Body signals during social play in free-ranging rhesus macaques (Macaca mulatta): A systematic analysis

Social play involves one of the most sophisticated types of communication, that is, the use of play signals. Most primate research on play signals has focused on the use of the play face. However, some species appear to exhibit a variety of play signals. For example, rhesus monkeys (Macaca mulatta) have been reported to use body movements or postures that might have signal value during social play, in addition to the play face. However, it is not clear whether these body signals actually meet several criteria necessary to label them as "play signals." Here we examine the forms and possible functions of seven candidate signals that we observed exclusively during social play contexts among free-ranging rhesus monkeys on Cayo Santiago. We aim to (1) distinguish them from actual play behavior (play involving contact or chasing) using loglinear analysis and (2) determine whether they predict playful behavior using modified PC-MC methods. Two candidate signals did not resemble any behaviors used in actual play. The other five signals contained elements that lasted longer or increased their conspicuousness over similar play behaviors, suggesting ritualized characteristics. Youngsters were likely to initiate contact or chasing play significantly sooner after candidate signals than in their absence. Thus, these candidate signals appear to meet critical criteria of signals that promote, moderate or facilitate play. As such, these findings open the door to questions about why multiple play signals have evolved.