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The male common chimp is up to 1.7 metres (5.6 ft) high when standing, and weighs as much as 70 kilograms (150 lb); the female is somewhat smaller. The common chimp’s long arms, when extended, have a span one and a half times as long as the body’s height and a chimpanzee's arms are longer than its legs.[8] The bonobo is a little shorter and thinner than the common chimpanzee, but has longer limbs. Both species use their long, powerful arms for climbing in trees. On the ground, chimpanzees usually walk on all fours using their knuckles for support with their hands clenched, a form of locomotion called knuckle-walking. Chimpanzee feet are better suited for walking than are those of the orangutan because the chimp’s soles are broader and the toes shorter. Both the common chimpanzee and bonobo can walk upright on two legs when carrying objects with their hands and arms. The bonobo has proportionately longer upper limbs and tends to walk upright more often than the common chimpanzee. The coat is dark; the face, fingers, palms of the hands, and soles of the feet are hairless; and the chimp has no tail. The exposed skin of the face, hands and feet varies from pink to very dark in both species, but is generally lighter in younger individuals, darkening as maturity is reached. A University of Chicago Medical Centre study has found significant genetic differences between chimpanzee populations.[9] A bony shelf over the eyes gives the forehead a receding appearance, and the nose is flat. Although the jaws protrude, the lips are thrust out only when a chimp pouts.