Face Discriminations by Orangutans (Pongo spp.) Vary as a Function of Familiarity

Faces provide humans with information on the age, sex, individual identity, and emotional state of other individuals. Face discrimination was likely advantageous in the evolution of many group living species; however, little is known about how a species’ sociality relates to their face discrimination skills. This may be particularly interesting in the context of discriminating familiar versus unfamiliar faces, as species that do not spend as much time in social groups may not differentiate between these two categories as compared to species that spend more time in groups. Previous studies in humans and chimpanzees, both group living species, have found differences in performance based on familiarity such that performance decreases across changes in viewpoint when discriminating unfamiliar, but not familiar faces. In this study, we tested a less gregarious species, orangutans, to determine if face discrimination skills differed from these other primates. Using a matching-to-sample paradigm, we found that two of the three orangutans performed significantly above chance when discriminating novel photographs of familiar individuals but not novel photographs of unfamiliar individuals. Thus, while additional data are needed on species that are even less gregarious than orangutans, our results indicate that, at least within the primates, more and less gregarious species show the same bias towards better discrimination of familiar faces. Further examination is needed to understand social organization and other social factors that may be important in the evolution of face-processing skills.