Manual Preference in Varieties of Reaching in Squirrel Monkeys

Abstract

Primates, unlike other mammals, have the ability to grasp objects with one hand. Capability for one-handed prehension evolved as part of the early primate specialization for manual dexterity that eventually included development of pseudo and fully opposable thumbs and the ability for precision gripping (Napier & Napier, 1967). The subsequent intensive and skilled hand use provided an adaptive basis for lateralized specialization of function for the two hands. This manual lateralization is manifested by the right-handedness in about 90% of the human population (Hecaen & de Ajuriaguerra, 1964). Important theoretical questions involve whether motor or cognitive asymmetries were the first to appear during primate evolutionary development and the causal relationship between the two types of asymmetry during evolutionary development. If nonhuman primates display varieties of lateralized handedness, they may be preadaptations not only for highly skilled human right-handedness but also for some language related skills (MacNeilage, 1987). Progress in answering these questions has been impeded by the common assumption that nonhuman primate handedness is nonexistent. When nonhuman handedness has been reported, the finding has often been dismissed as a laboratory artifact of trivial importance. It is further reported to be situation specific and not stable over long periods of time (Warren, 1977, 1980).