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Abstract
The appearance of stone-assisted tooling and bipedalism is a milestone in human evolution. However, because of the fragmentary nature of the fossil record and challenges in reconstructing behavior in fossils, changes in posture and motor control that accompanied the evolution of this behavior have remained elusive. Quadrupedal nonhuman primates that adopt a bipedal stance while using stone hammers to process hard food provide a unique comparative reference point to investigate specific aspects of such changes. In my dissertation research, I investigated the multi-joint posture and movement control in a population of wild bearded capuchin monkeys, Sapajus libidinosus using anvil-and-hammer tools in bipedal stance. Some populations of Sapajus crack open palm nuts using stone hammers during routine foraging. In the first and second studies, I found that the monkeys modulate the kinematic parameters of individual strikes and the organization of successive strikes to accommodate the nuts resistance-to-fracture. In the third study, I found that the monkeys keep the strikes amplitude and the hammers velocity at impact constant with respect to hammer mass. However, in doing so, the hammers kinetic energy at impactwhich determines the propagation of a fracture/crack in a nut and is the key parameter that humans control while knapping stones to manufacture stone toolsvaries across hammers of different mass. In the fourth study, I found that the monkeys use joint synergies to stabilize the hammer trajectory while cracking nuts in bipedal stance. Together these findings show that although inadequate to manufacture stone tools like humans do, these monkeys solve the challenge of postural and movement control by controlling hammers velocity at impact instead of hammers kinetic energy at impact, and structure motor variability strategically. They highlight the importance of studying the evolution of bipedal striking, and more generally, research on tooling in nonhuman animals, using method of biomechanics and human movement science.