Dynamics of the temporal structures of playing clusters and cliques among wild chimpanzees in Mahale Mountains National Park.

The overall structure and temporally changing configuration of members of social play among the wild chimpanzees (Pan troglodytes schweinfurthii) of Mahale Mountains National Park, Tanzania, were described on both the microscopic 'clique' levels, conceptualized as directly connected configurations through social play behavior, and macroscopic 'cluster' levels, conceptualized as indirectly connected gatherings of members of adjacent multiple cliques at the same time and space. Most playing clusters started as dyads. Although the cumulative number of participants increased, each clique size remained at 2 for most of the observational units. Dyadic cliques were more stable and lasted longer than larger cliques. Of all clusters' playing fields, 64.7 % had maximum diameters of 3 m. In summary, chimpanzees played stably in dyadic pairs in most of the time. As time passed, other chimpanzees often joined in the playing groups to form large polyadic clusters. Even when all chimpanzees in a cluster played socially at the same time, they normally did so in separate dyadic pairs, forming multiple dyadic cliques simultaneously in a small space. These social play dynamics may be explained assuming a hypothesis based on a balance model among socially playing chimpanzees, as the balanced cliques are limited only to those in which all the existing pairs form the mutual dyads, and they tend to avoid unbalanced and maintain balanced relationships during social play. As a result, larger cliques were difficult to maintain for long periods and tended to transition into dyadic mutual cliques. Thus, Heider's balance theory can be one of the possible theories to explain not only human social phenomena, but also the proximate mechanism of the structure and the temporal change of social play among wild chimpanzees. Although both mutual and transitive relationships are known to be balanced in various human networks, only mutual relationships among socially playing chimpanzees were balanced.