Hand preference for spontaneously unimanual and bimanual coordinated tasks in wild Sichuan snub-nosed monkeys: Implication for hemispheric specialization.

Lateralized behaviour has been studied as an observable measure of cerebral functional asymmetry for many years, and interest in the evolutionary origins of lateralized behaviour in humans has prompted research into the study of manual laterality in nonhuman primates. The Sichuan snub-nosed monkey (Rhinopithecus roxellana) is a rarely studied species as far as laterality research is concerned. In this study, we investigated hand preference in spontaneously unimanual and bimanual coordinated tasks during grooming in wild R. roxellana in the Qinling Mountains of China. We chose 26 adult individuals (20 females and 6 males) for both tasks. Wild R. roxellana shows group-level left-handedness in bimanual coordinated grooming task, but not in unimanual grooming task, which is just found on the individual-level based on z-scores. Both the direction and strength of hand preference in the bimanual task are significantly stronger than that in unimanual task. Our findings present the first evidence of group-level handedness in wild monkeys although group-level handedness had been reported in wild apes [e.g. Pongo pygmaeus: Rogers LJ, Kaplan G. Hand preference and other lateral biases in rehabilitated orangutans, Pongo pygmaeus pygmaeus. Anim Behav 1996;51:13-25]. The results in R. roxellana support the theory of task complexity as well as the postural origin hypothesis.