Freestanding Bipedal posture and coordinated bimanual manipulation significantly influence lateralized hand use in rhesus monkeys (Macaca mulatta) and chimpanzees (Pan troglodytes)

Investigations of behavioral lateralization in nonhuman primates yield important insights into brain- behavior relationships. In turn, they provide clues about both proximal and distal factors that shape the development and expression of association between motor asymmetries and underlying neural substrates. Nonhuman primates afford unique comparative opportunities to evaluate potential routes for the evolution of handedness, as well as to uncover relationships between behavioral lateralization and underlying neural, genetic, and physiological correlates. We examined hand preference in 22 rhesus monkeys and 79 chimpanzees using unimanual reaching tasks varying in postural stability and in a coordinated bimanual task. The majority of rhesus monkeys and chimpanzees showed significant lateral biases when reaching from a freestanding posture and when engaged in a coordinated bimanual task. Population-level directional bias was not evident for any task for rhesus monkeys and was observed only in the bimanual task for chimpanzees. We did not find consistent relationships between an individual's hand preference for different types of tasks. Both freestanding bipedal posture and coordinated bimanual hand use elicited significantly stronger lateral biases in reaching when compared with quadrupedal reaching. These data support the hypothesis that both degrees of postural instability and complex manipulation, such as bimanual coordination, may influence the expression of behavioral asymmetries in primates. These results demonstrate robust lateralization occurs at the individual level. Our results also highlight the need for greater consideration of task type and descriptive data in studies aimed at evaluating brain- behavior relationships and individual differences associated with hand preference.