Quantifying ocular morphologies in extant primates for reliable interspecific comparisons

Cross-examining evidence from comparative morphology has become a way to support specific hypotheses in social cognition. In particular, ocular morphology has been compared across extant primate species in order to argue that humans present a unique morphology that enables also an uniquely human array of socio-cognitive functions—including a crucial role in the acquisition and practice of linguistic abilities. Even though some of these comparisons have relied on quantifiable dimensions of ocular morphology, other aspects of the comparison have been established qualitatively, based on subjective ratings. In this article, I present a new method that intends to restrain the focus of attention to one specific perceptual aspect of ocular morphology—namely, the contrast between scleral and iridal colors, as it is thought to enable (or ease) gaze following from other individuals. The method also allows for more reliable comparisons across extant primate species, as it relies on quantifiable measurements. I exemplify the potential of this method with a three-fold comparison of three great ape species ( Pongo pygmaeus , Pongo abelii , and Homo sapiens ).