Abstract
Human between-group interactions are highly variable, ranging from violent to tolerant and affiliative. Tolerance between groups is linked to our unique capacity for large-scale cooperation and cumulative culture, but its evolutionary origins are understudied. In chimpanzees, one of our closest living relatives, predominantly hostile between-group interactions impede cooperation and information flow across groups. In contrast, in our other closest living relative, the bonobo, tolerant between-group associations are observed. However, as these associations can be frequent and prolonged and involve social interactions that mirror those within groups, it is unclear whether these bonobos really do belong to separate groups. Alternatively, the bonobo grouping patterns may be homologous to observations from the large Ngogo chimpanzee community, where individuals form within-group neighborhoods despite sharing the same membership in the larger group. 

To characterize bonobo grouping patterns, we compare the social structure of the Kokolopori bonobos with the chimpanzee group of Ngogo. Using cluster analysis, we find temporally stable clusters only in bonobos. Despite the large spatial overlap and frequent interactions between the bonobo clusters, we identified significant association preference within but not between clusters and a unique space use of each cluster. Although bonobo associations are flexible (i.e., fission–fusion dynamics), cluster membership predicted the bonobo fission compositions and the spatial cohesion of individuals during encounters. 

These findings suggest the presence of a social system that combines clear in-group/out-group distinction and out-group tolerance in bonobos, offering a unique referential model for the evolution of tolerant between-group interactions in humans.

Keywords
Bonobo, intergroup relations, grouping patterns, chimpanzee, social structure

Significance
In striking contrast to chimpanzees, tolerant betweengroup interactions in bonobos, our other closest living relative, have been suggested as a pathway to understanding the evolution of human betweengroup peacemaking and cooperation. However, due to the lack of data on bonobo group dynamics and partitioning, bonobo between-group tolerance has been routinely challenged.
Here, we compared the social systems of bonobos and chimpanzees to find distinct and stable social groups in bonobos that interact regularly and peacefully. Despite their tolerant meetings that exceed the rates previously described between human groups, bonobo groups maintained exclusive social and spatial borders. Tolerance occurring between socially distinct groups highlights the potential of bonobos as a referential model for the evolution of between group cooperation in humans.