Fig. 3.
Proposed facial expression homologues in chimpanzees and humans. From left to right, the chimpanzee expressions show the bared-teeth display, pant-hoot, play face, scream face, and bulging-lip face. Corresponding human expressions are shown in the top row, along with the action units (AUs) shared by the expressions in both species. (Human photos from Ekman, Friesen, & Hager, 2002; chimpanzee photos courtesy L.A. Parr and the Living Links Center, Emory University.)