Common marmosets use body posture as multi-functional signal to solicit, maintain, and modify social play

Social play is a highly active social interaction, characterized by rapid exchanges of various behaviors with multiple partners. Many primates use bodily expressions during social play, yet the potential signaling function of these expressions remains unclear. This study investigated whether common marmosets (Callithrix jacchus) use body posture as signal to regulate play. We recorded play within three captive common marmoset family groups using multiple cameras simultaneously to capture the fast-paced and high frequency behaviors. Three distinct signals (i.e. supine, hide, stalk) and six distinct play types (i.e. wrestle, chase, pounce, touch, catch, pull) were identified. We used a multi-state time-to-event model to analyze the sequences of play, including short-and long-term transitions between different states (i.e. signal, play, or rest/nothing). Our data-driven approach accounted for uncertainty in the duration of play bouts, using probabilistic classification rather than arbitrary bout thresholds. The resulting classifications allowed us to assess the social function of signals by comparing play behavior to a resting state baseline. We found that the presence of a signal: (1) increases the probability to play; (2) extends the duration of play; (3) leads to more diverse play; and (4) increases the probability of play fighting. Marmosets also show turn-taking of signaling and initiating subsequential play. These results show that marmosets use postures as communicative signals to initiate and change play dynamics, and thereby establish a mutual understanding of the joint action. The two-fold contribution of this study concerns novel analytical methods and a deeper conceptual understanding of primate communication. Play and its signals are important elements in the evolution of language, and our research contributes to its further understanding.