Out-group threat promotes within-group affiliation in a cooperative fish

In social species, conflict with outsiders is predicted to affect within-group interactions and thus influence group dynamics and the evolution and maintenance of sociality. Although empirical evidence exists for a relationship between out-group conflict and intragroup behavior in humans, experimental tests in other animals are rare. In a model fish system, we show that simulated out-group intrusions cause postconflict increases in intragroup affiliation but no changes in postconflict intragroup aggression. Postconflict affiliation was greater following intrusions by neighboring compared with nonneighboring individuals; neighbors represent greater threats to the dominance rank and breeding success of residents, and they are visible in the aftermath of the intrusion. By providing strong evidence of a link between outgroup conflict and postconflict intragroup behavior and demonstrating that intragroup affiliation is affected by the nature of the out-group intrusion, our study shows the importance of considering postconflict behavior for our understanding of cooperation and social structure.