| Compared to many other mammals, primates have well developed visual abilities and demonstrate a number of highly specialized adaptations of the visual system. This dissertation addresses the role that visual adaptations have played in primate evolutionary history by examining (1) the relationship between adaptations for increased visual acuity and relative brain size across primates, (2) the nature and timing of the appearance of adaptations for high visual acuity in haplorhines, and (3) the comparative state of visual adaptations in strepsirrhines.; Three separate methodological approaches have been employed. First, optic foramen quotients (osteological proxies for retinal summation) were calculated for a sample of 120 primate species (110 extant, 10 extinct) in order to draw inferences regarding the visual adaptations of early primates. Second, relative optic foramen size and endocranial volume for 101 primate, 71 carnivoran, 50 sciurid, and 15 scandentian species were used to test the hypothesis that the total amount of neural visual input to the brain may influence relative brain size. Third, data on eye and cornea size in 55 primate species were used to examine the effect of activity pattern on eye morphology, and the effect of eye morphology on orbit size. These three analyses reveal that (1) omomyiforms and adapiforms probably lacked all-cone retinal foveae similar to those of living diurnal anthropoids and (presumably) parapithecoids, (2) the amount of visual input has a significant influence on relative brain size in primates, as well as other mammals, and (3) nocturnal and diurnal primates consistently differ in relative cornea size but may have similar relative eye sizes.; The results of these three analyses support the conclusion that the relatively high encephalization of primates is partly the result of specializations for high visual acuity that appeared in the primate stem lineage. The origin of haplorhines was probably associated with a transition from nocturnality to diurnality and the subsequent evolution of adaptations for extremely high visual acuity (e.g., a retinal fovea and small relative cornea size). Increased visual input from the fovea led in part to the high encephalization of living anthropoids, and decreases in eye and cornea size produced several distinctive features of the anthropoid orbit (i.e., high convergence and constriction of the orbital aperture). By contrast, strepsirrhines do not exhibit adaptations for extremely high visual acuity comparable to those of anthropoids, but do exhibit adaptations for diurnality (e.g., reduced relative cornea size) similar to those of other diurnal mammals. |
