Face scanning in chimpanzees and humans: continuity and discontinuity

How do chimpanzees, Pan troglodytes, the species with the closest evolutionary connection to humans, view faces? This study is the first to use the eye-tracking method to perform direct comparisons between humans and chimpanzees with regard to face scanning. Members of both species viewed the same sets of photographs representing conspecific and nonconspecific faces under the same experimental conditions. Chimpanzees and humans showed systematic and similar patterns of face scanning, including intensely viewing main facial features (i.e. eyes, nose and mouth) and inspecting the eyes and mouth, in that order. However, several differences between the species were also evident. For example, humans were more likely to show sequential refixations on the eye regions than were chimpanzees, whereas chimpanzees were more likely to engage in quick, vertical scanning over the eyes and mouth. Such species similarities and differences were consistent across conspecific and nonconspecific faces and were thus independent of the external morphologies of species-specific faces. Furthermore, when presented with facial expressions, chimpanzees changed their scanning patterns in response to those facial actions, whereas humans maintained intense eye viewing across the expressions. Finally, we discuss how these face-scanning patterns are related to species-specific forms of facial communication in chimpanzees and humans, and suggest that both species have unique eye movement strategies for interactions with conspecifics.