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.