The evolution of despotic societies in mammals

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Social hierarchies limit conflicts between the members of a group by regulating their access to resources; as a result, the social rank in the hierarchy affects nearly every aspect of an individual’s Darwinian fitness. Much research has investigated the causes and consequences of an individual’s social rank in group-living species, but little is known about why some animal societies are more hierarchical than others, and specifically why social hierarchies vary in how strictly individuals are tied to their rank and how steep the differences between ranks are. This project aims at understanding the evolutionary origins of such variation in dominance style in group-living mammals, showing a gradient from egalitarian to despotic societies, across multiple scales (individuals, groups, populations and species). We will test three main hypotheses, respectively positing that individual differences in dominance style (i.e., the frequency and pattern with which an individual distributes aggression across the social hierarchy) represent stable, heritable behavioural syndromes (H1), flexible individual responses to changes in the ecological or social environment (H2), or socially-learned responses to changes in the cultural environment, resulting in ‘egalitarian’ or ‘despotic’ cultures (H3). Specifically, we will investigate variations in dominance style through 4 main objectives: 1. across individuals, to characterize individual differences in dominance style and test their stability and heritability (H1), using high-resolution behavioural data from three long-term studies on wild chacma baboons (Papio ursinus), mandrills (Mandrillus sphinx) and spotted hyenas (Crocuta crocuta); 2. within and across social groups, by linking individual dominance styles to the group dominance style to test if they reflect flexible responses of individual group members to changes in the ecological, social (H2) or cultural (H3) environment, using a combination of modelling and empirical data from the same populations. Individuals may for instance adjust their dominance style following a change in their own social rank or in the alpha position to strengthen their position in the new hierarchy, or they may socially learn and copy the style of new group members; 3. across populations and species, to provide a synthesis of the evolutionary history of dominance style in mammalian societies and test H1-3 using a broad comparative framework ; and 4. between the sexes by capitalizing on Objectives 1-3 and on the socio-ecological contrasts offered by our study populations to investigate why male and female dominance styles differ. Ultimately, this project will shed new light on the evolutionary origins of social inequality within and across animal societies.