Manual pressure distribution patterns of knuckle-walking apes.

Differences in how the hands of gorillas and chimpanzees contact the ground while knuckle walking have been noted but generally not quantified. It is widely believed that gorillas maintain a pronated arm and contact the ground with digits 2-5 consistently, while chimpanzees have variable arm position and digit contact. To further test these generalizations, distribution of pressure across the manus, peak digital pressures, and hand position were quantified using a pressure mat in eight captive chimpanzees (Pan troglodytes) and seven gorillas (Gorilla gorilla). Chimpanzees and gorillas make initial ground contact with the ulnar aspect of the hand and pressure moves radially. They differ in which digit usually makes final contact and receives maximum pressure, and hand position during contact. Gorillas regularly use a palm-back hand position and touch-off with digit 2. They show less variation in pressure application across the digits. Chimpanzees are more variable in hand position and pressure application. In both, hand position plays a key role in determining which digit acts as the final touch-off element.