The gaze that grooms: Contribution of social factors to the evolution of primate eye morphology

There is a wide variation in the external eye morphology across species in primates, which is considered to reflect adaptation to ecological factors such as body size and habitat type. However, little attention has been paid to the contribution of social factors to the evolution of primate eye morphology. To explore this, we analyzed correlations among eye morphology, social factors (neocortex ratio and group size) and other factors (habitat type and body mass) in 30 living primate species including humans, using phylogenetically independent contrasts. The results indicated that parameters of primate eye morphology correlate with group size and neocortex ratio (Study 1). Further analysis of behavior indicated that the proportion of scanning with eyeball movement alone per total scanning correlated with group size and neocortex ratio (Study 2). The results support the view that the scanning with independent eyeball movement and its morphological basis is an adaptation to larger social groups. Communicative functions of the gaze signal other than the expression of aggression, observed in some primate species, may be based on features related to eye morphology. Furthermore, the evolution of a contact-free, social grooming function of gaze, especially predominant in humans, may reflect one extreme case of this kind of adaptation, which we call the "gaze-grooming" hypothesis.