Knuckle-walking

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A Gorilla knuckle-walking (Cincinnati Zoo).

Gorillas, and chimpanzees, use a style of locomotion called knuckle-walking, where they walk on all fours with the fingers of their forelimbs held in a partially flexed posture, putting pressure on their knuckles. Weight is borne on the back of the intermediate phalanges of the hand and the metacarpophalangeal joints are hyperextended. Orangutans practice a variation of knuckle-walking called fist-walking, bearing weight on the back of the proximal phalanges instead.

Giant anteaters and platypus are also knuckle-walkers. Pangolins also sometimes walk on their knuckles. The chalicotheres were large prehistoric knuckle-walkers that looked something like at cross between a horse and a gorilla. The ground sloths may have also walked on their knuckles.

Knuckle-walking tends to evolve when the fingers of the forelimb are specialized for tasks other than locomotion on the ground. In the gorilla the fingers are used for the manipulation of food, and in chimpanzees for the manipulation of food and for climbing. In anteaters and pangolins the fingers have large claws for opening the mounds of social insects. Platypus fingers are extensively webbed for swimming.