Chimpanzees, Pan troglodytes, and bonobos, Pan paniscus, differ greatly in their social relationships and psychology, as many chapters in this book show (e.g. Takahata et al., Chapter 11; de Waal, Chapter 12). Why they do so is not understood. Yet since these are the two closest relatives of humans, and since each species has a different set of similarities with humans, the question is especially important by virtue of its relevance to human behavior. Why, for instance, do humans and chimpanzees have similarly violent intergroup aggression? The answer will likely depend on understanding why bonobos do not.

In general, bonobos have more relaxed relationships than chimpanzees, with a more pervasive web of alliances or friendships linking community members, especially adult females. This set of differences is thought to depend crucially on the ecological costs of grouping. Thus, friendly social relationships among female bonobos are thought to be possible because their parties are relatively stable, with individuals rarely forced to be solitary. Equivalently friendly relationships among female chimpanzees, on the other hand, are prohibited because parties are regularly forced to fragment, as a result of feeding competition when fruits are scarce (Chapman et al., 1994). Over evolutionary time, such differences have led to differences in species psychology (e.g. Wrangham, 1993).

Resource-based sociality is the only framework so far proposed to explain the ultimate sources of behavioral differences between the two species.