Rough and tumble play of chimpanzees (Pan Troglodytes) at Kyoto City Zoo

抄録

Rough and tumble play (RTP) is one of the most common types of social play in mammals. Yet, the functions of this activity are unclear. Panksepp’s (1998) study in rats (Rattus norvegicus) suggests that RTP might serve to cultivate self-regulation in animals laying the foundation for social morality. By investigating how chimpanzees regulate their aggression during RTP, we can have some insights into the origin of our moral conduct, namely behaving fairly. 124 hours of behavioral observation of six chimpanzees was conducted at Kyoto City Zoo. Continuous sampling was used to record the duration of RTP, types of play solicitation, the role of players, mode of termination, play intensity, and the presence or absence of play retrieve. Play bouts were categorized into three types based on how chimpanzees regulated aggression: fair, normal domination, and transgression. The findings of this study support the hypothesis that when a socially dominant chimpanzee plays with a subordinate partner, fair play will account for at least 30% of their total bouts. This result may shed new light on the study of animals’ social play by hinting at the existence of morality in chimpanzees which is likely to emerge in RTP. Also, the presence of play retrieve and direct play solicitation significantly extended the duration of play while play intensity did not. On this basis, the connection between social play and morality should be considered when conducting comparative studies on rough and tumble play in non-human animals.