Behavioural and spatial flexibility in mountain gorillas (Gorilla beringei beringei) in response to intergroup encounters

Intergroup encounters are a powerful selective force shaping behaviour. Primates can adjust ranging, activity, and affiliation patterns to mitigate the costs associated with intergroup conflict or enhance intragroup cohesion to facilitate collective action. However, empirical evidence for these adaptive responses remains equivocal. We investigated how intergroup encounters influence the behaviour of mountain gorillas ( Gorilla beringei beringei ), a species that exhibits lethal intergroup aggression. We employed Global Positioning System and focal sampling to record ranging and behavioural data across eight groups in Volcanoes National Park, Rwanda. We used data collected in 2023–2024 to estimate how daily travel distances, activity budgets, and social behaviours change across a short-term 3-day period following encounters compared with baseline. We also used long-term data collected in 2003–2024 to calculate changes in encounter site fidelity and revisit frequency of up to a year post-event. We found that daily travel distances and time spent moving increased following intergroup encounters while site fidelity decreased, demonstrating that mountain gorillas use flexible behavioural mechanisms to avoid other groups. However, site revisit frequency did not decrease, indicating avoidance is not sustained long-term. Although we found no evidence of social buffering as time invested in socialising remained unchanged, intragroup cohesion increased after encounters, which may enhance protection and cooperation. As groups in this growing subpopulation of mountain gorillas increasingly compete for limited space and experience more frequent intergroup encounters, our findings underscore the need for effective conservation strategies addressing possible long-term fitness consequences linked to shifting social dynamics.