When groups of conspecifics are confined to the same territory, hierarchies dictate an individual’s priority in accessing the resources ( Wilson, 1975). To mitigate the risk of constant fighting for limited resources, animals adopted basic, albeit not mutually exclusive strategies, such as territoriality and the formation of social hierarchies ( Chase, 1974 Wilson, 1975). Under naturalistic settings, animals have to compete for limited resources, including food, mating opportunities, and nesting sites. Since hierarchies inevitably produce inequalities, it is legitimate to ask why natural selection has favored them. Even though Marmot’s and Sapolsky’s hypothesis is under debate, it calls attention to the relevance of social hierarchies for stress gradients and health in animal societies ( Marmot and Sapolsky, 2014). However, in unstable, despotic societies that require frequent physical reassertion of dominance, dominant individuals experience the highest levels of stress. For example, in highly stable, despotic societies where rank is maintained through intimidation, which is prominent among humans and baboons, subordinate individuals are under pressure. This gradient of stress is, however, not universal, and depends on the stability of the hierarchy and the mechanisms used for rank maintenance ( Sapolsky, 2005). Subordinate baboons displayed basal hypercortisolism in the range known to adversely impact blood pressure, insulin sensitivity, and immunity ( Sapolsky et al., 2000). Since their studies focused on humans or troops of baboons, the gradient of stress, due to the respective organization of these societies, tends to increase in subordinates. In this regard, Marmot and Sapolsky proposed a novel perspective, whereby gradients in psychosocial stressors would contribute to the observed gradient in health, even in the absence of substantial deprivation ( Marmot and Sapolsky, 2014). It seems obvious that low-status individuals have limited access to essential resources, partially explaining their health conditions, however, the social gradient in health persists in human populations free from extreme poverty, suggesting that other factors might underlie this phenomenon. Socioeconomic status is, therefore, the single strongest predictor of life expectancy and disease risk in humans ( Sapolsky, 2004, 2005). Similar correlations are reported in all rich, poor, and intermediate countries surveyed ( Victora et al., 2003 Hurt et al., 2004 Marmot, 2004). Examples of this can be found broadly in human societies, for example, in the Scottish city of Glasgow, there is a 28-year difference in life expectancy between the wealthiest and poorest suburbs ( Hanlon et al., 2006), and in the poorer neighborhoods of Washington DC, life expectancy decreases by 16 years compared to wealthier areas ( Murray et al., 2006). Seminal studies by Marmot and Sapolsky provided evidence that social hierarchies in primates, both human and non-human, lead to social gradients in health, whereby lower rank individuals exhibit progressively higher morbidity and mortality rates ( Marmot, 2004 Marmot and Sapolsky, 2014). Social status is recognized as a major social determinant of health ( Wilkinson and Marmot, 1998). Based on connectivity data we also discuss candidate regions that might inspire further investigation, as well as the caveats and strategies that have been used to further our understanding of the biological substrates underpinning social hierarchy and dominance. We start by summarizing the literature on the prefrontal cortex and other relevant brain regions to expand the current “prefrontal-centric” view of social hierarchy behaviors. In this review, we compare and integrate findings from rodent and primate studies to create a model of the neural and cellular networks supporting social hierarchies, both from a macro (i.e., circuits) to a micro-scale perspective (microcircuits and synapses). Available evidence suggests the prefrontal cortex is a keystone in this circuit, but upstream and downstream candidates are progressively emerging. Social status is recognized as a major determinant of social behavior and health among animals however, the neural circuits supporting the formation and navigation of social hierarchies remain under extensive research. 3Department of Life Sciences, University of Coimbra, Coimbra, Portugal.2Institute of Interdisciplinary Research (IIIUC), University of Coimbra, Coimbra, Portugal.1CNC-Center for Neuroscience and Cell Biology, University of Coimbra, Coimbra, Portugal.Emanuel Ferreira-Fernandes 1,2 João Peça 1,3*
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