Tackling Hominin Tickling: Bonobos Share the Social Features and Developmental Dynamics of Play Tickling With Humans.

It is under debate whether intersubjectivity-the capacity to experience a sense of togetherness around an action-is unique to humans. In humans, heavy tickling-a repeated body probing play that causes an automatic response including uncontrollable laughter (gargalesis)-has been linked to the emergence of intersubjectivity as it is aimed at making others laugh (self-generated responses are inhibited), it is often asymmetrical (older to younger subjects), and it elicits agent-dependent responses (pleasant/unpleasant depending on social bond). Intraspecific tickling and the related gargalesis response have been reported in humans, chimpanzees, and anecdotally in other great apes, potentially setting the line between hominids and other anthropoids. Here we investigated this phenomenon in bonobos and predicted that in this species (sharing with humans and chimpanzees the last common ancestor) the presence of tickling would be modulated depending on the players' age, play session initiators, and familiarity. In April-June 2018, we collected videos on play sessions-including tickling-on a bonobo group housed at La Vallée des Singes (France). We showed that tickling received decreased while tickling performed increased with age, with tickling being mostly directed from older to younger individuals. Moreover, tickling was mostly performed by the individuals that started the play interaction and most of it occurred in strongly bonded dyads, particularly mother-infant ones. Bonobo tickling features, especially age profile and social modulation, mirror those of heavy tickling in humans thus suggesting a common evolutionary origin and shared patterns of basic intersubjectivity in hominins.