The mountain gorilla (Gorilla beringei beringei) is one of two subspecies of Eastern Gorillas. It is only found in the Virunga volcanic mountains of Central Africa, within three national parks. Some claim that the Bwindi population in Uganda’s Bwindi Impenetrable Forest is a third subspecies.
A census taken in 2003 has shown a 17% increase in population size since 1989. There are now a total of 380 gorillas in 30 social groups. However, the mountain gorilla continues to be considered critically endangered on the IUCN Red List of Endangered Species. It faces an extremely high risk of extinction in the wild due to habitat loss, poaching, human disease, and war.
The mountain gorilla has longer and darker hair than other gorilla species. This enables it to live at high altitudes and travel into areas where temperatures drop below freezing. It has adapted to a life on the ground more than any other non-human primate. Its feet most resemble those of humans. Gorillas can be identified by nose prints unique to each individual. Researchers often use photographs and illustrations of noses for identification and monitoring.
The mountain gorilla, like all gorillas, is highly sexually dimorphic. The males usually weigh twice as much as females. Adult males also have more pronounced bony crests on the top and back of their skulls. This gives their heads a more conical shape. These crests anchor the massive muscles of their large jaws. Adult females also have these crests, but they are much less pronounced.
Adult males are called silverbacks. When they reach sexual maturity, a saddle of gray or silver-colored hair develops on their backs. The hair on their backs is shorter than on most other body parts, and their arm hair is especially long. Upright, males reach 5 to 6 feet (1.5 to1.8 m) in height, with an arm span of 7 ft 6 in (2.25 m). They weigh from 450 to 500 lbs (204 to 227 kg).
The mountain gorilla primarily lives on land and walks on four legs. However, it will climb into fruiting trees if the branches can carry its weight. Like all great apes, its arms are longer than its legs. It moves by knuckle walking supporting its weight on the backs of its curved fingers rather than its palms.
The mountain gorilla is most active between 6:00 a.m. and 6:00 p.m. Many of these hours are spent eating, as large quantities of food are needed to sustain its massive bulk. It forages in early morning, rests during the late morning and around midday. In the afternoon, it forages again before resting at night. Each gorilla builds a nest from surrounding vegetation to sleep in, constructing a new one every evening. Only infants sleep in the same nest as their mothers. They leave their sleeping sites when the sun rises at around 6 am, except when it is cold and overcast. They often stay longer in their nests.
Habitat and diet
The mountain gorilla is primarily an herbivore. The majority of its diet is composed of the leaves, shoots and stems (85.8%) of 142 plant species. It also feeds on bark (6.9%), roots (3.3%), flowers (2.3%), and fruit (1.7%). It will also eat larvae, snails and ants (0.1%). Adult males can eat up to 75 pounds of vegetation a day, while a female can eat as much as 40 pounds.
A newborn gorilla weighs about 4 lb (1.8 kg), and spends its first few months of life in constant physical contact with its mother. It begins to walk at around four or five months. It starts to put plant parts in its mouth between four and six months. At eight months it regularly ingest solid food. Weaning occurs around three years of age. Juveniles may remain with their mothers for years after that.
Young male and female gorillas are considered infants from birth until three years of age. Blackbacks are sexually immature males from around eight years until they have developed the silver saddle and large canines of maturity. Females begin to ovulate at 7 or 8 years of age, and have their first infant between the ages of 10 and 12. Males generally do not start breeding before the age of 15.
The mountain gorilla has no mating season and females usually initiate mating behavior. Females generally rear one infant every 6 to 8 years, and may leave only 2 to 6 offspring over a 40-year life span
The mountain gorilla is highly social. It lives in relatively stable, cohesive groups held together by long-term bonds between adult males and females. Relationships among females are relatively weak. These groups are not territorial. The silverback generally defends his group rather than his territory.
Sixty-one percent of groups are composed of one adult male and a number of females. Thirty-six percent contain more than one adult male. The remaining gorillas are either lone males or exclusively male groups. Group sizes vary from five to thirty, with an average of ten individuals.
The dominant silverback generally determines the movements of the group, leading it to appropriate feeding sites throughout the year. He also mediates conflicts within the group and protects it from external threats. 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 silverback is usually the one who looks after his abandoned offspring. He even allows them to sleep in his nest.
When the dominant silverback dies or is killed, the family group may be severely disrupted. Unless he leaves behind a male descendant capable of taking over his position, the group will either split up or be taken over in its entirety by an unrelated male. When a new silverback takes control of a family group, he may kill all of the infants of the dead silverback.
Severe aggression is rare in stable groups. Sometimes when two mountain gorilla groups meet, the two silverbacks can sometimes engage in a fight to the death. They use their canines to cause deep, gaping injuries
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. The females groom their offspring regularly. Young gorillas play more often in trees 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.