I presented a paper entitled "Allometry of facial mobility in anthropoid primates" at the 2008 meeting of the Northeastern Anthropological Association in Amherst, MA. Here's the abstract:
Body size is likely to be an important constraint on the evolution of facial expression in primates. This is because facial movements are more difficult to discern in smaller species. I tested the hypothesis that facial mobility is positively correlated with body size in a comparative sample of nonhuman anthropoids. Facial mobility, or the variety of facial movements a species can produce, was estimated using the human Facial Action Coding System (FACS). I used FACS to estimate facial mobility in 12 nonhuman anthropoid species, based on video recordings of facial activity in zoo animals. Body mass data were taken from the literature. I performed a multiple regression analysis with facial mobility as the dependent variable and two independent variables: log body mass and dummy-coded infraorder. Together, body mass and infraorder explain 92% of the variance in facial mobility. However, the partial effect of body mass is much stronger than for infraorder. The results of my study suggest that allometry is an important constraint on the evolution of facial mobility, which may explain why smaller taxa tend to exhibit fewer facial displays than larger taxa. More work is needed to clarify the structural bases of this allometric pattern.
Some of the other folks in my session were Richard Lawler and Stacey Matarazzo.