Gorilla

I

Introduction

Gorilla, largest of the great apes and one of the closest living relatives of the human species. Like chimpanzees, orangutans, and other great apes, gorillas are members of the primate order. Monkeys and humans also belong to this order. Until recently, accounts of the gorilla have portrayed a ferocious, powerful beast prone to attacking people. Such descriptions proliferated in various media, a prime example being the American motion-picture classic King Kong (1933). Only after 1960 did evidence emerge, from the field studies of American zoologists George B. Schaller and Dian Fossey, that the gorilla is a relatively gentle vegetarian who attacks only if directly threatened.

Fossils and genetic data indicate that the ancestral line of gorillas and chimpanzees split from that of humans about 5 million to 10 million years ago. This divergence occurred far more recently than did the divergence of the line of orangutans, the only great apes of non-African origin. Because of their more recent divergence, gorillas and chimpanzees are more similar to humans than are any other living species. These similarities are apparent in their high intelligence, the shape of their hands and feet, their reproductive physiology, and infant development. Although chimpanzees solve experimental problems more easily than gorillas, gorillas appear to be more adept at acquiring human sign language.

II

Types of Gorillas

For many years gorillas were classified as a single species within the genus Gorilla. However, genetic evidence suggests that this classification should be modified to distinguish two separate species: the western and eastern gorillas, each represented by several subspecies.

Western gorillas live in the western portion of central Africa (Gabon, Republic of the Congo, Cameroon, Central African Republic, and Mbini). Eastern gorillas live in eastern Democratic Republic of the Congo (DRC, formerly Zaire), Rwanda, and Uganda. Although nearly all gorillas found in zoos are western gorillas rather than eastern gorillas, a subspecies of the eastern gorilla commonly known as the mountain gorilla is most familiar due to extensive research and popular documentary and motion-picture portrayals of its life in the wild.

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III

Physical Characteristics of Gorillas

Gorillas are born small, each weighing about 2 kg (4.5 lb). Yet adult females grow to an average weight of 90 kg (200 lb) and a height of 1.5 m (5 ft), and adult males may reach a weight of 140 to 180 kg (310 to 400 lb) and a height of 1.8 m (5.9 ft). Gorilla coats are generally black, with the lower back region turning to gray in adult males. Some gorilla populations have reddish-brown patches on the forehead, and others show white patches around the ears. The adult gorilla has a large head, a broad, flattened nose with flared nostrils, long arms with massive forearms, short and stocky legs, and a protruding abdomen. Its eyes, ears, and genitals are small relative to overall body size.

IV

Behavior of Gorillas

Gorillas are chiefly land animals. They walk quadrupedally (using both feet and hands), with the knuckles of the hands carrying the weight of the upper body. In lowland regions with many fruiting trees, gorillas tend to spend more time off the ground, climbing trees and sometimes brachiating (swinging by their arms).

Unlike the other great apes, gorillas travel, eat, play, and sleep in stable family groups. These groups may range from several individuals to more than 50, but a typical family consists of one or two adult males, three or four unrelated females, and their young.

Gorillas spend much of their day eating, consuming a primarily vegetarian diet of leaves, stems, shoots, and fruit. They travel between feedings, covering a distance of several hundred yards to a mile or more in a day. In lowland forests where gorillas eat a substantial amount of fruit, the slow passage of seeds through their digestive tracts serves an important ecological process: the widespread dispersal and propagation of trees.

At dusk a family settles wherever it has finished feeding. Each member constructs its own nest, either on the ground or in a tree, by bending nearby vegetation to form a flexible platform. The home range in which gorillas move, eat, and sleep varies from 5 to 30 sq km (2 to 12 sq mi).

Apart from humans, gorillas have few or no predators. In some regions leopards have been known to occasionally attack gorillas. If threatened, adult gorillas—especially males—defend others in the group by roaring, screaming, beating their chest with cupped hands, and eventually charging if the threat is serious. However, research and tourist programs in the wild indicate that gorillas demonstrate extreme tolerance of people as long as people approach them respectfully.

The first reports of tool use by gorillas in the wild appeared in a paper published in 2005. Field studies in a remote region of the Republic of Congo documented a gorilla probing the depth of a pool of water with a stick. Another gorilla moving through swampy ground used a broken tree trunk as a support and then set it down as a bridge. Captive gorillas can learn to use objects as tools, usually by imitating humans. Scientists have also observed tool use in wild chimpanzees, bonobos, and orangutans. The ability to use tools may be an ancient trait among the great apes, the closest living relatives of humans.

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