Collective arousal when reuniting after temporary separation in Tonkean macaques.

Celebrations and bursts of communal joy can occur spontaneously in human communities based on mechanisms of emotional contagion. Some examples of similar collective excitement have been reported in animals when they reunite or anticipate rewards, but little is known about the processes and meaning of these multiple interactions. We experimentally studied such collective arousals in two captive groups of Tonkean macaques (Macaca tonkeana) within the context of reunions following the temporary separation of two subgroups. We compared the behaviors of individuals after separation periods of 2 and 48 h with a control period with no separation. This study showed that it is possible to reproducibly induce bursts of friendly interactions in which groupmates run around over a period of several minutes, embracing and grasping one another while displaying numerous affiliative vocalizations and facial expressions. The longer the period of separation, the higher and longer-lasting the rates of affiliative interactions were. Individuals affiliated more frequently with groupmates from a previously separated subgroup than with those having stayed in their own subgroup. Collective arousal was followed by a quieter period characterized by high rates of contact-sitting and social grooming. These results point at the role of collective arousals in social cohesion; they could resolve social tension and renew social relationships. We propose that the emotional state experienced by Tonkean macaques during such events represents a disposition similar to that giving rise to what we humans call "shared joy."