ChimpanSEE, ChimpanDO: Grooming and play contagion in chimpanzees.

Behavioural contagion-the onset of a species-typical behaviour soon after witnessing it in a conspecific-forms the foundation of behavioural synchrony and cohesive group living in social animals. Although past research has mostly focused on negative emotions or neutral contexts, the sharing of positive emotions in particular may be key for social affiliation. We investigated the contagion of two socially affiliative interactive behaviours, grooming and play, in chimpanzees.