Exploratory Assessment of Interunit Dominance Relationships in a Rwenzori colobus (Colobus angolensis ruwenzorii) Multilevel Society: The Importance of Social Network Position

Abstract

Multilevel social organizations necessitate close spatial proximity of social units, making dominance hierarchies between core units potentially important to control aggression. We examined whether an interunit dominance hierarchy was present for a Rwenzori Angolan colobus multilevel society (MLS) and its potential social correlates. We recorded the outcome of interunit interactions (IUIs, N = 167) over 59 days in a band of 14 core units at Nabugabo, Uganda. Core units were almost always closely clustered, so we defined IUIs as approaches that decreased the distance between units and lead to changes in behaviour. We assessed dominance using network-based ranking, because the hierarchy was not linear. Tiers of dominance were evident, and we examined these relative to demographics, clan identity (MLS tier 2), male anogenital distance (AGD – a correlate of in utero androgen exposure), and social network analyses (SNA) of interunit association. Unit displacements without aggression occurred in 31.7% of IUIs, and some aggression occurred in 64.7%. Dominance tier was not significantly associated with unit demographics, but units with males that had longer AGDs (more androgenized) tended to be more dominant. Social network analyses showed that higher-ranking units had greater strength, eigenvector centralities, and clustering coefficients than lower-ranking units. Interunit interactions between units in different clans were more aggressive than those within clans and clan identity correlated with dominance tier, indicating that one clan outranked the other. Overall, we demonstrate complex social and spatial factors determining priority-of-access to resources in a primate MLS.