Distinct Perceptuomotor Features of Percussive Tooling in Humans (Homo sapiens) and Wild Bearded Capuchin Monkeys (Sapajus libidinosus)

Adjustment of percussive movements to match the energetic requirements of the task serves as an index of skill in stone-knapping and nut-cracking. In this study, we compared strike-by-strike adjustment of percussive movements in expert bearded capuchin monkeys (Sapajus libidinosus) and (novice and expert) humans cracking palm nuts (Astrocaryum spp.) using stone hammers of varying mass. The monkeys regulated each strike's amplitude and hammer's velocity at impact to a small degree based on the propagation of a fracture in the nut's shell but overall struck a nut with consistent kinetic energy for a given hammer. They showed the smallest movement variability across strikes among all groups. Novice humans took the most strikes to crack a nut among all groups. Expert humans took the fewest strikes among all groups while showing the largest variation in hammer's kinetic energy at impact and largest movement variability across strikes. Overall, our findings confirm that expert humans can adjust their strikes more precisely to meet variable task demands and are more efficient than monkeys or novice humans in cracking nuts, but the findings also show a surprising advantage for expert monkeys over novice humans. The monkeys' strategy of striking with consistent amplitude and velocity is advantageous when hammer mass is adequate to generate sufficient kinetic energy at impact and crack open a nut in few strikes without requiring fine adjustments, explaining why the monkeys are particularly attentive to a stone's mass when choosing it as a hammer.