Knuckle-walk

Knuckle-walking is a way some land animals move. Knuckle-walking animals curl fingers or front toes like a fist and touch the ground using the bony part of the front foot, the knuckle. Gorillas, chimpanzees, anteaters and platypuses knuckle-walk. In general, animals knuckle walk if their hands or front paws have special shapes for doing something very different from walking. Apes use their hands to hold food and use tools, so they have fingers. They curl up their fingers when they knuckle-walk. Anteaters use their front paws to dig for ants, so they curl up their digging claws when they knuckle-walk. Platypuses use their front paws to swim, so they curl up their webbed toes when they knuckle-walk.

This western lowland gorilla is walking on its knuckles.

Large anteaters that walk on the ground, for example the giant anteater, knuckle-walk. Smaller anteaters that live in trees do not. Platypuses walk on their knuckles so that the webbing on their front feet will not get in the way.

Skeleton of a giant anteater with its front toes curled under for knuckle-walking.

Gorillas from the genus Gorilla and chimpanzees from the genus Pan both walk on their knuckles but not in the same way. Chimpanzees spend more time in trees than gorillas do, so they turn their wrists in many different ways. Gorillas usually keep their wrists straight when they knuckle-walk. Gorillas and chimpanzees knuckle-walk when they move on the ground because their bodies are adapted to move around in trees: their arms are longer than their legs, and their middles are cone-shaped. The bones in their arms are good for knuckle-walking: the radius can lock into one of the bones in the wrist so the two bones together can hold the animal's weight. Human arm and wrist bones do not do this.

Some fictional cavemen knuckle-walk or drag the knuckles of their long arms on the ground, but scientists do not agree on whether human beings evolved from ancestors who really knuckle-walked on the ground the way gorillas do. One team of scientists said humans evolved from a tree-living ancestor, like chimpanzees did, because of the way our wrist bones are. Another team of scientists wrote that Australopithecus anamensis and Australopithecus afarensis, extinct relatives of humans, probably knuckle-walked.