HEESY, C.P.; HALL, M.I.; NYCOM; Stony Brook Univ.: Morphological Convergence and Functional Nonconvergence between Orbit Orientation and Visual Field Overlap in Avians and Mammals

Orbit orientation has long been suggested to reflect eye position and to be correlated with the degree of binocular visual field overlap in vertebrates. Based on this assumption, a number of studies have used orbit orientation as a proxy for visual field shape in comparative studies of visual ecomorphology in mammals. However, the relationship between orbit orientation and visual field shape has only been evaluated in a small sample of mammals, and is virtually unstudied in other vertebrates. Interestingly, strigiform and falconiform avians were believed to have large binocular visual field overlap based on qualitative observations of eye position and orbit orientation. However, in vivo reflex ophthalmoscopic data show that these taxa actually have minimal binocularity when compared to mammals and other vertebrate taxa. In this study we quantify the relationship between orbit and visual field orientation in visually directed avians and mammals, two groups for which visual field shape data are available. 3-D coordinate data on orbit orientation were collected on skulls of mammals and avians. In general, orbit and visual field orientation are highly correlated and isometric in mammals. Taxa with high orbit convergence also have broad binocular visual field overlap whereas taxa with divergent orbits have panoramic visual fields. In contrast, binocular visual field overlap in birds is negatively allometric. Taxa such as strigiformes have larger binocular visual fields than other avians, but smaller than would be expected based upon a mammalian model. The differences between these two classes may reflect differences in visual ecology and morphology as well as possible spatial constraints in the avian skull.