Social history and exposure to pathogen signals modulate social status effects on gene regulation in rhesus macaques

A female-biased gene expression signature of dominance in cooperatively breeding meerkats

Dominance is a primary determinant of social dynamics and resource access in social animals. Recent studies show that dominance is also reflected in the gene regulatory profiles of peripheral immune cells. However, the strength and direction of this relationship differs across the species and sex combinations investigated, potentially due to variation in the predictors and energetic consequences of dominance status. Here, we investigated the association between social status and gene expression in the blood of wild meerkats (Suricata suricatta; n = 113 individuals), including in response to lipopolysaccharide, Gardiquimod (an agonist of TLR7, which detects single-stranded RNA in vivo) and glucocorticoid stimulation. Meerkats are cooperatively breeding social carnivores in which breeding females physically outcompete other females to suppress reproduction, resulting in high reproductive skew. They therefore present an opportunity to disentangle the effects of social dominance from those of sex per se. We identify a sex-specific signature of dominance, including 1045 differentially expressed genes in females but none in males. Dominant females exhibit elevated activity in innate immune pathways and a larger fold-change response to LPS challenge. Based on these results and a preliminary comparison to other mammals, we speculate that the gene regulatory signature of social status in the immune system depends on the determinants and energetic costs of social dominance, such that it is most pronounced in hierarchies where physical competition is important and reproductive skew is large. Such a pattern has the potential to mediate life history trade-offs between investment in reproduction versus somatic maintenance.

Immune cell composition varies by age, sex and exposure to social adversity in free-ranging Rhesus Macaques

Increasing age is associated with dysregulated immune function and increased inflammation-patterns that are also observed in individuals exposed to chronic social adversity. Yet we still know little about how social adversity impacts the immune system and how it might promote age-related diseases. Here, we investigated how immune cell diversity varied with age, sex and social adversity (operationalized as low social status) in free-ranging rhesus macaques. We found age-related signatures of immunosenescence, including lower proportions of CD20 + B cells, CD20 + /CD3 + ratio, and CD4 + /CD8 + T cell ratio - all signs of diminished antibody production. Age was associated with higher proportions of CD3 + /CD8 + Cytotoxic T cells, CD16 + /CD3- Natural Killer cells, CD3 + /CD4 + /CD25 + and CD3 + /CD8 + /CD25 + T cells, and CD14 + /CD16 + /HLA-DR + intermediate monocytes, and lower levels of CD14 + /CD16-/HLA-DR + classical monocytes, indicating greater amounts of inflammation and immune dysregulation. We also found a sex-dependent effect of exposure to social adversity (i.e., low social status). High-status males, relative to females, had higher CD20 + /CD3 + ratios and CD16 + /CD3 Natural Killer cell proportions, and lower proportions of CD8 + Cytotoxic T cells. Further, low-status females had higher proportions of cytotoxic T cells than high-status females, while the opposite was observed in males. High-status males had higher CD20 + /CD3 + ratios than low-status males. Together, our study identifies the strong age and sex-dependent effects of social adversity on immune cell proportions in a human-relevant primate model. Thus, these results provide novel insights into the combined effects of demography and social adversity on immunity and their potential contribution to age-related diseases in humans and other animals.

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.

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.

Demographic turnover can be a leading driver of hierarchy dynamics, and social inheritance modifies its effects

Individuals and societies are linked through a feedback loop of mutual influence. Demographic turnover shapes group composition and structure by adding and removing individuals, and social inheritance shapes social structure through the transmission of social traits from parents to offspring. Here I examine how these drivers of social structure feedback to influence individual outcomes. I explore these society-to-individual effects in systems with social inheritance of hierarchy position, as occur in many primates and spotted hyenas. Applying Markov chain models to empirical and simulated data reveals how demography and social inheritance interact to strongly shape individual hierarchy positions. In hyena societies, demographic processes-not status seeking-account for the majority of hierarchy dynamics and cause an on-average lifetime decline in social hierarchy position. Simulated societies clarify how social inheritance alters demographic effects-demographic processes cause hierarchy position to regress to the mean, but the addition of social inheritance modifies this pattern. Notably, the combination of social inheritance and rank-related reproductive success causes individuals to decline in rank over their lifespans, as seen in the hyena data. Further analyses explore how 'queens' escape this pattern of decline, and how variation in social inheritance generates variability in reproductive inequality. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Microglial synaptic pruning in the nucleus accumbens during adolescence sex-specifically influences splenic immune outcomes

Strong social support promotes a variety of positive health outcomes in humans and rodent models, while social isolation in rodents shortens lifespan, perceived social isolation (i.e. loneliness) can increase mortality by up to 50% in humans. How social relationships lead to these drastic health effects is unclear, but may involve modulation of the peripheral immune system. The reward circuitry of the brain and social behaviors undergo a critical period of development during adolescence. We published that microglia-mediated synaptic pruning occurs in the nucleus accumbens (NAc) reward region during adolescence to mediate social development in male and female rats. We hypothesized that if reward circuitry activity and social relationships directly impact the peripheral immune system, then natural developmental changes in the reward circuitry and social behaviors during adolescence should also directly impact the peripheral immune system. To test this, we inhibited microglial pruning in the NAc during adolescence, and then collected spleen tissue for mass spectrometry proteomic analysis and ELISA validation. We found that the global proteomic consequences of inhibiting microglial pruning in the NAc were similar between the sexes, but target-specific examination suggests that NAc pruning impacts Th1 cell-related immune markers in the spleen in males, but not females, and broad neurochemical systems in the spleen in females, but not males.

Distinct immune and transcriptomic profiles in dominant versus subordinate males in mouse social hierarchies

Social status is a critical factor determining health outcomes in human and nonhuman social species. In social hierarchies with reproductive skew, individuals compete to monopolize resources and increase mating opportunities. This can come at a significant energetic cost leading to trade-offs between different physiological systems. In particular, changes in energetic investment in the immune system can have significant short and long-term effects on fitness and health. We have previously found that dominant alpha male mice living in social hierarchies have increased metabolic demands related to territorial defense. In this study, we tested the hypothesis that high-ranking male mice favor adaptive immunity, while subordinate mice show higher investment in innate immunity. We housed 12 groups of 10 outbred CD-1 male mice in a social housing system. All formed linear social hierarchies and subordinate mice had higher concentrations of plasma corticosterone (CORT) than alpha males. This difference was heightened in highly despotic hierarchies. Using flow cytometry, we found that dominant status was associated with a significant shift in immunophenotypes towards favoring adaptive versus innate immunity. Using Tag-Seq to profile hepatic and splenic transcriptomes of alpha and subordinate males, we identified genes that regulate metabolic and immune defense pathways that are associated with status and/or CORT concentration. In the liver, dominant animals showed a relatively higher expression of specific genes involved in major urinary production and catabolic processes, whereas subordinate animals showed relatively higher expression of genes promoting biosynthetic processes, wound healing, and proinflammatory responses. In spleen, subordinate mice showed relatively higher expression of genes facilitating oxidative phosphorylation and DNA repair and CORT was negatively associated with genes involved in lymphocyte proliferation and activation. Together, our findings suggest that dominant and subordinate animals adaptively shift immune profiles and peripheral gene expression to match their contextual needs.

Agonism and grooming behaviour explain social status effects on physiology and gene regulation in rhesus macaques

Variation in social status predicts molecular, physiological and life-history outcomes across a broad range of species, including our own. Experimental studies indicate that some of these relationships persist even when the physical environment is held constant. Here, we draw on datasets from one such study-experimental manipulation of dominance rank in captive female rhesus macaques-to investigate how social status shapes the lived experience of these animals to alter gene regulation, glucocorticoid physiology and mitochondrial DNA phenotypes. We focus specifically on dominance rank-associated dimensions of the social environment, including both competitive and affiliative interactions. Our results show that simple summaries of rank-associated behavioural interactions are often better predictors of molecular and physiological outcomes than dominance rank itself. However, while measures of immune function are best explained by agonism rates, glucocorticoid-related phenotypes tend to be more closely linked to affiliative behaviour. We conclude that dominance rank serves as a useful summary for investigating social environmental effects on downstream outcomes. Nevertheless, the behavioural interactions that define an individual's daily experiences reveal the proximate drivers of social status-related differences and are especially relevant for understanding why individuals who share the same social status sometimes appear physiologically distinct. 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'.

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'.

The dynamics of dominance: open questions, challenges and solutions

Although social hierarchies are recognized as dynamic systems, they are typically treated as static entities for practical reasons. Here, we ask what we can learn from a dynamical view of dominance, and provide a research agenda for the next decades. We identify five broad questions at the individual, dyadic and group levels, exploring the causes and consequences of individual changes in rank, the dynamics underlying dyadic dominance relationships, and the origins and impacts of social instability. Although challenges remain, we propose avenues for overcoming them. We suggest distinguishing between different types of social mobility to provide conceptual clarity about hierarchy dynamics at the individual level, and emphasize the need to explore how these dynamic processes produce dominance trajectories over individual lifespans and impact selection on status-seeking behaviour. At the dyadic level, there is scope for deeper exploration of decision-making processes leading to observed interactions, and how stable but malleable relationships emerge from these interactions. Across scales, model systems where rank is manipulable will be extremely useful for testing hypotheses about dominance dynamics. Long-term individual-based studies will also be critical for understanding the impact of rare events, and for interrogating dynamics that unfold over lifetimes and generations. 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'.

Holistic Rehabilitation: Biological Embedding of Social Adversity and Its Health Implications

Human health is affected by lived experiences, both past and present. The environments we encounter throughout our lives, therefore, shape how we respond to new challenges, how we maintain a healthy immune system, and even how we respond to treatment and rehabilitation. Early in life and throughout adulthood, social experiences-such as exposure to various forms of adversity-can alter how cells in our body function, with far-reaching consequences for human health, disease, and treatment. This Perspective highlights studies from an ever-growing body of literature on the social determinants of health, with a focus on exposure to social adversities, such as social isolation, discrimination, or low social status, experienced both early in life and adulthood and how they variably impact health. By focusing on recent observational studies in humans and experimental studies on social nonhuman animals, this article details how social adversity can become biologically embedded in our cells at the molecular level. Given that humans are social animals, it is no surprise that social adversity can negatively impact our health, and experimental animal studies have helped us to uncover some of the causal mechanistic pathways underlying the link between social adversity and health outcomes. These molecular consequences can have far-reaching implications and, when combined with our growing knowledge on the social determinants of health, should inform how we approach treatment and rehabilitation.

Sensitive periods during the development and expression of vertebrate sexual signals: A systematic review

Many sexually selected traits exhibit phenotypic plasticity. Despite a growing appreciation for the ecological context in which sexual selection occurs, and for the role of plasticity in shaping traits associated with local adaptation and divergence, there is an important gap in knowledge about the onset and duration of plasticity in sexual trait expression. Integrating this temporal dimension of plasticity into models of sexual selection informs our understanding of the information conveyed by sexual traits and our predictions related to trait evolution, and is critical in this time of unprecedented and rapid environmental change. We conducted a systematic review of 869 studies to ask how trait modalities (e.g., visual and chemical) relate to the onset and duration of plasticity in vertebrate sexual signals. We show that this literature is dominated by studies of coloration in birds and fish, and most studies take place during the breeding season. Where possible, we integrate results across studies to link physiology of specific trait modalities with the life stage (e.g., juvenile, breeding, or nonbreeding) during which plasticity occurs in well-studied traits. Limitations of our review included a lack of replication in our dataset, which precluded formal analysis. We argue that the timing of trait plasticity, in addition to environmental context, is critical for determining whether and how various communication signals are associated with ecological context, because plasticity may be ongoing or occur at only one point in an individual's lifetime, and determining a fixed trajectory of trait expression. We advocate for careful consideration of the onset and duration of plasticity when analyzing how environmental variation affects sexual trait expression and associated evolutionary outcomes.

The Immunology of Stress and the Impact of Inflammation on the Brain and Behavior

Exposure to acute versus chronic stressors and threats activates the immune system in adaptive and maladaptive manners, respectively. While acute activation of the immune system in response to threat is homeostatically regulated by glucocorticoid negative feedback, chronic activation of the immune system arising from persistent stress exposure can contribute to an allostatic load with an inflammatory diathesis that has been implicated in stress-related psychopathology, including of depression and anxiety. Increased inflammation in the periphery and in the brain arising from chronic stress exposure can alter neurotransmitter metabolism and impact activation of brain regions to increase adverse behavioral health symptoms (e.g. anhedonia, anxiety, fatigue) and emotion dysregulation. While interventions targeting the immune system and its downstream effects on the brain for the treatment of depression and other psychiatric disorders has been of great interest as they have shown some efficacy in treating stress-related behavioral health disorders, future studies are necessary to better characterize the contexts under which anti-inflammatory agents should be used to treat stress-related psychopathology.

Where Are the Disease-Associated eQTLs?

Most disease-associated variants, although located in putatively regulatory regions, do not have detectable effects on gene expression. One explanation could be that we have not examined gene expression in the cell types or conditions that are most relevant for disease. Even large-scale efforts to study gene expression across tissues are limited to human samples obtained opportunistically or postmortem, mostly from adults. In this review we evaluate recent findings and suggest an alternative strategy, drawing on the dynamic and highly context-specific nature of gene regulation. We discuss new technologies that can extend the standard regulatory mapping framework to more diverse, disease-relevant cell types and states.

Sickness and the Social Brain: Love in the Time of COVID

As a highly social species, inclusion in social networks and the presence of strong social bonds are critical to our health and well-being. Indeed, impaired social functioning is a component of numerous neuropsychiatric disorders including depression, anxiety, and substance use disorder. During the current COVID-19 pandemic, our social networks are at risk of fracture and many are vulnerable to the negative consequences of social isolation. Importantly, infection itself leads to changes in social behavior as a component of "sickness behavior." Furthermore, as in the case of COVID-19, males and females often differ in their immunological response to infection, and, therefore, in their susceptibility to negative outcomes. In this review, we discuss the many ways in which infection changes social behavior-sometimes to the benefit of the host, and in some instances for the sake of the pathogen-in species ranging from eusocial insects to humans. We also explore the neuroimmune mechanisms by which these changes in social behavior occur. Finally, we touch upon the ways in which the social environment (group living, social isolation, etc.) shapes the immune system and its ability to respond to challenge. Throughout we emphasize how males and females differ in their response to immune activation, both behaviorally and physiologically.

SARS-CoV-2 morbidity and mortality in racial/ethnic minority populations: A window into the stress related inflammatory basis of health disparities?

Health disparity related to race/ethnicity has been cited as “the most serious and shameful health care issue of our time”(Peterson et al., 2018). A portion of the now recognized disproportionate impact of the COVID-19 pandemic among Black, Indigenous and People of Color (BIPOC) communities is attributable to social determinants such as socioeconomic status (SES), physical living situation, health care access, and the psychosocial factors associated with socioenvironmental circumstances such as bias, victimization, trauma and toxic stress as well as structural factors that reduce the capacity to practice physical distancing (Agurs-Collins et al., 2019).

In this paper, we hypothesize that, prior to the COVID-19 pandemic, disproportionate socio-economic and environmental stressors in the BIPOC population promoted heightened stress-associated neurobiological activity (Stress-NbA). This chronic elevation in Stress-NbA results in down-stream complications of chronic stress including underactivation of anti-viral type I IFN pathway genes. This results in an increase in susceptibility to viral diseases, including coronavirus illnesses. Additionally, Stress-NbA chronically potentiates systemic inflammation (from hematopoietic system activation with myelopoiesis) increasing the prevalence of metabolic syndrome (MetS) and setting the stage for stress-related chronic non-communicable diseases (NCDs).

This process was propelled by overactivation of immune cell gene expression in the nuclear factor κ-light-chain-enhancer of activated B cells (NF-kB) activation pathway and underactivation of gene expression in the anti-viral type I interferon (IFN) pathway. The higher prevalence of MetS and NCDs in minority populations turned out to be predictive of the elevated risk they would face in the presence of a highly contagious viral pandemic. The stress-related generation of a chronic non-pathogen associated molecular pattern (non-PAMP) immunoactivation state led to decreased viral immune defense and increased susceptibility to SARS-CoV-2 infection with increased risk of severe illness induced by cytokine storm syndrome (CSS).

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There is a disproportionate impact of the COVID-19 pandemic among Black, Indigenous and People of Color (BIPOC) communities.

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Prior to the COVID-19 pandemic and during the pandemic, existing disproportionate structural, socio-economic and environmental stressors in the BIPOC communities may have resulted in heightened stress-associated neurobiological activity (Stress-NbA).

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In this paper, we hypothesize that a combination of chronic elevation of stress-NbA, systemic inflammation, stress response and immune response related factors aligned against BIPIC communities are potential drivers of excess morbidity and mortality.

Biological embedding of experience: A primer on epigenetics

Biological embedding occurs when life experience alters biological processes to affect later life health and well-being. Although extensive correlative data exist supporting the notion that epigenetic mechanisms such as DNA methylation underlie biological embedding, causal data are lacking. We describe specific epigenetic mechanisms and their potential roles in the biological embedding of experience. We also consider the nuanced relationships between the genome, the epigenome, and gene expression. Our ability to connect biological embedding to the epigenetic landscape in its complexity is challenging and complicated by the influence of multiple factors. These include cell type, age, the timing of experience, sex, and DNA sequence. Recent advances in molecular profiling and epigenome editing, combined with the use of comparative animal and human longitudinal studies, should enable this field to transition from correlative to causal analyses.

Genes and environments, development and time

A now substantial body of science implicates a dynamic interplay between genetic and environmental variation in the development of individual differences in behavior and health. Such outcomes are affected by molecular, often epigenetic, processes involving gene-environment (G-E) interplay that can influence gene expression. Early environments with exposures to poverty, chronic adversities, and acutely stressful events have been linked to maladaptive development and compromised health and behavior. Genetic differences can impart either enhanced or blunted susceptibility to the effects of such pathogenic environments. However, largely missing from present discourse regarding G-E interplay is the role of time, a "third factor" guiding the emergence of complex developmental endpoints across different scales of time. Trajectories of development increasingly appear best accounted for by a complex, dynamic interchange among the highly linked elements of genes, contexts, and time at multiple scales, including neurobiological (minutes to milliseconds), genomic (hours to minutes), developmental (years and months), and evolutionary (centuries and millennia) time. This special issue of PNAS thus explores time and timing among G-E transactions: The importance of timing and timescales in plasticity and critical periods of brain development; epigenetics and the molecular underpinnings of biologically embedded experience; the encoding of experience across time and biological levels of organization; and gene-regulatory networks in behavior and development and their linkages to neuronal networks. Taken together, the collection of papers offers perspectives on how G-E interplay operates contingently within and against a backdrop of time and timescales.

Rhesus macaques as a tractable physiological model of human ageing

Research in the basic biology of ageing is increasingly identifying mechanisms and modifiers of ageing in short-lived organisms such as worms and mice. The ultimate goal of such work is to improve human health, particularly in the growing segment of the population surviving into old age. Thus far, few interventions have robustly transcended species boundaries in the laboratory, suggesting that changes in approach are needed to avoid costly failures in translational human research. In this review, we discuss both well-established and alternative model organisms for ageing research and outline how research in nonhuman primates is sorely needed, first, to translate findings from short-lived organisms to humans, and second, to understand key aspects of ageing that are unique to primate biology. We focus on rhesus macaques as a particularly promising model organism for ageing research owing to their social and physiological similarity to humans as well as the existence of key resources that have been developed for this species. As a case study, we compare gene regulatory signatures of ageing in the peripheral immune system between humans and rhesus macaques from a free-ranging study population in Cayo Santiago. We show that both mRNA expression and DNA methylation signatures of immune ageing are broadly shared between macaques and humans, indicating strong conservation of the trajectory of ageing in the immune system. We conclude with a review of key issues in the biology of ageing for which macaques and other nonhuman primates may uniquely contribute valuable insights, including the effects of social gradients on health and ageing. We anticipate that continuing research in rhesus macaques and other nonhuman primates will play a critical role in conjunction with the model organism and human biodemographic research in ultimately improving translational outcomes and extending health and longevity in our ageing population. This article is part of the theme issue 'Evolution of the primate ageing process'.

Prime time for primate functional genomics

Functional genomics research is continually improving our understanding of genotype-phenotype relationships in humans, and comparative genomics perspectives can provide additional insight into the evolutionary histories of such relationships. To specifically identify conservation or species-specific divergence in humans, we must look to our closest extant evolutionary relatives. Primate functional genomics research has been steadily advancing and expanding, in spite of several limitations and challenges that this field faces. New technologies and cheaper sequencing provide a unique opportunity to enhance and expand primate comparative studies, and we outline possible paths going forward. The potential human-specific insights that can be gained from primate functional genomics research are substantial, and we propose that now is a prime time to expand such endeavors.

Broadening primate genomics: new insights into the ecology and evolution of primate gene regulation

Comparative analyses have played a key role in understanding how gene regulatory evolution contributes to primate phenotypic diversity. Recently, these studies have expanded to include a wider range of species, within-population as well as interspecific analyses, and research on wild as well as captive individuals. This expansion provides context for understanding genetic and environmental effects on gene regulation in humans, including the importance of the pathogen and social environments. Although taxonomic representation remains biased, inclusion of more species has also begun to reveal the evolutionary processes that explain whether and when gene regulation is conserved. Together, this work highlights how studies in other primates contribute to understanding evolution in our own lineage, and we conclude by identifying promising avenues for future work.

Social determinants of health and survival in humans and other animals

The social environment, both in early life and adulthood, is one of the strongest predictors of morbidity and mortality risk in humans. Evidence from long-term studies of other social mammals indicates that this relationship is similar across many species. In addition, experimental studies show that social interactions can causally alter animal physiology, disease risk, and life span itself. These findings highlight the importance of the social environment to health and mortality as well as Darwinian fitness-outcomes of interest to social scientists and biologists alike. They thus emphasize the utility of cross-species analysis for understanding the predictors of, and mechanisms underlying, social gradients in health.

Juvenile rank acquisition is associated with fitness independent of adult rank

Social rank is a significant determinant of fitness in a variety of species. The importance of social rank suggests that the process by which juveniles come to establish their position in the social hierarchy is a critical component of development. Here, we use the highly predictable process of rank acquisition in spotted hyenas to study the consequences of variation in rank acquisition in early life. In spotted hyenas, rank is 'inherited' through a learning process called 'maternal rank inheritance.' This pattern is very consistent: approximately 80% of juveniles acquire the exact rank expected under the rules of maternal rank inheritance. The predictable nature of rank acquisition in these societies allows the process of rank acquisition to be studied independently from the ultimate rank that each juvenile attains. In this study, we use Elo-deviance scores, a novel application of the Elo-rating method, to calculate each juvenile's deviation from the expected pattern of maternal rank inheritance during development. Despite variability in rank acquisition among juveniles, most of these juveniles come to attain the exact rank expected of them according to the rules of maternal rank inheritance. Nevertheless, we find that transient variation in rank acquisition in early life is associated with long-term fitness consequences for these individuals: juveniles 'underperforming' their expected ranks show reduced survival and lower lifetime reproductive success than better-performing peers, and this relationship is independent of both maternal rank and rank achieved in adulthood. We also find that multiple sources of early life adversity have cumulative, but not compounding, effects on fitness. Future work is needed to determine if variation in rank acquisition directly affects fitness, or if some other variable, such as maternal investment or juvenile condition, causes variation in both of these outcomes.

Social history and exposure to pathogen signals modulate social status effects on gene regulation in rhesus macaques

Social experience is an important predictor of disease susceptibility and survival in humans and other social mammals. Chronic social stress is thought to generate a proinflammatory state characterized by elevated antibacterial defenses and reduced investment in antiviral defense. Here we manipulated long-term social status in female rhesus macaques to show that social subordination alters the gene expression response to ex vivo bacterial and viral challenge. As predicted by current models, bacterial lipopolysaccharide polarizes the immune response such that low status corresponds to higher expression of genes in NF-κB-dependent proinflammatory pathways and lower expression of genes involved in the antiviral response and type I IFN signaling. Counter to predictions, however, low status drives more exaggerated expression of both NF-κB- and IFN-associated genes after cells are exposed to the viral mimic Gardiquimod. Status-driven gene expression patterns are linked not only to social status at the time of sampling, but also to social history (i.e., past social status), especially in unstimulated cells. However, for a subset of genes, we observed interaction effects in which females who fell in rank were more strongly affected by current social status than those who climbed the social hierarchy. Taken together, our results indicate that the effects of social status on immune cell gene expression depend on pathogen exposure, pathogen type, and social history-in support of social experience-mediated biological embedding in adulthood, even in the conventionally memory-less innate immune system.

Social Status and Gene Regulation: Conservation and Context Dependence in Primates

Current models suggest that low social status affects immune function by increasing inflammation and compromising antiviral defense. While this pattern appears to be somewhat conserved, recent studies argue that the gene regulatory signature of social status also depends on the local environment and the nature of social hierarchies.