Ecomorphology of orbit orientation and the adaptative significance of binocular vision in primates and other mammals

HEESY, C.P. 2008. Ecomorphology of orbit orientation and the adaptative significance of binocular vision in primates and other mammals. Brain, Behaviou and Evolution 71:54-67. doi: 10.1159/000108621

Abstract. Primates are characterized by forward-facing, or convergent, orbits and associated binocular field overlap. Hypotheses explaining the adaptive significance of these traits often relate to ecological factors, such as arboreality, nocturnal visual predation, or saltatory locomotion in a complex nocturnal, arboreal environment. This study re-examines the ecological factors that are associated with high orbit convergence in mammals. Orbit orientation data were collected for 321 extant taxa from sixteen ordens of metatherian (marsupial) and eutherian mammals. These taxa were coded for activity pattern, degree of faunivory, and substrate preference. Results demonstrate that nocturnal and cathemeral mammals have significantly more convergent orbits than diurnal taxa, both within and across ordens. Faunivorous eutherians (noth nocturnal and diurnal) have higher mean orbit convergence that opportunistically foraging or non-faunivorous taxa. However, substrate preference is not associated with higher orbit convergence and, by extension, greater binocular visual field overlap. These results are consistent with the hypothesis that mammalian predators evolved higher orbit convergence, binocular vision, and stereopsis to counter camouflage in prey inhabiting a nocturnal environment. Strepsirhine primates have a range of orbit convergence values similar to nocturnal or cathemeral prodatory non-primates mammals. These data are entirely consistent with the nocturnal visual predation hypothesis of primate origins.