research :: facial expression categorization
Chimpanzees have a wide variety of facial expressions and they are used in flexible ways, bridging social contexts and often appearing in graded intensities and/or blends between several categories.
Figure 1 below shows some examples of prototypical facial expression categories in chimpanzees.
There are several excellent ethograms describing chimpanzee facial expressions, including those of Dr. Jane Goodall (Goodall, 1968) and Professor Jan van Hooff (1973). We have also published an ethogram of chimpanzee facial expressions and vocalizations (Parr, Cohen & de Waal, 2005).
Click here to download a PDF of this ethogram.
Very little is understood about how chimpanzees discriminate among these highly salient social signals, and what they may communicate. Our lab has conducted some of the only studies on facial expression categorization in chimpanzees utilizing the MTS paradigm. Subjects were able to discriminate among most categories of expressions without any additional training. They appear to do this using a combination of distinctive features, such as the visibility of teeth, or mouth opening. However, overall facial configuration seems also to be important (Parr, Hopkins & de Waal, 1998).