Impact of Aggression on Bystanders: Quadratic Post-Conflict Affiliation in Chimpanzees (Pan troglodytes)

In social animals, aggression is a group matter not involving only the opponents. Witnessing a conflict can induce tension and distress in bystanders (i.e., individuals not involved in either the conflict or post-conflict affiliation with the aggressor and aggressee). For this reason, bystanders can engage in post-conflict affiliative exchanges to reduce tension and distress, a phenomenon known as Quadratic Post-Conflict Affiliation (QPCA). This study investigated the occurrence of QPCA in a group of chimpanzees (Pan troglodytes, N = 15) housed at ZooParc de Beauval, France. Our findings confirmed the presence of QPCA in chimpanzees under study (group QPCA tendency: 5.60% +/- 2.55 SE). QPCA was primarily directed towards males, who usually tended to be more influenced by the ongoing aggression and could potentially redirect further aggression towards bystanders. High-ranking bystanders were contacted more frequently than low-ranking ones, as the former can potentially provide immediate protection against other aggressors and offer greater tolerance. Additionally, bystanders were less frequently targeted by aggression when QPCA was present than when it was absent. Thus, QPCA may function as a protective mechanism against aggression by other group members by reducing the chance that bystanders become victims for redirected aggression (Bystander Protection Hypothesis). However, QPCA failed in reducing the levels of bystanders' anxiety-related behaviors. In conclusion, QPCA may be one of the behavioral strategies used by chimpanzees to navigate social challenges, maintain group cohesion, and mitigate aggression.