Also, I show that metabolic genes may have a higher adaptation rate than their nonmetabolic counterparts, possibly due to recent changes in diet in primate evolution. Overall, the MK regression is a powerful approach to elucidate the genomic basis of adaptation.

Maternal stress effects on infant development in wild Verreaux's sifaka (Propithecus verreauxi)

Abstract

Maternal effects mediated by nutrients or specific endocrine states of the mother can affect infant development. Specifically, pre- and postnatal maternal stress associated with elevated glucocorticoid (GC) output is known to influence the phenotype of the offspring, including their physical and behavioral development. These developmental processes, however, remain relatively poorly studied in wild vertebrates, including primates with their relatively slow life histories. Here, we investigated the effects of maternal stress, assessed by fecal glucocorticoid output, on infant development in wild Verreaux’s sifakas (Propithecus verreauxi), a group-living Malagasy primate. In a first step, we investigated factors predicting maternal fecal glucocorticoid metabolite (fGCM) concentrations, how they impact infants’ physical and behavioral development during the first 6 months of postnatal life as well as early survival during the first 1.5 years of postnatal life. We collected fecal samples of mothers for hormone assays and behavioral data of 12 infants from two birth cohorts, for which we also assessed growth rates. Maternal fGCM concentrations were higher during the late prenatal but lower during the postnatal period compared to the early/mid prenatal period and were higher during periods of low rainfall. Infants of mothers with higher prenatal fGCM concentrations exhibited faster growth rates and were more explorative in terms of independent foraging and play. Infants of mothers with high pre- and postnatal fGCM concentrations were carried less and spent more time in nipple contact. Time mothers spent carrying infants predicted infant survival: infants that were more carried had lower survival, suggesting that they were likely in poorer condition and had to be cared for longer. Thus, the physical and behavioral development of these young primates were impacted by variation in maternal fGCM concentrations during the first 6 months of their lives, presumably as an adaptive response to living in a highly seasonal, but unpredictable environment.

Significance statement

The early development of infants can be impacted by variation in maternal condition. These maternal effects can be mediated by maternal stress (glucocorticoid hormones) and are known to have downstream consequences for behavior, physiology, survival, and reproductive success well into adulthood. However, the direction of the effects of maternal physiological GC output on offspring development is highly variable, even within the same species. We contribute comparative data on maternal stress effects on infant development in a Critically Endangered primate from Madagascar. We describe variation in maternal glucocorticoid output as a function of ecological and reproductive factors and show that patterns of infant growth, behavioral development, and early survival are predicted by maternal glucocorticoids. Our study demonstrates how mothers can influence offspring fitness in response to challenging environmental conditions.

Early maternal loss leads to short- but not long-term effects on diurnal cortisol slopes in wild chimpanzees

The biological embedding model (BEM) suggests that fitness costs of maternal loss arise when early-life experience embeds long-term alterations to hypothalamic-pituitary-adrenal (HPA) axis activity. Alternatively, the adaptive calibration model (ACM) regards physiological changes during ontogeny as short-term adaptations. Both models have been tested in humans but rarely in wild, long-lived animals. We assessed whether, as in humans, maternal loss had short- and long-term impacts on orphan wild chimpanzee urinary cortisol levels and diurnal urinary cortisol slopes, both indicative of HPA axis functioning. Immature chimpanzees recently orphaned and/or orphaned early in life had diurnal cortisol slopes reflecting heightened activation of the HPA axis. However, these effects appeared short-term, with no consistent differences between orphan and non-orphan cortisol profiles in mature males, suggesting stronger support for the ACM than the BEM in wild chimpanzees. Compensatory mechanisms, such as adoption, may buffer against certain physiological effects of maternal loss in this species.

Glucocorticoid exposure predicts survival in female baboons

Are differences in hypothalamic-pituitary-adrenal (HPA) axis activation across the adult life span linked to differences in survival? This question has been the subject of considerable debate. We analyze the link between survival and fecal glucocorticoid (GC) measures in a wild primate population, leveraging an unusually extensive longitudinal dataset of 14,173 GC measurements from 242 adult female baboons over 1634 female years. We document a powerful link between GCs and survival: Females with relatively high current GCs or high lifelong cumulative GCs face an elevated risk of death. A hypothetical female who maintained GCs in the top 90% for her age across adulthood would be expected to lose 5.4 years of life relative to a female who maintained GCs in the bottom 10% for her age. Hence, differences among individuals in HPA axis activity provide valuable prognostic information about disparities in life span.

A biologist's guide to model selection and causal inference

A goal of many research programmes in biology is to extract meaningful insights from large, complex datasets. Researchers in ecology, evolution and behavior (EEB) often grapple with long-term, observational datasets from which they construct models to test causal hypotheses about biological processes. Similarly, epidemiologists analyse large, complex observational datasets to understand the distribution and determinants of human health. A key difference in the analytical workflows for these two distinct areas of biology is the delineation of data analysis tasks and explicit use of causal directed acyclic graphs (DAGs), widely adopted by epidemiologists. Here, we review the most recent causal inference literature and describe an analytical workflow that has direct applications for EEB. We start this commentary by defining four distinct analytical tasks (description, prediction, association, causal inference). The remainder of the text is dedicated to causal inference, specifically focusing on the use of DAGs to inform the modelling strategy. Given the increasing interest in causal inference and misperceptions regarding this task, we seek to facilitate an exchange of ideas between disciplinary silos and provide an analytical framework that is particularly relevant for making causal inference from observational data.

Social bonds do not mediate the relationship between early adversity and adult glucocorticoids in wild baboons

In humans and other animals, harsh conditions in early life can have profound effects on adult physiology, including the stress response. This relationship may be mediated by a lack of supportive relationships in adulthood. That is, early life adversity may inhibit the formation of supportive social ties, and weak social support is itself often linked to dysregulated stress responses. Here, we use prospective, longitudinal data from wild baboons in Kenya to test the links between early adversity, adult social bonds, and adult fecal glucocorticoid hormone concentrations (a measure of hypothalamic-pituitary-adrenal [HPA] axis activation and the stress response). Using a causal inference framework, we found that experiencing one or more sources of early adversity led to a 9 to 14% increase in females' glucocorticoid concentrations across adulthood. However, these effects were not mediated by weak social bonds: The direct effects of early adversity on adult glucocorticoid concentrations were 11 times stronger than the effects mediated by social bonds. This pattern occurred, in part, because the effect of social bonds on glucocorticoids was weak compared to the powerful effects of early adversity on glucocorticoid levels in adulthood. Hence, in female baboons, weak social bonds in adulthood are not enough to explain the effects of early adversity on glucocorticoid concentrations. Together, our results support the well-established notions that early adversity and weak social bonds both predict poor adult health. However, the magnitudes of these two effects differ considerably, and they may act independently of one another.