The perception of two-tone Mooney faces in chimpanzees (Pan troglodytes)

Neurological experiments have revealed a complex network of areas in the human brain that respond more to faces than to other categories of objects and thus have been implemented in face categorization. The aim of this study was to investigate whether chimpanzees (n = 5), our closest living relatives, detect and categorize faces on the basis of first-order information, and whether this sensitivity is specific to faces or generalizes to other objects. In service to this aim, we created multiple categories of two-tone 'Mooney' objects (chimpanzee faces, shoes, human hands), because, by maximizing contrast, the Mooney transformation selectively degrades second-order information (the basis for individual discrimination in humans), leaving only first-order information intact. Two experiments used a 2AFC MTS procedure. The first experiment provided strong evidence that, like humans, chimpanzees categorize Mooney faces as faces. However, without second-order information, the chimpanzees could not match Mooney faces at the individual level. In Experiment 2, four of the five chimpanzees found it easier to categorize Mooney faces than Mooney shoes. Neurological evidence strongly indicates a dedicated neural mechanism for face categorization in the human brain, and our data suggest that chimpanzees share this level of specialization.