SEPARATE, YET CONNECTED: SOCIAL CONTAGION WITHIN AND BETWEEN TWO GROUPS OF CHIMPANZEES

Behavioural or social contagion is the involuntary replication of others' actions and may inform the transmission of the emotional states that underlie such actions. Interactive behavior contagion is understudied. We investigated presence, timing, and networking of social contagion in adult chimpanzees, for aggression, allogrooming, and social play (target behaviours). We extracted 118.5 hrs of observational data from videos recorded simultaneously on two captive chimpanzee groups in visual/acoustic contact, housed at Mona Foundation (Spain). When a chimpanzee (trigger) started a target behaviour, we recorded which individuals (in-group/out-group) detected it (detection/non-detection condition) and whether and when they performed the same behaviour within 3-min. Grooming and aggression contagion were present and peaked in the first two minutes whereas play contagion occurred at comparable rates across all minutes. Aggression tended to be more contagious within groups. Grooming and aggression contagion were highest in young adults and -for aggression- in males. Aggression can propagate within groups and young adults can form alliances via grooming and - especially if males - compete for dominant positions. Low play contagion in adults may explain why it was no modulation factor emerged. Social network analyses suggested a double valence of social play because individual Eigenvector centrality values in the play and aggression contagion networks were positively correlated, and a correlation trend was found between play and grooming contagion networks. This study, focusing on chimpanzees as our closest relatives, sheds light on the biological roots of behaviour transmission and potentially the emotions (linked to arousal, care, reward systems) behind it.