Play face in Japanese macaques reflects the sender's play motivation

Abstract

Animals often initiate social interactions by exchanging signals. Especially when initiating amicable interactions, signaling one's friendly stance toward others in advance may be important to avoid being misunderstood as having hostile intentions. We used data on dyadic play fighting in a group of Japanese macaques, Macaca fuscata, to examine the function of "play face" at the opening of a play session. We found no support for the previously proposed hypothesis that play face expression is likelier before entering risky situations (e.g., before gaining an undue advantage over the partner) to avoid being misunderstood. The results showed that play face expression was likelier in male juveniles before initiating play with other males than in females before initiating play with males or other females and that juveniles were likelier to express play face before initiating play with others closer in age. As male Japanese macaques play more frequently than females, and juveniles prefer to play with individuals closer in age, play face expression before play initiation may reflect the individual's motivation for subsequent play interactions. This interpretation is supported by our observation that play bouts lasted longer when initiated with bidirectional play face by both participants than when initiated without play face. We also argued that since there was no tendency that play face was likelier to be expressed toward individuals with low play propensity (e.g., females) or infrequent partners to play with (e.g., individuals more distant in age), Japanese macaques may not tactically deploy this signal to recruit reluctant partners.

Keywords: Japanese macaque; Peer play; Play face; Play signal; Social cognition.

Fig. 1

The mean probability of the expression of play face according to the a sex combination between a subject player and a partner, b a subject player’s age, and c absolute age difference between players. Error bars represent standard errors. Sample size: N = 226. *p < 0.05; ns non-significant

Fig. 2

The mean duration of a play bout according to the a direction of play face and b absolute age difference between players. Error bars represent standard errors. Sample size: N = 113. ***p < 0.01; †p < 0.1; ns non-significant (Tukey’s post hoc test)

Fig. 3

The mean proportion of time during which a subject player maintained an advantage according to the sex combination between a subject player and a partner. Error bars represent standard errors. Sample size: N = 226. **p < 0.01; ns non-significant