How do consistent inter-individual differences drive female friendships in feral water buffalo?

Affiliative interactions in social species are vital for survival. Empirical evidence suggests that friendships (strong affiliative bonds) benefit the involved parties and can have large implications for maintaining social structures. Yet, what drives animals to choose their friends is relatively understudied. Homophily in consistent inter-individual differences or personalities is considered a cognitively less demanding mechanism for choosing a friend. However, findings are mostly limited to ‘model’ species, like non-human primates. We studied a feral and free-ranging population of highly social species, water buffalo (Bubalus bubalis), to investigate whether personalities drive friendships (n=30). We assessed friendships by calculating a composite dyadic index comprised of behavioural variables, proximity and body contact. To assess personalities, we conducted focal observations (720 min/focal) with a ‘bottom-up’ approach (consisting of 54 variables), followed by repeatability checking and a principal component analysis. Three personality traits were evident, and individual scores were extracted. We found significant associations between dyadic personality score differences (PSD) and friendships, where individuals with lower PSD were more likely to be friends than individuals with higher PSD. Thus, personality homophily drives friendships in water buffalo. Our findings from a ‘non-model’ species provide valuable insights into the proximate mechanisms and, broadly, the evolution of friendships.