Homophily in social and demographic traits predict association patterns in female western and mountain gorillas

Affiliative relationships are a hallmark of social relationships in gregarious mammals, but what drives variation of association patterns remain to be investigated. Gorillas, who exhibit female dispersal, provide an interesting counterpoint to female philopatric species to examine the factors influencing variation in association patterns. We examined demographic and social factors that may predict association patterns of female western (Gorilla gorilla gorilla; Loango, Gabon) and mountain gorillas (Gorilla beringei beringei; Bwindi, Uganda). We looked at dyadic and individual strength scores of social proximity data (37 group-years). For individual female gorillas, high dominance rank increased association scores and being newly emigrated decreased them. For dyads, higher mean dominance rank, and both partners having a dependant infant increased association scores, whereas a partner being newly emigrated decreased association scores. Further analysis of time-matched birth and emigration events confirmed the temporal nature of these associations. Overall, female gorilla association patterns show flexibility in association strength based on real time contingencies, namely social and demographic traits. Our results suggest that affiliative relationships in species with female secondary dispersal are governed by homophily like that of modern humans. Understanding the female gorilla social structure can enhance our knowledge of the evolutionary origins of sociality.

Methods
This data relates to data collected on mountain and western gorilla females association patterns and the predictor variables. Continuous variables have been z-transformed. Please refer to the manuscript for full details of the methods.