It has been suggested that primates with more specialized visual systems tend to have relatively large brains due to selection for enlarged cortical visual processing areas. This “visual specialization hypothesis” is supported by several comparative studies. However, the behavioral bases of these co-evolutionary patterns remain unclear. If the visual specialization hypothesis is correct, then variation in visually-oriented behaviors should correlate with variation in brain size. The purpose of this study is to test this prediction by examining the co-evolution of facial expression complexity and relative brain size in extant anthropoids. Facial expression complexity is a function of facial mobility, or the number of visually distinct facial movements a species can produce. Data on facial mobility are currently available for 12 species. These data were combined with published estimates of endocranial volume and body mass. Phylogenetically-informed partial correlation analyses were used to examine the association between facial mobility and endocranial volume after controlling for body mass. Male and female data were analyzed separately. The results of this study provide broad support for the visual specialization hypothesis. Facial mobility is positively correlated with endocranial volume after controlling for body mass in males (partial r = 0.65; p = .031). However, females do not exhibit a significant partial correlation between facial mobility and endocranial volume (partial r = 0.10; p = .396). These findings suggest that male, but not female, brain size evolution is influenced by selection for facial expression processing.This presentation is part of an invited poster symposium entitled Ears, Eyes, and Noses: Revisiting the Evolution and Ecology of the Primate Special Senses. The session is organized by Carrie Veilleux, Eva Garrett, and Rachel Jacobs.
Wednesday, February 16, 2011
I will present a poster entitled "Co-evolution of facial expression and brain size: a test of the visual specialization hypothesis" at this year's annual meeting of the American Association of Physical Anthropologists. Here's the abstract: