Gorillas are the largest of the great apes. A male can weigh from 450-500 pounds and, when standing upright, can measure up to six feet. Females are smaller, weighing about 200 pounds.
There are three different subspecies of gorillas. The most abundant species is the western lowland gorilla; populations are estimated at about 40,000. Eastern lowland gorillas, found only in Zaire, number from 3,000 to 5,000. The mountain gorilla is the most endangered of the three subspecies. There are no mountain gorillas in zoos today.
Mountain gorillas live in social groups ranging from two to more than 35 individuals. The leader of each group is a mature male called a "silverback," because of the silver hair on his back. A male's hair will start to turn silver around age 11. The silver-back is the center of attention in the group, with the other gorillas congregating around him during periods of rest and play. An average group of 10 individuals might consist of the silverback, one "blackback" (an 8-12 year old male), three or four sexually mature females, and three to six juveniles and infants.
Females first breed at about age 10; males first breed around age 15. Gestation is from eight to nine months, resulting in a baby weighing four to five pounds. A baby is dependent on its mother for three years while it learns to build nests and forage for food. The mother holds her infant until it is about four months old, after which it is able to ride on her back, clinging to her fur. Females bear a single baby every four to five years.
Mountain gorillas eat large amounts of vegetation from more than 70 different plant species, including wild celery, gallium, vines, berries, bamboo, roots and bark. When the bamboo plant sprouts in June and November, bamboo shoots can make up to 90 percent of the gorilla's diet. A silverback can eat up to 75 pounds of bamboo each day; females might eat about 40 pounds per day. Gorillas spend about 30 percent of the day eating, 40 percent resting and sleeping and 30 percent traveling. Led by the silver-back, gorillas travel around a large range, foraging for food. Each night they build fresh grass nests.
Each mountain gorilla is truly an individual. Scientists use the distinctive "nose prints" of gorillas to identify them. The late Dr. Dian Fossey, the scientist whose observations of gorillas in the wild brought their plight international recognition, documented the gorillas' unique nose patterns, using photographs and illustrations in much the same way that scientists use photographs of flukes to identify individual humpback whales.
The Rwandan government recognizes the importance of both the mountain gorillas and the montane forest ecosystem on which they depend. Constant vigilance is needed to protect gorillas from the snares of poachers, to protect established park boundaries from encroachment and to ensure that human activity in the parks does not threaten the health of gorillas.
To contain threats to their gorilla populations, government agencies and researchers in Rwanda have divided mountain gorilla groups into three categories: research, tourist and wild gorillas. The research groups were originally studied by Dr. Dian Fossey. Her years of field observations among them helped condition them to the presence of humans. Other gorilla groups were "habituated" to the presence of people in order to create opportunities for tourists to view them. This has represented an important source of foreign revenue for Rwandan agencies charged with protecting parks and gorillas. Groups of wild gorillas live in the far reaches of the forest, occasionally interacting with the researchers and tourists.