HEESY, C.P.; New York College of Osteopathic Medicine: Hunters and the Hunted: Ecological Influences on Mammalian Orbit Orientation

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. To date, although the nocturnal visual predation hypothesis is the consensus explanation for primate orbit convergence, it is not universally accepted. This study re-examines the ecological factors that are associated with high orbit convergence in mammals. Orbit orientation data were collected for over 331 extant taxa from sixteen orders of metatherian and eutherian mammals. These taxa were also 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 orders. Faunivorous eutherians (both nocturnal and diurnal) have higher mean orbit convergence than opportunistic or non-faunivorous taxa. However, substrate preference is not associated with greater visual field overlap. These results are consistent with the suggestion that mammalian predators evolved binocular vision and stereopsis to counter camouflage as well as possibly improving distance judgments for prey capture. Strepsirrhine primates have a range of orbit convergence values similar to nocturnal or cathemeral predatory non-primate mammals. These data are entirely consistent with the nocturnal visual predation hypothesis of primate origins.