The self-domestication hypothesis: evolution of bonobo psychology is due to selection against aggression

Experiments indicate that selection against aggression in mammals can have multiple effects on their morphology, physiology, behaviour and psychology, and that these results resemble a syndrome of changes observed in domestic animals. We hypothesize that selection against aggression in some wild species can operate in a similar way. Here we consider the bonobo, Pan paniscus, as a candidate for having experienced this ‘self-domestication’ process. We first detail the changes typically seen in domesticated species including shifts in development. We then show that bonobos show less severe forms of aggression than chimpanzees, Pan troglodytes, and suggest that this difference evolved because of relaxed feeding competition. We next review evidence that phenotypic differences in morphology and behaviour between bonobos and chimpanzees are analogous to differences between domesticates and their wild ancestors. We then synthesize the first set of a priori experimental tests of the self-domestication hypothesis comparing the psychology of bonobos and chimpanzees. Again, bonobo traits echo those of domesticates, including juvenilized patterns of development. We conclude that the self-domestication hypothesis provides a plausible account of the origin of numerous differences between bonobos and chimpanzees, and note that many of these appear to have arisen as incidental by-products rather than adaptations. These results raise the possibility that self-domestication has been a widespread process in mammalian evolution, and suggest the need for research into the regulatory genes responsible for shifts in developmental trajectories in species that have undergone selection against aggression.