Kinematics and energetics of nut-cracking in wild cacpuhin monkeys (Cebus libidinosus) in Piaui, Brazil

Wild bearded capuchins (Cebus libidinosus) in Brazil crack nuts using large stones. The biomechanics of the nut-cracking action of two males and two females were examined. From a bipedal stance, the monkeys raised a heavy hammer stone (1.46 kg and 1.32 kg) to an average height of 0.333 meters, 60% of body length. The stone was then rapidly lowered by flexing the lower extremities and the trunk until the stone contacted the nut. A hit consisting of an upward phase and a downward phase averaged 0.744 seconds in duration. The upward lifting phase lasted 69% of hit duration. The two males added substantial energy to the stone in the downward phase. The monkeys developed individualized kinematic strategies. Kinematic results are compared to human weight-lifters and chimpanzees, indicating nut-cracking is strenuous for capuchins. The current study provides a reference point for the evolution of percussive tool use and bipedality in primates.