Grooming interventions in female rhesus macaques as social niche construction

Social animals invest time and resources into adapting their social environment, which emerges not only from their own but also from the decisions of other group members. Thus, individuals have to monitor interactions between others and potentially decide when and how to interfere to prevent damage to their own investment. These interventions can be subtle, as in the case of affiliative interactions such as grooming, but they can inform us about how animals structure their world and influence other group members. Here, we used interventions into grooming bouts in female rhesus macaques, Macaca mulatta, to determine who intervened in which grooming bouts, and what determined intervention outcomes, based on kinship, dominance rank and affiliative relationships between groomers and (potential) interveners. We show that high dominance rank of groomers reduced the risk of intervention. Bystanders, particularly when high ranking, intervened in grooming of their kin, close affiliates and close-ranked competitors. Interveners gained access to their close affiliates for subsequent grooming. Reduced aggression risk facilitated grooming involving three individuals, which was more common when a strong affiliative relationship existed and when interveners were lower in rank than the groomers. Thus, interventions in this species involved the monitoring of grooming interactions, decision making based on several individual and dyadic characteristics, and potentially allowed individuals to broaden their access to grooming partners, protect their own relationships and influence their social niche.