Gorillas lack hair on the face, hands, and feet, and the chest of old males is bare. The hair of G. beringei beringei is longer than that of the other three subspecies. Adult males have a prominent crest on top of the skull and a “saddle” of gray or silver hairs on the lower part of the back—hence the term silverback, which is commonly used to refer to mature males. This saddle is much more conspicuous in eastern gorillas (G. beringei), which are jet black, than in western gorillas (G. gorilla), which are more of a deep gray-brown.

Source: Britannica