Hand preferences in unimanual and bimanual feeding by wild vervet monkeys (Cercopithecus aethiops)

Lateral preference was examined in spontaneous feeding actions in 2 troops of wild vervet monkeys (Cercopithecus aethiops). Processing of 4 foods (ter mites, leaf shoots, sugarcane, and fruit) was studied. Actions included uni manual reaching to moving objects, operating from an unstable posture, and coordinated bimanual processing. Between 19 and 31 subjects were available, according to the task. In, 2 tasks, laterality of 2 independent stages was measured separately, giving 6 measures in all. On 4 of these measures, mos t monkeys were ambipreferent, and only a few showed significant hand prefer ences. Only for termite feeding and detaching material from fruits did the majority show significant lateralization; no tasks elicited exclusive use o f 1 hand. Preference appeared labile, because in 2 tasks, population trends reversed with increasing age. No population trends to left or right were f ound; instead, these monkeys showed ambilaterality, with lateralization ass ociated with task complexity.