Figure 8.

An illustration of five individual component movements, that is, action units, used to describe the prototypical chimpanzee facial expressions. From left to right these include AU10–upper lip retractor, AU12–lip corner puller (bared-teeth, play face2, whimper, scream), AU16- lower lip depressor (bared-teeth, scream), AU22–lip funneler (pant-hoot and whimper) and AU27–mouth stretch (play face2 and scream). An illustration of each individual action unit, from left to right AU10–upper lip raiser, AU12–lip corner puller, AU16–lower lip depressor, AU22–lip funneler, and AU27–mouth stretch. Each chimpanzee facial expression contains a unique combination of some or most of these action units.