Face-to-face opening phase in Japanese macaques' social play enhances and sustains participants' engagement in subsequent play interaction.

A face-to-face "opening phase" in human interaction serves as a platform for the interactants to initiate and manage their interaction collaboratively. This study investigated whether, as is the case in humans, a face-to-face opening phase in animal interaction serves to manage a subsequent interaction and establish interactants' engagement. We compared the dyadic play fighting of Japanese macaques (Macaca fuscata) initiated with and without a face-to-face opening phase. Our observations showed that play sessions with a face-to-face opening phase lasted longer than did sessions without one. Furthermore, our results indicate that facing toward playmates was a sign of interactants' engagement. In sessions with a face-to-face opening phase, both players were likely to gain an advantage over their playmates, whereas in sessions without such an opening phase, only an individual who unidirectionally faced toward another individual who looked away when play began was likely to maintain an advantage over a long period. Our findings demonstrate that a face-to-face opening phase has a socio-cognitive function to establish and sustain interactants' social engagement during subsequent interaction not only in humans but also in Japanese macaques.