Between-group competition has long been thought to be a key factor influencing within-group social dynamics. In humans, it has been suggested that between-group competition may favour the emergence of within-group cooperation. However, between-group competition can also be hypothesized to induce social tension and cause within-group social relationships to deteriorate. So far, little research has focused on these possible effects in animal societies. In this study, we used an experimental set-up to manipulate the opportunity for visual aggressive interactions between two adjacent groups of captive tufted capuchin monkeys, Cebus apella, to examine the group members' behavioural adjustment in terms of affiliation and aggression. The 'cooperative hypothesis' predicted social relationships to be positively reinforced by the visual exposure to the adjacent group to buffer between-group hostility. By contrast, the 'induced-tension hypothesis' predicted social relationships to be negatively affected by the visual exposure to the adjacent group because of the higher level of social tension induced by between-group hostility. The experimental manipulation did not produce changes in affiliative behaviours, whereas within-group aggression was higher and the dominance hierarchy steeper when visual aggressive interactions between groups were allowed. Overall, interactions with a neighbouring group caused social relationships to deteriorate and partly confirmed the induced-tension hypothesis.
