Disparate social structures are underpinned by distinct social rules across a primate radiation
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Over six decades of research on wild baboons and their close relatives (collectively, the African papionins) has uncovered substantial variation in their behavior and social organization. While most papionins form discrete social groups (single-level societies), a few others form small social units nested within larger aggregations (multi-level societies). To understand the social processes that shape this variation, a more systematic, comparative analysis of social structure is needed. Here, we constructed a database of behavioral and demographic records spanning 135 group-years across 13 long-term papionin field studies to (i) quantify variation in grooming network structure, and (ii) identify the factors (e.g., sex, kinship, and social status effects) that underlie these differences. We detected considerable variation in grooming network structure across the papionins, even within the classic single-level societies. The papionins could be best divided into three broad categories: single-level cohesive , single-level cliquish , and multi-level . The cohesive single-level societies formed networks that were dense, moderately kin-biased, and weakly rank-structured, while the cliquish single-level societies formed networks that were relatively modular, highly kin-biased, and more strongly rank-structured. As expected, multi-level networks were highly modular and shaped by females’ ties to specific dominant males but varied in their kin biases. Taken together, these data suggest that: (i) discrete typologies obscure variation in social structure; and (ii) similarities in social structure are sometimes, but not always, shaped by similar social processes.
SIGNIFICANCE STATEMENT
Do all primate groups fit the same social mold? While factors like kinship and dominance shape the social lives of many of our close relatives, it remains unclear how their effects differ across species. Using a new database representing decades of field research, we found that baboons and their close relatives fell into one of three general patterns: one in which groups were cohesive and only somewhat nepotistic (i.e., kin- and rank-biased), another in which groups were more cliquish and nepotistic, and a final pattern in which groups were divided into clusters centered on dominant males. Distinct primate societies may thus reflect differences in the strength of females’ social biases towards kin and the degree of males’ social influence.
