“Visual Predation,” Habitat Structure, and the Ancestral Primate Niche

Abstract

Many of the characters of the basic suite of primate adaptations have at one time or the other been ascribed to arboreality. Frederick Wood Jones (1916) proposed that tree climbing lead to selection for a grasping forelimb, and Grafton Elliott Smith (1924) that grasping feet and hands were an adaptation for agility in a complex three-dimensional environment. Smith argued that these developments lead to replacement of the sense of smell by the senses of vision and touch, and consequent reduction of the snout. Collins (1921) and Le Gros Clarke (1959) noted a link between frontally-directed, convergent orbits and stereoscopic vision, which, Le Gros Clarke suggested, was advantageous for estimation of distances in arboreal leaping. But Cartmill (1972; 1974a & b; 1992) has challenged this “arboreal hypothesis” (Howells, 1947), indicating that the primate facility in arboreal movement per se could be matched, or even exceeded, by animals such as squirrels, with clawed, convergent digits, enhanced sense of smell, and laterally-facing orbits. He showed that clawed cheiridia actually offer locomotor advantages on large diameter vertical supports over clawless, grasping cheiridia (Cartmill, 1972a), and noted that “Despite their laterally directed eyes (and presumed lack of stereoscopy) squirrels of several genera may leap 13 to 17 body lengths from tree to tree, which compares favorably with the 20 body lengths reported for the saltatory lemuroid Propithecus verreauxi” (Cartmill, 1974b).