The function of greetings in male Tonkean macaques.

Males living in multi-male groups are both strong rivals and potential allies. In several primate species they regularly interact through ritualized exchanges known as greetings. We studied five captive groups of Tonkean macaques (Macaca tonkeana) to test five hypotheses regarding the social function of greetings. We found that greetings were mostly reciprocal interactions, and that they often involved physical contact and facial displays. Although they were mostly initiated by the higher-ranking individual in each dyad, subordinates could initiate approximately one-third of greetings, which indicates that greetings do not serve as a formal acknowledgement of dominance relationships. Although greeting frequencies were negatively correlated to the frequency of supplantations and conflicts, they were not significantly influenced by age and dominance status, showing that greetings are not used to appease partners or decrease tension between males. Males most frequently greeted partners with whom they spent more time in proximity and body contact, and this is consistent with the hypothesis that greetings play a role in reinforcing social relationships. Lastly, greetings increased in frequency between individuals involved in challenges for rank, lending support to the hypothesis that males assess the state of their social relationships through greetings.