The natural history of model organisms: the rhesus macaque as a success story of the Anthropocene

Food-washing monkeys recognize the law of diminishing returns

Few animals have the cognitive faculties or prehensile abilities needed to eliminate tooth-damaging grit from food surfaces. Some populations of monkeys wash sand from foods when standing water is readily accessible, but this propensity varies within groups for reasons unknown. Spontaneous food-washing emerged recently in a group of long-tailed macaques (Macaca fascicularis) inhabiting Koram Island, Thailand, and it motivated us to explore the factors that drive individual variability. We measured the mineral and physical properties of contaminant sands and conducted a field experiment, eliciting 1282 food-handling bouts by 42 monkeys. Our results verify two long-standing presumptions: monkeys have a strong aversion to sand and removing it is intentional. Reinforcing this result, we found that monkeys clean foods beyond the point of diminishing returns, a suboptimal behavior that varied with social rank. Dominant monkeys abstained from washing, a choice consistent with the impulses of dominant monkeys elsewhere: to prioritize rapid food intake and greater reproductive fitness over the long-term benefits of prolonging tooth function.

Pigtail Macaque (Macaca nemestrina) Performance Differs Across Multiple Cognitive Domains in Comparison to Rhesus Macaques (Macaca mulatta)

Rhesus and pigtail macaques are closely related and have similar social structures, yet differences in their behavior, socio-ecology, and personality have been observed, although not systematically documented. Given these differences, it is important to assess pigtail macaque cognition independently, rather than relying on rhesus macaque findings as a proxy. To gain a better understanding of pigtail macaque cognition, we used a battery of three cognitive tasks. Rhesus macaques were tested on the same tasks to validate our methods and to allow for comparison. Across just three tasks, we found significant differences between the two closely related species. In the three cups task, which tests short-term memory, both pigtail and rhesus macaques performed significantly better when they had to recall the location of a hidden food reward after a 0 s delay compared to a 15 s delay. However, in the 15 s delay condition, only rhesus macaques performed above chance levels, whereas pigtail macaques did not. In the reversal learning task, which tested rule learning and cognitive flexibility, we found species differences in learning performance. For the quantity discrimination task, which tests numerosity, we found that both rhesus and pigtail macaques were more accurate at discriminating "easy" ratios of foods (e.g., 1 vs. 5 or 2 vs. 6) than the "hard" ratios (e.g., 2 vs. 3 or 4 vs. 5). However, pigtail macaques were more accurate than rhesus macaques in the hard ratio trials. These contribute to a novel understanding of cognition in pigtail macaques while also increasing research rigor in translational research.

Integrated analysis of the complete sequence of a macaque genome

The crab-eating macaques (Macaca fascicularis) and rhesus macaques (Macaca mulatta) are pivotal in biomedical and evolutionary research1-3. However, their genomic complexity and interspecies genetic differences remain unclear4. Here, we present a complete genome assembly of a crab-eating macaque, revealing 46% fewer segmental duplications and 3.83 times longer centromeres than those of humans5,6. We also characterize 93 large-scale genomic differences between macaques and humans at a single-base-pair resolution, highlighting their impact on gene regulation in primate evolution. Using ten long-read macaque genomes, hundreds of short-read macaque genomes and full-length transcriptome data, we identified roughly 2 Mbp of fixed-genetic variants, roughly 240 Mbp of complex loci, 16.76 Mbp genetic differentiation regions and 110 alternative splice events, potentially associated with various phenotypic differences between the two macaque species. In summary, the integrated genetic analysis enhances understanding of lineage-specific phenotypes, adaptation and primate evolution, thereby improving their biomedical applications in human disease research.

MacaqueNet: Advancing comparative behavioural research through large-scale collaboration

There is a vast and ever-accumulating amount of behavioural data on individually recognised animals, an incredible resource to shed light on the ecological and evolutionary drivers of variation in animal behaviour. Yet, the full potential of such data lies in comparative research across taxa with distinct life histories and ecologies. Substantial challenges impede systematic comparisons, one of which is the lack of persistent, accessible and standardised databases. Big-team approaches to building standardised databases offer a solution to facilitating reliable cross-species comparisons. By sharing both data and expertise among researchers, these approaches ensure that valuable data, which might otherwise go unused, become easier to discover, repurpose and synthesise. Additionally, such large-scale collaborations promote a culture of sharing within the research community, incentivising researchers to contribute their data by ensuring their interests are considered through clear sharing guidelines. Active communication with the data contributors during the standardisation process also helps avoid misinterpretation of the data, ultimately improving the reliability of comparative databases. Here, we introduce MacaqueNet, a global collaboration of over 100 researchers (https://macaquenet.github.io/) aimed at unlocking the wealth of cross-species data for research on macaque social behaviour. The MacaqueNet database encompasses data from 1981 to the present on 61 populations across 14 species and is the first publicly searchable and standardised database on affiliative and agonistic animal social behaviour. We describe the establishment of MacaqueNet, from the steps we took to start a large-scale collective, to the creation of a cross-species collaborative database and the implementation of data entry and retrieval protocols. We share MacaqueNet's component resources: an R package for data standardisation, website code, the relational database structure, a glossary and data sharing terms of use. With all these components openly accessible, MacaqueNet can act as a fully replicable template for future endeavours establishing large-scale collaborative comparative databases.

Personality Assessment of Synanthropic Rhesus Macaques: Implications and Challenges

Answers to the question of what characteristics allow animals to thrive in human-dominated environments remain elusive. Consistent interindividual differences or personalities can potentially explain the functional significance of habitat-specific traits that enable animals to coexist with humans. Rhesus macaques (Macaca mulatta) are the most successful nonhuman primates in the Anthropocene, living in diverse climatic and environmental conditions. Studying the personalities of synanthropic rhesus macaques, that is, those that thrive in anthropogenic habitats, can provide insights into the biological traits facilitating their success. We planned a multi-method "bottom-up" approach of behavioral observations and novelty experiments, standardized for assessing captive nonhuman primates, to evaluate the personalities of adult rhesus macaques (N = 52). Novelty experiments encountered significant challenges, limiting their effectiveness. So, we continued with behavioral observations in the form of focal sampling, that revealed two repeatable traits, subjectively labeled as social tension and meekness. We found an association of sex with social tension, where males exhibited higher social tension than females. In an additional analysis, we found that individuals that obtained food through contact provisioning had higher scores for the meekness trait than individuals that obtained food through noncontact provisioning. We discuss how the observed personality traits may offer adaptive advantages in human-dominated environments, where despotic rhesus macaques face both benefits and costs (including social) of living in an anthropogenic setting. We also emphasize that protocols designed for captive conditions may not be directly applicable to free-living animals. The study underscores the need to reconsider behavioral experiments to obtain comparable measures between captive and non-captive populations. This would enhance the ecological validity of personality assessments. Nevertheless, empirically identifying traits using observations in synanthropic species can provide valuable insights into the mechanisms that enable certain animals to thrive amidst a rapid expansion of anthropogenic activities.

Linking individual variation in facial musculature to facial behavior in rhesus macaques

Facial expression is a key component of primate communication, and primates (including humans) have a complex system of facial musculature underpinning this behavior. Human facial musculature is highly variable across individuals, but to date, whether other primate species exhibit a similar level of inter-individual variation is unknown. Whether individual-level variation in facial musculature covaries with significant differences in facial movement within the same individual is also unknown. Here, we use facial dissection data from 31 adult rhesus macaques, the largest sample to date, to quantify inter-individual variation in facial muscle presence. We used a subsample of eight individuals to measure covariation between facial muscle presence and the presence of external facial movements (action units in the Facial Action Coding System, or FACS). We found, in contrast to humans, limited inter-individual variation in muscle presence, but the zygomatic region exhibited more gross anatomical variation in muscle presence and morphology than any other region of the macaque face. We also found a good correspondence between facial muscle presence and the presence of the associated action units. Our results indicate that the observed variation in rhesus macaque facial expressivity is not likely driven primarily by variation in facial muscle presence but may instead be due to other factors such as learned behavior and/or physiological differences. These findings provide insight into the anatomical basis of inter-individual variation in facial behavior in primates and suggest potential differences in variation between humans and other primate species.

A Macaque Model for the Effects of Hybridization on Body Size

OBJECTIVES

Genomics research has uncovered recurrent hybridization between hominin species, yet its morphological impact remains understudied. Non-human primate research has suggested a morphological signature of hybrid ancestry, which could be used to identify hybrids in the hominin fossil record. This pattern may include extreme size, heightened variation, and markers of developmental instability, but factors affecting these characteristics are poorly understood. Studies of non-mammalian taxa suggest that extreme morphology is more likely in early-generation hybrids and with a greater parental distance. To understand hybridization in hominins, therefore, we must use appropriate proxy taxa.

MATERIALS AND METHODS

Here, we use Chinese × Indian Macaca mulatta hybrids with a comparable divergence time in generations to Homo sapiens/Neanderthals and wide variation in admixture. Measuring limb lengths, body length, and weight, we investigate the relationship between admixture and size/variation.

RESULTS

Compared to previous work with more phylogenetically distant primate taxa and a focus on early generation hybrids, we found no evidence of a relationship between admixture and extreme large size, nor with increased size variation. Hybrids in our sample are relatively small but within the range of variation of the smaller parental taxon.

CONCLUSIONS

Our results suggest that hybridization between closely related taxa, such as Neanderthals and H. sapiens, may lead to more subtle morphological patterns than previously anticipated. It will be necessary, however, to better understand the factors governing primate hybrid morphology before we can produce robust inferences on how hybridization has affected hominin evolution.

The Rhesus Macaque as an Animal Model for Human Nutrition: An Ecological-Evolutionary Perspective

Nutrition is a complex and contested area in biomedicine, which requires diverse evidence sources. Nonhuman primate models are considered an important biomedical research tool because of their biological similarities to humans, but they are typically used with little explicit consideration of their ecology and evolution. Using the rhesus macaque (RM), we consider the potential of nutritional ecology for enriching the use of primates as models for human nutrition. We introduce some relevant aspects of RM evolutionary and social ecology and discuss two examples where they have been used in biomedical research: obesity and aging. We next consider how insights from nutritional ecology can help inform and direct the use of RM as a biomedical model. We conclude by illustrating how conceptual tools might inform the use of RM as a model for human nutrition and extracting insights from RM that might be relevant to broader theoretical considerations around animal model systems.

Associations between social behaviour and proinflammatory immune activation are modulated by age in a free-ranging primate population

The effect of the social environment on the proinflammatory immune response may mediate the relationship between social environment and fitness but remains understudied outside captive animals and human populations. Age can also influence both immune function and social behaviour, and hence may modulate their relationships. This study investigates the role of social interactions in driving the concentrations of two urinary markers of proinflammatory immune activation, neopterin and soluble urokinase plasminogen activator receptor (suPAR), in a free-ranging population of rhesus macaques, Macaca mulatta. We collected 854 urine samples from 172 adult monkeys and quantified how urinary suPAR and neopterin concentrations were related to affiliative behaviour and agonistic behaviour received over 60 days. In females, but not in males, higher rates of affiliative interactions were associated with lower neopterin concentrations, while conversely, experiencing more agonistic interactions was associated with higher neopterin concentrations. The association between affiliation and neopterin concentration was modulated by age, with older females experiencing a stronger negative association between affiliative behaviour and neopterin concentration. There were no associations between suPAR concentration and social environment for either sex. This study demonstrates that proinflammatory immune activity is a potential mechanism mediating the association between social environment and fitness under naturalistic conditions and that age can be an important modulator of the effect of social environment on the immune system.

The Influence of Provisioning on the Intergroup Relationships of Rhesus Macaque in Hainan, China

Intergroup competition for limited resources is a significant selection pressure that drives the evolution of animal society. The rhesus macaque (Macaca Mulatta) is the most widely distributed nonhuman primate in the world and can adapt well to environments disturbed by humans. In some areas, human provisioning provides ample food resources for rhesus macaques, leading to an increase in their population size, inevitably affecting competition patterns within and between groups. In this study, we focused on seven provisioned groups of rhesus macaque in an eco-tourism park in Hainan, China, to verify how provisioning impacted their intergroup relationships. The results showed that: (1) Peaceful coexistence was the most common form of Intergroup contacts; (2) Provisioning led to an increase in intergroup contact and conflicts, but monkeys tended to avoid direct contact with other groups at main-provisioned sites with high conflict risk. (3) Larger groups did not interfere with each other's space use in the park, but smaller groups were more easily tolerated by other groups. (4) There were no strict linear dominance relationships among monkey groups. Overall, intensive provisioning satisfied the energy requirement of all monkeys in our study site, leading to a reduction in the relative benefit of intergroup conflict. Consequently, monkeys have adopted an intergroup contact strategy that avoids direct conflicts and prevents conflict escalation. We should pay more attention to the behavior patterns of provisioned animal populations, which will help us better understand how resources such as food have influenced the evolution of social strategies of animal groups, as well as how to manage such human disturbed animal populations in the future.

Social ageing can protect against infectious disease in a group-living primate

The benefits of social living are well established, but sociality also comes with costs, including infectious disease risk. This cost-benefit ratio of sociality is expected to change across individuals' lifespans, which may drive changes in social behaviour with age. To explore this idea, we combine data from a group-living primate for which social ageing has been described with epidemiological models to show that having lower social connectedness when older can protect against the costs of a hypothetical, directly transmitted endemic pathogen. Assuming no age differences in epidemiological characteristics (susceptibility to, severity and duration of infection), older individuals suffered lower infection costs, which was explained largely because they were less connected in their social networks than younger individuals. This benefit of 'social ageing' depended on epidemiological characteristics and was greatest when infection severity increased with age. When infection duration increased with age, social ageing was beneficial only when pathogen transmissibility was low. Older individuals benefited most from having a lower frequency of interactions (strength) and network embeddedness (closeness) and benefited less from having fewer social partners (degree). Our study provides a first examination of the epidemiology of social ageing, demonstrating the potential for pathogens to influence the evolutionary dynamics of social ageing in natural populations.This article is part of the discussion meeting issue 'Understanding age and society using natural populations'.

Social status as a latent variable in the amygdala of observers of social interactions

Successful integration into a hierarchical social group requires knowledge of the status of each individual and of the rules that govern social interactions within the group. In species that lack morphological indicators of status, social status can be inferred by observing the signals exchanged between individuals. We simulated social interactions between macaques by juxtaposing videos of aggressive and appeasing displays, where two individuals appeared in each other's line of sight and their displays were timed to suggest the reciprocation of dominant and subordinate signals. Viewers of these videos successfully inferred the social status of the interacting characters. Dominant individuals attracted more social attention from viewers even when they were not engaged in social displays. Neurons in the viewers' amygdala signaled the status of both the attended (fixated) and the unattended individuals, suggesting that in third-party observers of social interactions, the amygdala jointly signals the status of interacting parties.

Genetic Architecture of Immune Cell DNA Methylation in the Rhesus Macaque

Genetic variation that impacts gene regulation, rather than protein function, can have strong effects on trait variation both within and between species. Epigenetic mechanisms, such as DNA methylation, are often an important intermediate link between genotype and phenotype, yet genetic effects on DNA methylation remain understudied in natural populations. To address this gap, we used reduced representation bisulfite sequencing to measure DNA methylation levels at 555,856 CpGs in peripheral whole blood of 573 samples collected from free-ranging rhesus macaques (Macaca mulatta) living on the island of Cayo Santiago, Puerto Rico. We used allele-specific methods to map cis-methylation quantitative trait loci (meQTL) and tested for effects of 243,389 single nucleotide polymorphisms (SNPs) on local DNA methylation levels. Of 776,092 tested SNP-CpG pairs, we identified 516,213 meQTL, with 69.12% of CpGs having at least one meQTL (FDR < 5%). On average, meQTL explained 21.2% of nearby methylation variance, significantly more than age or sex. meQTL were enriched in genomic compartments where methylation is likely to impact gene expression, for example, promoters, enhancers and binding sites for methylation-sensitive transcription factors. In support, using mRNA-seq data from 172 samples, we confirmed 332 meQTL as whole blood cis-expression QTL (eQTL) in the population, and found meQTL-eQTL genes were enriched for immune response functions, like antigen presentation and inflammation. Overall, our study takes an important step towards understanding the genetic architecture of DNA methylation in natural populations, and more generally points to the biological mechanisms driving phenotypic variation in our close relatives.

Shared Accountability Shaping the Destinies of Individual and Groups of Nonhuman Primates

At the California National Primate Research Center (CNPRC), the preferred housing for rhesus macaques involves maintaining them in complex social groups outdoors, primarily for breeding purposes. This functionally appropriate environment promotes effective coping through the expression of species-typical behaviors and important aspects of species-typical social structure, thus enabling normal animal development, higher reproductive success, and the production of high-quality biological models. Despite the benefits, social housing introduces challenges like trauma from aggressive interactions. These challenges necessitate a delicate balance between tolerating some aggression and preventing repeated targeting of individuals. Therefore, the CNPRC has established a multidisciplinary working group of behavioral management experts, veterinarians, animal care, and researchers that meets regularly to review cases of animals that may need to be removed from their social group. We discuss the criteria and decision-making processes employed to manage and mitigate aggression. We describe the systematic approach to identifying at-risk individuals and the comprehensive evaluation process that guides whether to relocate an animal from their groups or not. Considerations include the welfare of the individual and the group's social stability. This paper provides case studies demonstrating how the working group applies these criteria and processes in practical scenarios, highlighting the complexities and challenges of such decisions.

Social Status as a Latent Variable in the Amygdala of Observers of Social Interactions

Successful integration into a hierarchical social group requires knowledge of the status of each individual and of the rules that govern social interactions within the group. In species that lack morphological indicators of status, social status can be inferred by observing the signals exchanged between individuals. We simulated social interactions between macaques by juxtaposing videos of aggressive and appeasing displays where two individuals appeared in each other's line of sight and their displays were timed to suggest the reciprocation of dominant and subordinate signals. Viewers of these videos successfully inferred the social status of the interacting characters. Dominant individuals attracted more social attention from viewers even when they were not engaged in social displays. Neurons in the viewers' amygdala signaled the status of both the attended (fixated) and the unattended individuals suggesting that in third party observers of social interactions, the amygdala signals jointly the status of interacting parties.

Highlights

Monkeys infer the social status of conspecifics from videos of simulated dyadic interactionsDuring fixations neural populations signal the social status of the attended individualsNeurons in the amygdala jointly encode the status of interacting individuals.

In brief

Third party-viewers of pairwise dominant-subordinate interactions infer social status from the observed behaviors. Neurons in the amygdala are tuned to the inferred dominant/subordinate status of both individuals.

Evolutionary and biomedical implications of sex differences in the primate brain transcriptome

Humans exhibit sex differences in the prevalence of many neurodevelopmental disorders and neurodegenerative diseases. Here, we generated one of the largest multi-brain-region bulk transcriptional datasets for the rhesus macaque and characterized sex-biased gene expression patterns to investigate the translatability of this species for sex-biased neurological conditions. We identify patterns similar to those in humans, which are associated with overlapping regulatory mechanisms, biological processes, and genes implicated in sex-biased human disorders, including autism. We also show that sex-biased genes exhibit greater genetic variance for expression and more tissue-specific expression patterns, which may facilitate rapid evolution of sex-biased genes. Our findings provide insights into the biological mechanisms underlying sex-biased disease and support the rhesus macaque model for the translational study of these conditions.

Comprehensive Flow Cytometric, Immunohistologic, and Molecular Assessment of Thymus Function in Rhesus Macaques

The critical importance of the thymus for generating new naive T cells that protect against novel infections and are tolerant to self-antigens has led to a recent revival of interest in monitoring thymic function in species other than humans and mice. Nonhuman primates such as rhesus macaques (Macaca mulatta) provide particularly useful animal models for translational research in immunology. In this study, we tested the performance of a 15-marker multicolor Ab panel for flow cytometric phenotyping of lymphocyte subsets directly from rhesus whole blood, with validation by thymectomy and T cell depletion. Immunohistochemical and multiplex RNA expression analysis of thymus tissue biopsies and molecular assays on PBMCs were used to further validate thymus function. Results identify Ab panels that can accurately classify rhesus naive T cells (CD3+CD45RA+CD197+ or CD3+CD28+CD95-) and recent thymic emigrants (CD8+CD28+CD95-CD103+CD197+) using just 100 µl of whole blood and commercially available fluorescent Abs. An immunohistochemical panel reactive with pan-cytokeratin (CK), CK14, CD3, Ki-67, CCL21, and TdT provides histologic evidence of thymopoiesis from formalin-fixed, paraffin-embedded thymus tissues. Identification of mRNAs characteristic of both functioning thymic epithelial cells and developing thymocytes and/or molecular detection of products of TCR gene rearrangement provide additional complementary methods to evaluate thymopoiesis, without requiring specific Abs. Combinations of multiparameter flow cytometry, immunohistochemistry, multiplex gene expression, and TCR excision circle assays can comprehensively evaluate thymus function in rhesus macaques while requiring only minimal amounts of peripheral blood or biopsied thymus tissue.

Ecological disturbance alters the adaptive benefits of social ties

Extreme weather events radically alter ecosystems. When ecological damage persists, selective pressures on individuals can change, leading to phenotypic adjustments. For group-living animals, social relationships may be a mechanism enabling adaptation to ecosystem disturbance. Yet whether such events alter selection on sociality and whether group-living animals can, as a result, adaptively change their social relationships remain untested. We leveraged 10 years of data collected on rhesus macaques before and after a category 4 hurricane caused persistent deforestation, exacerbating monkeys' exposure to intense heat. In response, macaques demonstrated persistently increased tolerance and decreased aggression toward other monkeys, facilitating access to scarce shade critical for thermoregulation. Social tolerance predicted individual survival after the hurricane, but not before it, revealing a shift in the adaptive function of sociality.

A natural disaster exacerbates and redistributes disease risk across free-ranging macaques by altering social structure

Climate change is intensifying extreme weather events, with severe implications for ecosystem dynamics. A key behavioural mechanism whereby animals may cope with such events is by increasing social cohesion to improve access to scarce resources like refuges, which in turn could exacerbate epidemic risk due to increased close contact. However, how and to what extent natural disasters affect disease risk via changes in sociality remains unexplored in animal populations. By modelling disease spread in free-living rhesus macaque groups (Macaca mulatta) before and after a hurricane, we demonstrate doubled pathogen transmission rates up to five years following the disaster, equivalent to an increase in pathogen infectivity from 10% to 20%. Moreover, the hurricane redistributed the risk of infection across the population, decreasing status-related differences found in pre-hurricane years. These findings demonstrate that natural disasters can exacerbate and homogenise epidemic risk in an animal population via changes in sociality. These observations provide unexpected further mechanisms by which extreme weather events can threaten wildlife health, population viability, and spillover to humans.

Maintenance of caecal homeostasis by diverse adaptive immune cells in the rhesus macaque

Objectives

The caecum bridges the small and large intestine and plays a front-line role in discriminating gastrointestinal antigens. Although dysregulated in acute and chronic conditions, the tissue is often overlooked immunologically.

Methods

To address this issue, we applied single-cell transcriptomic-V(D)J sequencing to FACS-isolated CD45+ caecal patch/lamina propria leukocytes from a healthy (5-year-old) female rhesus macaque ex vivo and coupled these data to VDJ deep sequencing reads from haematopoietic tissues.

Results

We found caecal NK cells and ILC3s to co-exist with a spectrum of effector T cells partially derived from SOX4 + recent thymic emigrants. Tolerogenic Vγ8Vδ1-T cells, plastic CD4+ T helper cells and GZMK + EOMES + and TMIGD2 + tissue-resident memory CD8+ T cells were present and differed metabolically. An IL13 + GATA3 + Th2 subset expressing eicosanoid pathway enzymes was accompanied by IL1RL1 + GATA3 + regulatory T cells and a minor proportion of IgE+ plasma cells (PCs), illustrating tightly regulated type 2 immunity devoid of ILC2s. In terms of B lymphocyte lineages, caecal patch antigen-presenting memory B cells sat alongside germinal centre cells undergoing somatic hypermutation and differentiation into IGF1 + PCs. Prototypic gene expression signatures decreased across PC clusters, and notably, expanded IgA clonotypes could be traced in VDJ deep sequencing reads from additional compartments, including the bone marrow, supporting that these cells contribute a steady stream of systemic antibodies.

Conclusions

The data advance our understanding of caecal immunological function, revealing processes involved in barrier maintenance and molecular networks relevant to disease.

Lethal dog attacks on adult rhesus macaques (Macaca mulatta) in an anthropogenic landscape

For nonhuman primates living in anthropogenic areas, predation by larger predators is relatively rare. However, smaller predators, such as free-ranging as well as domesticated dogs, can shape the socioecology of urban nonhuman primates, either directly by attacking and killing them or indirectly by modifying their activity patterns. Here, we describe three (two probably fatal) cases of dog attacks on adult rhesus macaques inhabiting an anthropogenic landscape in Northern India and the circumstances surrounding these incidents. We discuss the importance of considering human presence and intervention in dog-nonhuman primate relationships while studying nonhuman primate populations across anthropogenic gradients, and its potential influences on group social dynamics and transmission of zoonotic agents.

Neural signatures of natural behaviour in socializing macaques

Our understanding of the neurobiology of primate behaviour largely derives from artificial tasks in highly controlled laboratory settings, overlooking most natural behaviours that primate brains evolved to produce1-3. How primates navigate the multidimensional social relationships that structure daily life4 and shape survival and reproductive success5 remains largely unclear at the single-neuron level. Here we combine ethological analysis, computer vision and wireless recording technologies to identify neural signatures of natural behaviour in unrestrained, socially interacting pairs of rhesus macaques. Single-neuron and population activity in the prefrontal and temporal cortex robustly encoded 24 species-typical behaviours, as well as social context. Male-female partners demonstrated near-perfect reciprocity in grooming, a key behavioural mechanism supporting friendships and alliances6, and neural activity maintained a running account of these social investments. Confronted with an aggressive intruder, behavioural and neural population responses reflected empathy and were buffered by the presence of a partner. Our findings reveal a highly distributed neurophysiological ledger of social dynamics, a potential computational foundation supporting communal life in primate societies, including our own.

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.

A Retrospective Comparison of Electrocardiographic Parameters in Ketamine and Tiletamine-Zolazepam Anesthetized Indian Rhesus Monkeys (Macaca mulatta)

Electrocardiographic evaluation is performed in rhesus monkeys to establish the cardiovascular safety of candidate molecules before progressing to clinical trials. These animals are usually immobilized chemically by ketamine (KTM) and tiletamine-zolazepam (TZ) to obtain a steady-state heart rate and to ensure adequate human safety. The present study aimed to evaluate the effect of these anesthetic regimens on different electrocardiographic parameters. Statistically significant lower HR and higher P-wave duration, RR, QRS, and QT intervals were observed in the KTM-anesthetized group in comparison to TZ-anesthetized animals. No significant changes were noticed in the PR interval and p-wave amplitude. Sex-based significance amongst these parameters was observed in male and female animals of TZ- and KTM-anesthetized groups. Regression analysis of four QTc formulas in TZ-anesthetized rhesus monkeys revealed that QTcNAK (Nakayama) better corrected the QT interval than QTcHAS (Hassimoto), QTcBZT (Bazett), and QTcFRD (Fridericia) formulas. QTcNAK exhibited the least correlation with the RR interval (slope closest to zero and r = .01) and displayed no statistical significance between male and female animals. These data will prove useful in the selection of anesthetic regimens for chemical restraint of rhesus monkeys in nonclinical safety evaluation studies.

Natural disaster alters the adaptive benefits of sociality in a primate

Weather-related disasters can radically alter ecosystems. When disaster-driven ecological damage persists, the selective pressures exerted on individuals can change, eventually leading to phenotypic adjustments. For group-living animals, social relationships are believed to help individuals cope with environmental challenges and may be a critical mechanism enabling adaptation to ecosystems degraded by disasters. Yet, whether natural disasters alter selective pressures on patterns of social interactions and whether group-living animals can, as a result, adaptively change their social relationships remains untested. Here, we leveraged unique data collected on rhesus macaques from 5 years before to 5 years after a category 4 hurricane, leading to persistent deforestation which exacerbated monkeys' exposure to intense heat. In response, macaques increased tolerance for and decreased aggression toward other monkeys, facilitating access to scarce shade critical for thermoregulation. Social tolerance predicted individual survival for 5 years after the hurricane, but not before it, revealing a clear shift in the adaptive function of social relationships in this population. We demonstrate that an extreme climatic event altered selection on sociality and triggered substantial and persistent changes in the social structure of a primate species. Our findings unveil the function and adaptive flexibility of social relationships in degraded ecosystems and identify natural disasters as potential evolutionary drivers of sociality.

One-Sentence Summary

Testard et al. show that a natural disaster altered selection on sociality in group-living primates triggering persistent changes in their social structure.

Towards an integrated understanding of dietary phenotypes

Diet and nutrition comprise a complex, multi-faceted interface between animal biology and food environments. With accumulating information on the many facets of this association arises a need for systems-based approaches that integrate dietary components and their links with ecology, feeding, post-ingestive processes and the functional and ecological consequences of these interactions. We briefly show how a modelling approach, nutritional geometry, has used the experimental control afforded in laboratory studies to begin to unravel these links. Laboratory studies, however, have limited ability to establish whether and how the feeding and physiological mechanisms interface with realistic ecological environments. We next provide an overview of observational field studies of free-ranging primates that have examined this, producing largely correlative data suggesting that similar feeding mechanisms operate in the wild as in the laboratory. Significant challenges remain, however, in establishing causal links between feeding, resource variation and physiological processes in the wild. We end with a more detailed account of two studies of temperate primates that have capitalized on the discrete variation provided by seasonal environments to strengthen causal inference in field studies and link patterns of intake to dynamics of nutrient processing. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

The biology of aging in a social world: Insights from free-ranging rhesus macaques

Social adversity can increase the age-associated risk of disease and death, yet the biological mechanisms that link social adversities to aging remain poorly understood. Long-term naturalistic studies of nonhuman animals are crucial for integrating observations of social behavior throughout an individual's life with detailed anatomical, physiological, and molecular measurements. Here, we synthesize the body of research from one such naturalistic study system, Cayo Santiago, which is home to the world's longest continuously monitored free-ranging population of rhesus macaques (Macaca mulatta). We review recent studies of age-related variation in morphology, gene regulation, microbiome composition, and immune function. We also discuss ecological and social modifiers of age-markers in this population. In particular, we summarize how a major natural disaster, Hurricane Maria, affected rhesus macaque physiology and social structure and highlight the context-dependent and domain-specific nature of aging modifiers. Finally, we conclude by providing directions for future study, on Cayo Santiago and elsewhere, that will further our understanding of aging across different domains and how social adversity modifies aging processes.

Neural signatures of natural behavior in socializing macaques

Our understanding of the neurobiology of primate behavior largely derives from artificial tasks in highly-controlled laboratory settings, overlooking most natural behaviors primate brains evolved to produce1. In particular, how primates navigate the multidimensional social relationships that structure daily life and shape survival and reproductive success remains largely unexplored at the single neuron level. Here, we combine ethological analysis with new wireless recording technologies to uncover neural signatures of natural behavior in unrestrained, socially interacting pairs of rhesus macaques within a larger colony. Population decoding of single neuron activity in prefrontal and temporal cortex unveiled robust encoding of 24 species-typical behaviors, which was strongly modulated by the presence and identity of surrounding monkeys. Male-female partners demonstrated near-perfect reciprocity in grooming, a key behavioral mechanism supporting friendships and alliances, and neural activity maintained a running account of these social investments. When confronted with an aggressive intruder, behavioral and neural population responses reflected empathy and were buffered by the presence of a partner. Surprisingly, neural signatures in prefrontal and temporal cortex were largely indistinguishable and irreducible to visual and motor contingencies. By employing an ethological approach to the study of primate neurobiology, we reveal a highly-distributed neurophysiological record of social dynamics, a potential computational foundation supporting communal life in primate societies, including our own.

An Evolutionary Model for the Ancient Origins of Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a common endocrinopathy of reproductive-aged women, characterized by hyperandrogenism, oligo-anovulation and insulin resistance and closely linked with preferential abdominal fat accumulation. As an ancestral primate trait, PCOS was likely further selected in humans when scarcity of food in hunter-gatherers of the late Pleistocene additionally programmed for enhanced fat storage to meet the metabolic demands of reproduction in later life. As an evolutionary model for PCOS, healthy normal-weight women with hyperandrogenic PCOS have subcutaneous (SC) abdominal adipose stem cells that favor fat storage through exaggerated lipid accumulation during development to adipocytes in vitro. In turn, fat storage is counterbalanced by reduced insulin sensitivity and preferential accumulation of highly lipolytic intra-abdominal fat in vivo. This metabolic adaptation in PCOS balances energy storage with glucose availability and fatty acid oxidation for optimal energy use during reproduction; its accompanying oligo-anovulation allowed PCOS women from antiquity sufficient time and strength for childrearing of fewer offspring with a greater likelihood of childhood survival. Heritable PCOS characteristics are affected by today's contemporary environment through epigenetic events that predispose women to lipotoxicity, with excess weight gain and pregnancy complications, calling for an emphasis on preventive healthcare to optimize the long-term, endocrine-metabolic health of PCOS women in today's obesogenic environment.

Single-Cell Transcriptomics of Mtb/HIV Co-Infection

Tuberculosis (TB) and Human Immunodeficiency Virus (HIV) co-infection continues to pose a significant healthcare burden. HIV co-infection during TB predisposes the host to the reactivation of latent TB infection (LTBI), worsening disease conditions and mortality. There is a lack of biomarkers of LTBI reactivation and/or immune-related transcriptional signatures to distinguish active TB from LTBI and predict TB reactivation upon HIV co-infection. Characterizing individual cells using next-generation sequencing-based technologies has facilitated novel biological discoveries about infectious diseases, including TB and HIV pathogenesis. Compared to the more conventional sequencing techniques that provide a bulk assessment, single-cell RNA sequencing (scRNA-seq) can reveal complex and new cell types and identify more high-resolution cellular heterogeneity. This review will summarize the progress made in defining the immune atlas of TB and HIV infections using scRNA-seq, including host-pathogen interactions, heterogeneity in HIV pathogenesis, and the animal models employed to model disease. This review will also address the tools needed to bridge the gap between disease outcomes in single infection vs. co-infection. Finally, it will elaborate on the translational benefits of single-cell sequencing in TB/HIV diagnosis in humans.

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.

Current state of stem cell research in non-human primates: an overview

The remarkable similarity between non-human primates (NHPs) and humans establishes them as essential models for understanding human biology and diseases, as well as for developing novel therapeutic strategies, thereby providing more comprehensive reference data for clinical treatment. Pluripotent stem cells such as embryonic stem cells and induced pluripotent stem cells provide unprecedented opportunities for cell therapies against intractable diseases and injuries. As continue to harness the potential of these biotechnological therapies, NHPs are increasingly being employed in preclinical trials, serving as a pivotal tool to evaluate the safety and efficacy of these interventions. Here, we review the recent advancements in the fundamental research of stem cells and the progress made in studies involving NHPs.

Going beyond established model systems of Alzheimer's disease: companion animals provide novel insights into the neurobiology of aging

Alzheimer's disease (AD) is characterized by brain plaques, tangles, and cognitive impairment. AD is one of the most common age-related dementias in humans. Progress in characterizing AD and other age-related disorders is hindered by a perceived dearth of animal models that naturally reproduce diseases observed in humans. Mice and nonhuman primates are model systems used to understand human diseases. Still, these model systems lack many of the biological characteristics of Alzheimer-like diseases (e.g., plaques, tangles) as they grow older. In contrast, companion animal models (cats and dogs) age in ways that resemble humans. Both companion animal models and humans show evidence of brain atrophy, plaques, and tangles, as well as cognitive decline with age. We embrace a One Health perspective, which recognizes that the health of humans is connected to those of animals, and we illustrate how such a perspective can work synergistically to enhance human and animal health. A comparative biology perspective is ideally suited to integrate insights across veterinary and human medical disciplines and solve long-standing problems in aging.

The biology of aging in a social world:insights from free-ranging rhesus macaques

Social adversity can increase the age-associated risk of disease and death, yet the biological mechanisms that link social adversities to aging remain poorly understood. Long-term naturalistic studies of nonhuman animals are crucial for integrating observations of social behavior throughout an individual's life with detailed anatomical, physiological, and molecular measurements. Here, we synthesize the body of research from one such naturalistic study system, Cayo Santiago Island, which is home to the world's longest continuously monitored free-ranging population of rhesus macaques. We review recent studies of age-related variation in morphology, gene regulation, microbiome composition, and immune function. We also discuss ecological and social modifiers of age-markers in this population. In particular, we summarize how a major natural disaster, Hurricane Maria, affected rhesus macaque physiology and social structure and highlight the context-dependent and domain-specific nature of aging modifiers. Finally, we conclude by providing directions for future study, on Cayo Santiago and elsewhere, that will further our understanding of aging across different domains and how social adversity modifies aging processes.

Concentrations of urinary neopterin, but not suPAR, positively correlate with age in rhesus macaques

Identifying biomarkers of age-related changes in immune system functioning that can be measured non-invasively is a significant step in progressing research on immunosenescence and inflammaging in free-ranging and wild animal populations. In the present study, we aimed to investigate the suitability of two urinary compounds, neopterin and suPAR, as biomarkers of age-related changes in immune activation and inflammation in a free-ranging rhesus macaque (Macaca mulatta) population. We also investigated age-associated variation in gene transcription from blood samples to understand the underlying proximate mechanisms that drive age-related changes in urinary neopterin or suPAR. Neopterin was significantly positively correlated with age, and had a moderate within-individual repeatability, indicating it is applicable as a biomarker of age-related changes. The age-related changes in urinary neopterin are not apparently driven by an age-related increase in the primary signaler of neopterin, IFN-y, but may be driven instead by an age-related increase in both CD14+ and CD14− monocytes. suPAR was not correlated with age, and had low repeatability within-individuals, indicating that it is likely better suited to measure acute inflammation rather than chronic age-related increases in inflammation (i.e., “inflammaging”). Neopterin and suPAR had a correlation of 25%, indicating that they likely often signal different processes, which if disentangled could provide a nuanced picture of immune-system function and inflammation when measured in tandem.