Maintaining homeostasis in the group: a possible role of agonistic support in geladas

Agonistic support occurs when a bystander intervenes in an ongoing conflict to help one of the two participants. In addition to the clear benefit to the recipient, agonistic support can provide benefits to the bystander who intervenes (e.g. help gain access to resources or maintain hierarchical status). However, the consequences of agonistic support provided to the victim might be different from that given to the aggressor. We hypothesized that agonistic support for the victim may not only provide a protective function for the supported subject, but also have a homeostatic role limiting the escalation of aggression among group members. To test this hypothesis, we selected Theropithecus gelada , a highly social species well equipped with behavioural mechanisms aimed at maintaining cohesion in the group. Our results show that agonistic support was preferentially directed towards the victim rather than towards the aggressor. The best supporters for the victim were high-ranking individuals. Supporting victims significantly reduced the frequency of subsequent aggression both towards the victim and further aggression in the group. In contrast, support for aggressors increased the probability of further aggression. Further, the highest levels of support were provided to the victims who were unrelated and without strong social bonds to aggressors. Agonistic support may be a social tool which comes into play when affiliative post-conflict mechanisms are less likely to occur, for instance, when the two participants are not kin or friends. This peacekeeping role (both at individual and at group level) of agonistic support in geladas could be favoured by natural selection on the basis of the self-interest and motivation of the supporter. However, we cannot exclude other selective forces, especially those that are not mutually exclusive, such as kin selection or reciprocal altruism, in the evolution of this complex and variable behaviour.