A practical approach to understanding lateral asymmetries in body, brain, and cognition would be to examine the performance advantages/disadvantages associated with the corresponding functions and behavior. In the present study, we examined whether the division of labor in hand usage, marked by the preferential usage of the two hands across manual operations requiring maneuvering in three-dimensional space (e.g., reaching for food, grooming, and hitting an opponent) and those requiring physical strength (e.g., climbing), as described by Mangalam et al. , is associated with higher hand performance in free-ranging bonnet macaques, Macaca radiata. We determined the extent to which (a) the macaques exhibit laterality in hand usage in an experimental unimanual and a bimanual food-reaching task, and (b) manual laterality is associated with hand performance in an experimental hand-performance-differentiation task. We found strong negative relationships between (a) the performance of the preferred hand in the hand-performance-differentiation task (measured as the latency in food extraction; lower latency = higher performance), the preferred hand determined using the bimanual food-reaching task, and the normalized difference in the performance between the two hands (measured as the difference in the latency in food extraction between them normalized by the latency in food extraction using the preferred hand), and (b) the normalized difference in the performance between the two hands and the manual specialization (measured as the absolute difference in the laterality in hand usage between the unimanual and the bimanual food-reaching tasks; lesser difference = higher manual specialization). These observations demonstrate that the division of labor between the two hands is associated with higher hand performance.
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