BackgroundAlthough a wealth of literature points to the importance of social factors on health, a detailed understanding of the complex interplay between social and biological systems is lacking. Social status is one aspect of social life that is made up of multiple structural (humans: income, education; animals: mating system, dominance rank) and relational components (perceived social status, dominance interactions). In a nonhuman primate model we use novel network techniques to decouple two components of social status, dominance rank (a commonly used measure of social status in animal models) and dominance certainty (the relative certainty vs. ambiguity of an individual’s status), allowing for a more complex examination of how social status impacts health.MethodsBehavioral observations were conducted on three outdoor captive groups of rhesus macaques (N = 252 subjects). Subjects’ general physical health (diarrhea) was assessed twice weekly, and blood was drawn once to assess biomarkers of inflammation (interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and C-reactive protein (CRP)).ResultsDominance rank alone did not fully account for the complex way that social status exerted its effect on health. Instead, dominance certainty modified the impact of rank on biomarkers of inflammation. Specifically, high-ranked animals with more ambiguous status relationships had higher levels of inflammation than low-ranked animals, whereas little effect of rank was seen for animals with more certain status relationships. The impact of status on physical health was more straightforward: individuals with more ambiguous status relationships had more frequent diarrhea; there was marginal evidence that high-ranked animals had less frequent diarrhea.DiscussionSocial status has a complex and multi-faceted impact on individual health. Our work suggests an important role of uncertainty in one’s social status in status-health research. This work also suggests that in order to fully explore the mechanisms for how social life influences health, more complex metrics of social systems and their dynamics are needed.

The notion of dominance is ubiquitous across the animal kingdom, wherein some species/groups such relationships are strictly hierarchical and others are not. Modern approaches for measuring dominance have emerged in recent years taking advantage of increased computational power. One such technique, named Percolation and Conductance (Perc), uses both direct and indirect information about the flow of dominance relationships to generate hierarchical rank order that makes no assumptions about the linearity of these relationships. It also provides a new metric, known as ‘dominance certainty’, which is a complimentary measure to dominance rank that assesses the degree of ambiguity of rank relationships at the individual, dyadic and group levels. In this focused review, we will (i) describe how Perc measures dominance rank while accounting for both nonlinear hierarchical structure as well as sparsity in data—here we also provide a metric of dominance certainty estimated by Perc, which can be used to compliment the information dominance rank supplies; (ii) summarize a series of studies by our research team reflecting the importance of ‘dominance certainty’ on individual and societal health in large captive rhesus macaque breeding groups; and (iii) provide some concluding remarks and suggestions for future directions for dominance hierarchy research. 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’.

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Positive social relationships in humans are known to have health promoting effects while negative social relationships have detrimental effects. Features of the broader social network, including indirect connections, also impact health. However, complicating our ability to examine these features, human networks are diverse and difficult to fully quantify. Animal models where social networks can be fully characterized are useful in examining how structurally similar yet functionally different relationships can differentially relate to biomarkers of health. For example, in nonhuman primates, grooming serves two main functions, to maintain social bonds (family/friends networks) or gain access to resources/support (political networks). We examined whether an individual’s position in these two network types was differentially related to biomarkers of inflammation and physiological stress in female rhesus macaques (Macaca mulatta). Consistent with predictions, females with higher family/friends centrality had lower IL-6/TNF-α levels, while females with high political centrality showed elevated levels. Middle-ranking females with high political centrality showed elevated hair cortisol yet little to no benefit of family/friend centrality. These results indicate that while grooming interactions are structurally similar, they may be functionally distinct and therefore have very different, even opposite, effects on health. Affiliative interactions occurring within the context of an established relationship (i.e., family/friends) can provide opportunities for social buffering. In contrast, interactions among individuals without established relationships, even friendly interactions, may ultimately be physiologically costly. Ultimately, these results indicate that while social relationships may appear similar, the underlying functionality can have fundamentally diverse physiological outcomes.

Due to primate adaptations for sociality, captive rhesus macaques have optimal welfare and utility as a biomedical model when they can be maintained in outdoor social groups. As a despotic species; however, aggression can result in costly injuries and may result in temporary or permanent removal of specific individuals from social housing. Enrichment items, such as toys, climbing structures, and foraging material, are employed to keep captive animals occupied. We hypothesized that produce enrichment that requires more processing to extract may reduce socially-derived injuries by keeping animals occupied. We tested the effects of additional weekly produce (corn-in-husk, whole melon, or whole squash) on trauma incidence in an outdoor social group of rhesus macaques across two distinct seasons (mating and birthing seasons) at the California National Primate Research Center. Aggression and status behavioral data, food resource use and proximity, and trauma incidence were collected over two 16-week periods, with eight control and treatment conditions alternating biweekly. Mixed-effects regression modeling was used to determine the best predictors of trauma risk and severe aggression at the group level and at an individual level. We found that food resource use was an important predictor of trauma risk at both group and individual levels; greater use of food resources reduced trauma risk. Produce enrichment did not; however, reduce severe aggression. We suggest that other captive social groups of rhesus macaques with high levels of trauma may benefit from supplemental produce enrichment that increases animal engagement with food resources.