Enabling a group split in captivity through proactive enclosure design and reactive social management

Abstract Fissioning from one group into two or more is a solution to spatial and social tension. Captive settings, however, usually do not facilitate fission through the naturalistic process of individual self-selection of group membership. We studied a mixed-sex, outdoor-housed group (N = 184) of rhesus macaques (Macaca mulatta) and implemented an enclosure design to promote self-selected fissioning. There were four study periods: baseline in the home enclosure, acclimatization to an expanded enclosure, separation and post-split observations. The social overthrow of the alpha matriline in the separation phase necessitated intervention via a reactive, human-induced split into two groups, informed by subjective expertise and animal behaviour observations up until this event. Here, we examined whether behaviour supported our rapidly (less than 24 h) decided group memberships using pre- and post-split location usage, agonistic dynamics and community formation as well as composition in temporal networks. Pre-induced split, we found changes in spatial usage and directional agonism consistent with exclusion based on future group memberships. We found increased hierarchical ordering, community structures and assortativity along the lines of future group structures. Post-induced split observations saw a decline of within-group aggression rates and retention of hierarchical network structure. Our study design enabled these social dynamics to emerge, but circumstances necessitated human intervention.