The Primate Page

Social Structure

Mountain gorillas are highly social, and live in relatively stable, cohesive groups held together by long-term bonds between adult males and females. Relationships among females are relatively weak (Stewart and Harcourt 1987). These groups are nonterritorial; the silverback generally defends his group rather than his territory. In the Virunga Mountain Gorillas, the average length of tenure for a dominant silverback is 4.7 years (Robbins 1995).

61% of groups are composed of one adult male and a number of females and 36% contain more than one adult male. The remaining gorillas are either loan males or exclusively male groups, usually made up of one mature male and a few younger males (Harcourt 1988). Group sizes vary from five to thirty, with an average of ten individuals. A typical group contains: one silverback, who is the group's undisputed leader; one or two blackbacks, who act as sentries; three to four sexually mature females, who are ordinarily bonded to the dominant silverback for life; and, from three to six juveniles and infants (Fossey 1983).

Most males and about 60% of females leave their natal group. Males leave when they are about 11 years old, and often the separation process is slow: they spend more and more time on the edge of the group until they leave altogether (Lindsley and Sorin 2001). They may travel alone or with all-male group for 2-5 years before they can attract females to join them and form a new group. Females typically emigrate when they are about 8 years old, either transferring directly to an established group or beginning a new one with a lone male. Females often transfer to a new group several times before they settle down with a certain silverback male (Watts 1990).

Social Behavior

The dominant silverback generally determines the movements of the group, leading them to appropriate feeding sites throughout the year. He also mediates conflicts within the group and protects them from external threats ("Life of Mountain Gorillas" 2002). Experienced silverbacks are capable of removing poachers' snares from the hands or feet of their group members (Berggorilla & Regenwald Direkthilfe [BRD]). He is the center of attention during rest sessions, and young animals frequently stay close to him and include him in their games. If a mother dies or leaves the group, the sliverback is usually the one who looks after his abandoned offspring, even allowing them to sleep in his nest (Stewart 2001).

Aggressive: Severe aggression is rare in stable groups, but when two mountain gorilla groups meet, the leading silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries (Fossey 1983). For this reason, conflicts are most often resolved by displays and other threat behaviors that are intended to intimidate without becoming physical. The ritualized charge display is unique to gorillas (SNZP). The entire sequence has nine steps: (1) hooting slow to fast, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running, two-legged to four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms to end display (Maple and Hoff 1982).

Affiliative: The midday rest period is an important time for establishing and reinforcing relationships within the group. Mutual grooming reinforces social bonds, and helps keep hair free from dirt and parasites. It is not as common among gorillas as in other primates, although females groom their offspring regularly. Young gorillas play often and are more arboreal than the large adults. Playing helps them learn how to communicate and behave within the group. Activities include wrestling, chasing and somersaults. The silverback and his females tolerate and even participate if encouraged (SNZP).

Vocalizations: Twenty-five distinct vocalizations are recognized, many of which are used primarily for group communication within dense vegetation. Sounds classified as grunts and barks are heard most frequently while traveling, and indicate the whereabouts of individual group members (Harcourt et al. 1993). They may also be used during social interactions when discipline is required. Screams and roars signal alarm or warning, and are produced most often by silverbacks. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intragroup communication (Fossey 1983).