Abstract
The direction of the orbits in mammals, especially in primates, was examined to explain orbital convergence in primates. The orbital axes of old world monkeys are between 40°∼50°, while those of new world monkeys exceed 50°. The orbital axes tend. to be even larger in callithricids. In Prosimians, the axis angle ranges from 60°∼100°, and is clearly larger than those of the anthropoids. The orbital axis angle of carnivores is between those of prosimians and anthropoids. However, their orbital planes have not turned to the front, because the olfactory sense is also important for them. Ungulates have large orbital axis angles over 100°. It is clear that arboreality is possible even if the orbit has not turned to the front as it is in anthropoids, because tree shrews or squirrels do not have orbits rotated to the front as in anthropoids. Carnivores, although they are terrestrial mammals, have orbital axis angle as small as in primates. As a result, the frontal rotation of the orbit was not caused simply by the adaptation to arboreal life, supporting the visual predation hypothesis advocated by Cartmill (1972).
