Quantifying within-group variation in sociality—covariation among metrics and patterns across primate groups and species

Facial expressivity in dominant macaques is linked to group cohesion

Social living affords primates (including humans) many benefits. Communication has been proposed to be the key mechanism used to bond social connections, which could explain why primates have evolved such expressive faces. We assessed whether the facial expressivity of the dominant male (quantified from the coding of anatomically based facial movement) was related to social network properties (based on social proximity and grooming) in nine groups of captive rhesus macaques (Macaca mulatta) housed in uniform physical and social environments. More facially expressive dominant male macaques were more socially connected and had more cohesive social groups. These findings show that inter-individual differences in facial expressivity are related to differential social outcomes at both an individual and group level. More expressive individuals occupy more beneficial social positions, which could help explain the selection for complex facial communication in primates.

The dynamics of sociality and glucocorticoids in wild male Assamese macaques

For males of gregarious species, dominance status and the strength of affiliative relationships can have major fitness consequences. Social dynamics also impose costs by affecting glucocorticoids, mediators of homeostasis and indicators of the physiological response to challenges and within-group competition. We investigated the relationships between dominance, social bonds, seasonal challenges, and faecal glucocorticoid metabolite (fGC) measures in wild Assamese macaques (Macaca assamensis) at Phu Khieo Wildlife Sanctuary, Thailand, combining behavioural data with 4129 samples from 62 adult males over 15 years. Our previous work on this population suggested that increased competition during the mating season was associated with elevated fGC levels and that, unusually for male primates, lower rank position correlated with higher fGC levels. With a much larger dataset and dynamic measures of sociality, we re-examined these relationships and additionally tested the potentially fGC-attenuating effect of social support. Contrary to our previous study, yet consistent with the majority of work on male primates, dominance rank had a positive relationship with fGC levels, as high status correlated with elevated glucocorticoid measures. fGC levels were increased at the onset of the mating season. We demonstrated an fGC-reducing effect of supportive relationships in males and showed that dynamics in affiliation can correlate with dynamics in physiological responses. Our results suggest that in a system with intermediate contest potential, high dominance status can impose physiological costs on males that may potentially be moderated by social relationships. We highlight the need to consider the dynamics of sociality and competition that influence hormonal processes.

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.

Social network shrinking is explained by active and passive effects but not increasing selectivity with age in wild macaques

Evidence of social disengagement, network narrowing and social selectivity with advancing age in several non-human animals challenges our understanding of the causes of social ageing. Natural animal populations are needed to test whether social ageing and selectivity occur under natural predation and extrinsic mortality pressures, and longitudinal studies are particularly valuable to disentangle the contribution of within-individual ageing from the demographic processes that shape social ageing at the population level. Data on wild Assamese macaques (Macaca assamensis) were collected between 2013 and 2020 at the Phu Khieo Wildlife Sanctuary, Thailand. We investigated the social behaviour of 61 adult females observed for 13 270 h to test several mechanistic hypotheses of social ageing and evaluated the consistency between patterns from mixed-longitudinal and within-individual analyses. With advancing age, females reduced the size of their social network, which could not be explained by an overall increase in the time spent alone, but by an age-related decline in mostly active, but also passive, behaviour, best demonstrated by within-individual analyses. A selective tendency to approach preferred partners was maintained into old age but did not increase. Our results contribute to our understanding of the driver of social ageing in natural animal populations and suggest that social disengagement and selectivity follow independent trajectories during ageing.

Effect of behavioural sampling methods on local and global social network metrics: a case-study of three macaque species

Social network analysis (SNA) is a powerful, quantitative tool to measure animals' direct and indirect social connectedness in the context of social groups. However, the extent to which behavioural sampling methods influence SNA metrics remains unclear. To fill this gap, here we compare network indices of grooming, huddling, and aggression calculated from data collected from three macaque species through two sampling methods: focal animal sampling (FAS) and all-occurrences behaviour sampling (ABS). We found that measures of direct connectedness (degree centrality, and network density) were correlated between FAS and ABS for all social behaviours. Eigenvector and betweenness centralities were correlated for grooming and aggression networks across all species. By contrast, for huddling, we found a correlation only for betweenness centrality while eigenvector centralities were correlated only for the tolerant bonnet macaque but not so for the despotic rhesus macaque. Grooming and huddling network modularity and centralization were correlated between FAS and ABS for all but three of the eight groups. By contrast, for aggression network, we found a correlation for network centralization but not modularity between the sampling methodologies. We discuss how our findings provide researchers with new guidelines regarding choosing the appropriate sampling method to estimate social network metrics.

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

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

Genetic, maternal, and environmental influences on sociality in a pedigreed primate population

Various aspects of sociality in mammals (e.g., dyadic connectedness) are linked with measures of biological fitness (e.g., longevity). How within- and between-individual variation in relevant social traits arises in uncontrolled wild populations is challenging to determine but is crucial for understanding constraints on the evolution of sociality. We use an advanced statistical method, known as the 'animal model', which incorporates pedigree information, to look at social, genetic, and environmental influences on sociality in a long-lived wild primate. We leverage a longitudinal database spanning 20 years of observation on individually recognized white-faced capuchin monkeys (Cebus capucinus imitator), with a multi-generational pedigree. We analyze two measures of spatial association, using repeat sampling of 376 individuals (mean: 53.5 months per subject, range: 6-185 months per subject). Conditioned on the effects of age, sex, group size, seasonality, and El Niño-Southern Oscillation phases, we show low to moderate long-term repeatability (across years) of the proportion of time spent social (posterior mode [95% Highest Posterior Density interval]: 0.207 [0.169, 0.265]) and of average number of partners (0.144 [0.113, 0.181]) (latent scale). Most of this long-term repeatability could be explained by modest heritability (h²_social: 0.152 [0.094, 0.207]; h²_partners: 0.113 [0.076, 0.149]) with small long-term maternal effects (m²_social: 0.000 [0.000, 0.045]; m²_partners: 0.000 [0.000, 0.041]). Our models capture the majority of variance in our behavioral traits, with much of the variance explained by temporally changing factors, such as group of residence, highlighting potential limits to the evolvability of our trait due to social and environmental constraints.

Impact of joint interactions with humans and social interactions with conspecifics on the risk of zooanthroponotic outbreaks among wildlife populations

Pandemics caused by pathogens that originate in wildlife highlight the importance of understanding the behavioral ecology of disease outbreaks at human–wildlife interfaces. Specifically, the relative effects of human–wildlife and wildlife-wildlife interactions on disease outbreaks among wildlife populations in urban and peri-urban environments remain unclear. We used social network analysis and epidemiological Susceptible-Infected-Recovered models to simulate zooanthroponotic outbreaks, through wild animals’ joint propensities to co-interact with humans, and their social grooming of conspecifics. On 10 groups of macaques (Macaca spp.) in peri-urban environments in Asia, we collected behavioral data using event sampling of human–macaque interactions within the same time and space, and focal sampling of macaques’ social interactions with conspecifics and overall anthropogenic exposure. Model-predicted outbreak sizes were related to structural features of macaques’ networks. For all three species, and for both anthropogenic (co-interactions) and social (grooming) contexts, outbreak sizes were positively correlated to the network centrality of first-infected macaques. Across host species and contexts, the above effects were stronger through macaques’ human co-interaction networks than through their grooming networks, particularly for rhesus and bonnet macaques. Long-tailed macaques appeared to show intraspecific variation in these effects. Our findings suggest that among wildlife in anthropogenically-impacted environments, the structure of their aggregations around anthropogenic factors makes them more vulnerable to zooanthroponotic outbreaks than their social structure. The global features of these networks that influence disease outbreaks, and their underlying socio-ecological covariates, need further investigation. Animals that consistently interact with both humans and their conspecifics are important targets for disease control.