Facial displays provide information about what an individual is likely to do next. This information is essential for negotiation since the appropriate response to a facial display can inhibit aggression and/or promote affiliation. The “social cohesion hypothesis” posits that facial expression has been designed by natural selection to maintain cohesion among group members. The purpose of this paper is to test the comparative predictions of this hypothesis in diurnal anthropoids.
I used two measures of signal effectiveness: facial mobility, and published data on facial nucleus volume (N = 26). Facial mobility data were collected from video recordings of zoo animals. The facial-action coding system (FACS) was used to count the number of facial movements in each species (N = 12). Socioecological, body mass, and additional brain component data were taken from the literature. Partial effects were examined using multiple regression analyses of independent contrasts. Body mass and group size were entered as predictor variables in the facial mobility analysis. Medulla volume, group size, and grooming time were used in analyses of facial nucleus volume.
The social cohesion hypothesis is supported by the results of this study. Facial mobility is positively correlated with group size in anthropoids, controlling for size and phylogeny. Similarly, facial nucleus volume is correlated with group size and grooming time. However, these effects are only significant after excluding platyrrhines and the outlier Pongo pygmaeus. Thus, while facial expression likely evolved for the purposes of social cohesion, this particular selection pressure may have been confined to catarrhines.
Research supported by NSF (#0424160) and Sigma-Xi.