Bonobo but not chimpanzee infants use socio-sexual contact with peers

Bonobos have been observed to use socio-sexual behavior at higher frequency than chimpanzees. Little is known about the developmental influences that shape this behavior in bonobos. We compared the social sexual behavior of wild-born bonobo (n = 8) and chimpanzee (n = 16) infants in an experimental feeding test. Subjects of both species were orphans of the bushmeat trade living at sanctuaries in peer groups. During the experiment, chimpanzee infants never had socio-sexual interactions with one another. In contrast, bonobo infants had socio-sexual interactions significantly more than the chimpanzee infants and more often when food was presented. During these socio-sexual interactions, bonobo infants did not show a preference for heterosexual partners or genital-genital positioning that is reproductive in adults (e.g. a dorso-ventral posture). These findings suggest that the socio-sexual behavior previously observed in various captive and wild bonobos is species-typical. Wild-born bonobos originating from a large geographical range develop this behavior long before puberty and without the need for adults initiating such behavior or acting as models for observational learning. Meanwhile, chimpanzee infants of the same age with similar rearing history show no signs of the same socio-sexual behavior. Results are interpreted regarding hypotheses for the evolution of bonobo psychology.