Chimpanzees, but not bonobos, attend more to infant than adult conspecifics

Abstract

Primate infants have several physical features that distinguish them from adults in appearance. One such feature is the well-known ‘baby schema,’ in which the infant has a relatively large head and eyes, and a small nose and mouth. Humans typically prefer infants of both conspecifics and heterospecifics with a strong baby schema. In nonhuman primates, infants of some (e.g. chimpanzees, Pan troglodytes) but not all species (e.g. bonobos, Pan paniscus) have species-specific conspicuous skin and fur coloration, in addition to species-common infantile features. This study examined whether closely related chimpanzees and bonobos have a preference for infants, and, if so, whether they are attracted to species-common features (e.g. the relatively large head and eyes) or species-specific features (e.g. conspicuous coloration). We used eye tracking to measure eye movements while participants viewed naturalistic images of a mother–infant pair. We adopted a cross-species design, so that both species viewed images of chimpanzees, bonobos and an outgroup primate species. We found that chimpanzees viewed the faces of infants for longer than those of adults when presented with conspecific images, but not when presented with heterospecific images. Chimpanzees also did not show any preference when facial coloration was matched between conspecific infants and adults in a follow-up experiment. Chimpanzees thus seem to respond particularly to the special infantile coloration of conspecifics. Bonobos clearly differed from chimpanzees in that they did not view conspecific infants for longer than adults and viewed heterospecific adults for longer than infants. These observed species differences may be related to their species-typical social and rearing styles. In conclusion, the preference for infants is species-specific in bonobos and chimpanzees, with a general lack of preference for the species-common baby schema.