Meeting Abstract
A few wild populations of bearded capuchin monkeys, Sapajus libidinosus use stones and wood pieces in their natural form for pounding palm nuts and other encased food. In the present study, we investigated the biomechanical strategies employed by wild monkeys at Fazenda Boa Vista, Piauí, Brazil to coordinate their movement to control heavy hammers while cracking nuts. We performed the UCM analysis on joint angle movements of BC monkeys of different body mass striking nuts with hammers of different mass. To crack open a nut, monkeys must strike it with a hammer with adequate value of the hammer’s kinetic energy at impact within a range of the point and angle of impact. However, the monkeys kept the strike’s amplitude and the hammer’s velocity at impact, but not the hammer’s kinetic energy at impact unchanged with respect to hammer mass. Surprisingly, the monkeys showed very small variability in the location of the hammer’s center of mass, although the latter appeared to increase with hammer mass. UCM analysis revealed that joint angle variability was greater in the controlled subspace compared to the uncontrolled subspace. In other words, well-defined joint synergies characterized the striking movements of the monkeys. Body mass and hammer mass appeared to influence the strength of the synergy, that is, the ratio of joint angle variability in the controlled and uncontrolled subspace. These results suggest that wild bearded capuchin monkeys have discovered unique motor solutions to cracking nuts through joint synergies that allow them to control the trajectory of heavy hammers. We discuss these results in the light of orgasmic, environmental, and task constraints on monkeys’ striking movements.
