Social play fosters cooperation in wild adult chimpanzees.

Adult social play is a universal human trait, promoting the tolerance, bonding, cooperation, and collective action that sustain our large and complex societies. Play serves as a conduit for transmitting positive emotions, thereby stimulating psychological resilience to stressors and facilitating the positive intent and trust essential for cooperation emergence. In contrast, non-human adult social play is considered rare, and its role in cooperation remains unknown. We address this gap by studying the play behavior of 57 adult chimpanzees (Pan troglodytes) in Taï National Park, Côte d'Ivoire, where adult social play and collective action regularly occur. We show that adult female and male chimpanzees play more during times of increased mate competition (with males mainly playing with immatures) and with adult partners they had recent disputes with, highlighting the role of play in regulating social tension that can undermine cooperation. Chimpanzees also preferred playing with adult partners with whom they share strong affiliative bonds, aligning with the idea that play is associated with social familiarity and trust. Finally, adult chimpanzees were more likely to play before collectively defending their territory against outsiders and hunting monkeys. Those who played together were subsequently more likely to collaborate, reinforcing the notion that the positive feedback signaled via play can facilitate cooperation. Our findings demonstrate the sustained significance of adult social play throughout the chimpanzee lifespan, providing valuable insights into the evolution of adult social play and its societal functions, from diffusing tension to supporting social bonds and collective action.