The gorilla, Gorilla, is a member of the family Pongidae, or great apes, which also includes the chimpanzee and orangutan. Together with the lesser apes — a group that includes the gibbon and the siamang — the great apes are close relatives of the monkeys and of humans, and all belong to the order primates.

The gorilla is the largest living primate: males sometimes reach 180 cm (6 ft) in height and in nature weigh between 135 and 180 kg (300 and 400 lb). Females are shorter and weigh about half as much as the males. In years past, some captive males attained weights greater than 270 kg (600 lb), but for health reasons captive gorillas are no longer allowed to become so overweight.

Gorillas become sexually mature at 6-9 years of age and live about half as long as a human. Single infants are born following a pregnancy of 8 1/2 months, are partially weaned by 1 year, but remain with the mother for at least 3 years. The hair on the male's back turns silver-gray with maturity, giving rise to the term "silverback" for adult males.

There are three geographic variants, or races, of gorilla, all of which are found in the African rain forest: the western lowland gorilla (Gorilla gorilla gorilla), the mountain gorilla (G. gorilla beringei), and the eastern lowland gorilla (G. gorilla graueri). The mountain gorilla, in comparison to the western lowland gorilla, has longer, thicker, more intensely black hair, suited to its colder, montane forest habitat, less flaring of the nostrils, a broader chest, and shorter arms and legs. The eastern lowland gorilla is intermediate between the other two races with respect to these physical characteristics.

Gorillas are the most terrestrial of the great apes, their bulky size making them ill-suited to tree dwelling. Locomotion on the ground is quadrupedal, and the knuckles of the hands are used to support the upper body, in contrast to the monkeys, who use the palms. Youngsters and females with infants demonstrate their arboreal heritage, however, by building sleeping nests in trees each night.

Gorillas live in family groups that consist of a single male leader, or silverback; some younger, black-backed males, possibly sons of the silverback; several adult females; and varied numbers of juvenile and infant offspring. The size of the group varies with geographic location and the availability of food.

Contrary to popular misconceptions, gorillas are not aggressive, bloodthirsty monsters, but rather peaceful vegetarians. They spend most of their day, apart from a midday rest period, foraging for food. When gorilla groups meet, fighting is rare. Females and youngsters of different groups frequently intermingle for periods of time. When tension does develop between silverbacks of two groups, it is generally relieved in nonaggressive ways such as impressive displays of chest beating, which serve to reduce actual conflict.

The gorilla is threatened with extinction in its natural habitat by various human activities, and its last chances for survival may be a few gorilla sanctuaries in Africa and the zoos and other captive environments that maintain gorillas in other parts of the world.

Ronald D. Nadler

Bibliography: Fossey, Dian, Gorillas in the Mist (1983); Norton, Boyd, The Mountain Gorilla (1990); Schaller, George B., The Mountain Gorilla: Ecology and Behavior (1963; repr. 1988) and The Year of the Gorilla (1988).