Chicago Zoological Society

The Greatest Great Ape

Gorillas 101
Gorillas are the largest and most powerful of the great apes, which also include orangutans, chimpanzees, and bonobos.

Gorillas have large, muscular bodies. Adult males are particularly massive in the chest and shoulders. Gorillas’ arms are longer than their legs, a feature that enables them to walk comfortably on all fours. They "knuckle-walk"--resting their body weight on the backs of their strong fingers, rather than walking on their palms. Although well-adapted for ground dwelling, they have opposable thumbs and toes that allow them to grasp objects such as tree branches.

There are three subspecies of gorilla: the eastern lowland gorilla, the mountain gorilla, and the western lowland gorilla. Brookfield Zoo is home to western lowland gorillas, which are slightly smaller and lighter in color than the other two subspecies.

Troop dynamics
Gorilla family groups, called troops, are usually made up of between two and twenty individuals: an adult male, several adult females, and offspring ranging from infants to young adults. The silverback is at the center of the social structure; he is the only fully adult male in the troop. Named for the broad saddle of silver fur across his back, the silverback is usually twice as big as the adult females in the group. His presence is the glue that keeps the troop together. He protects them from predators that are a threat to young gorillas, and from other silverbacks.

Females in the same troop are often unrelated, so the strong sisterly bonds that unify most social mammal groups do not occur in gorillas. When they are old enough to reproduce, young female gorillas leave their birth group to join a new troop. Females are choosy about what troop to join—only a silverback that can protect their offspring from danger is selected.

Big, but not bad
As they mature, western lowland gorillas make an astounding transformation, increasing in size a hundredfold. From four pounds to about four hundred (in the case of the silverback), they develop from helpless newborns into adults powerful enough to defend themselves against any predator, other than humans.

Usually the first things that come to mind about gorillas are their great size and strength, and fierce appearance. But despite their formidable reputation, there's little aggression in gorilla society. The most serious conflicts, usually between males from different troops, are often resolved through displays. When they spot a stranger, males go into a dramatic bout of chest beating. In addition to pounding their chest with their cupped hands, they'll throw leaves and branches into the air, and roar threateningly—but actual fighting is rare.

The rhythm of the day
Besides displays, gorillas use as many as 16 different kinds of vocalizations to communicate. They hoot to warn other troops to stay away. Grunts and barks keep the troop together, which is especially important during the troop's daily movements to new feeding sites. They can also communicate through facial expressions.

Western lowland gorillas at Brookfield Zoo
Brookfield Zoo participates in the western lowland gorilla Species Survival Plan (SSP). SSPs are cooperative breeding, education, and conservation programs between North American Zoos. One of the goals of the western lowland gorilla SSP is to produce genetically healthy populations for the future of this endangered primate. Right now there are about 360 western lowland gorillas that are part of the SSP.

You can see Brookfield Zoo's troop of western lowland gorillas, including Ramar the silverback, in Tropic World: Africa.