Social Structure

Gorillas are flexible in their nature and behavior. Thus, the information found here is only a general guide.

Gorillas live in groups, or troops, from two to over 30 members. But data seems to indicate that western lowland gorillas live in smaller groups, averaging about five individuals. Groups are generally composed of a silverback male, one or more black back males, several adult females and their infant and juvenile offspring. This group composition varies greatly due to births, deaths and the immigration and emigration of individuals.

Mature offspring typically leave their birth group to find a mate. At about eight years old, females generally emigrate into a new group of their choosing. They seem to choose which silverback to join based on such attributes as size and quality of his home range.

A female may change family groups a number of times throughout her life. When leaving their birth group, some sexually mature males may attempt to replace the silverback in an already established group. However, they usually spend a few years as bachelor males. Nevertheless, a new troop can be easily formed when one or more non-related females join a lone male.

An adult, dominant, silverback males leads the group. He has exclusive breeding rights to the females though at times, he may allow other sub-adult males in the group to mate with females. The silverback mediates disputes and also determines the group's home range. He regulates what time they wake up, eat and go to sleep.

All gorillas over three years of age make both day nests for resting and night nests for sleeping. Infants share their mothers' nests. Gorillas form nests by sitting in one place and pulling down and tucking branches, leaves or other vegetation around themselves. Adult males usually nest on the ground. Females may nest on the ground or in trees. Juveniles are more apt to nest in trees. Studies of western lowland gorillas have shown that the number of nests found at a site does not necessarily coincide with the number of weaned animals observed in a group.

The western lowland gorilla is a quiet, peaceful and non-aggressive animal. It never attacks unless provoked. However, males do fight over females, and the new leader of a group may kill unrelated infants. This causes the females to begin cycling sooner. An adult male protecting his group may attempt to intimidate his aggressor by standing on his legs and slapping its chest with cupped or flat hands while roaring and screaming.

If this elaborate display is unsuccessful and the intruder persists, the male may rear his head back violently several times. He may also drop on all fours and charge toward the intruder. In general, when they charge they do not hit the intruder. Instead, they merely pass them by. This display of aggression maintains order among separate troops and reduces the possibility of injury. It is thought that size plays an important role in determining the winner of an encounter between males, with the larger male winning.

Gorillas exhibit complex and dynamic relationships. They interact using grooming behaviors, although less than most other primates. Affiliation may also be shown through physical proximity. Young gorillas play often and are more arboreal than the large adults. Adults, even the silverback, tolerate infant play behavior. He also tolerates and often participates in the play of older juveniles and black back males. Because of gorilla variability, some or all of these behaviors may or may not be seen.