Chimpanzees can walk on two feet, but they prefer to move about on all four legs. On the ground, they walk on their hind feet and knuckles. They have hands that look like human hands, but their thumbs are shorter than those of humans. At night, chimpanzees sleep in nests that they make on tree branches. They bend twigs and tuck in leaves to make a soft platform to rest in a place that is safe from enemies on the ground. The gestation period of chimpanzees lasts between six and eight months. Usually only one offspring is produced; they rarely have twins. Chimpanzees live up to 60 years in the wild.