Differences in the Mutual Eye Gaze of Bonobos (Pan paniscus) an Chimpanzees (Pan troglodytes)

Eye gaze is widespread in nonhuman primate taxa and important for social cognition and communicative signaling. Bonobos and chimpanzees, two closely related primate species, differ in social organization, behavior, and cognition. Chimpanzees' eye gaze and gaze following has been studied extensively, whereas less is known about bonobos' eye gaze. To examine species differences using a more ecologically relevant measure than videos or pictures, the current study compared bonobo and chimpanzee mutual eye gaze with a human observer. A multivariate analysis of variance revealed significant species differences in frequency and total duration, but not bout length, of mutual eye gaze (p < .001). Specifically, bonobos engage in mutual eye gaze more frequently and for longer total duration than chimpanzees. These results are likely related to species differences in social behavior and temperament and are consistent with eye-tracking studies in which bonobos looked at the eye region of conspecifics (in pictures and videos) longer than chimpanzees. Future research should examine the relationship between mutual eye gaze and gaze following, as well as examine its genetic and neurological correlates.