The Mountain Gorillas are the rarest of all and are on the verge of extinction. The world's gorilla population is relatively small and still declining. All three gorilla subspecies are listed as endangered by the U.S. Fish and Wildlife Service and by the Convention on International Trade for Endangered Species. There are currently about 50,000 western lowland gorillas living in the wild in West Central Africa. This gorilla is also the type most often seen in zoos. The eastern lowland gorilla population has declined significantly in recent decades. An estimated 5,000-15,000 lived in the eastern Congolese rainforest around 1960. Today only about 2,500 remain in the wild, and only a few dozen live in the world's zoos. The mountain gorillas are the rarest of all and are on the verge of extinction. Only about 600 of these magnificent animals are left in the wild, about 320 in the Virunga Mountains and another 300 in the Bwindi Impenetrable Forest National Park in Uganda. None are found in captivity.
The population of mountain gorillas in the Virunga Mountains has been watched closely for the last half-century and shows the effects of human interaction, both good and bad. George Schaller estimated that about 450 mountain gorillas lived in the Virungas in 1960. Hunting and poaching reduced their numbers to about 250 by 1981, when the protection efforts of the late Dian Fossey and others brought the decline to a halt. Today about 320 mountain gorillas inhabit the Virungas, but their long-term survival continues to be threatened by natural changes and disasters, hunters and poachers, and the chronic political instability that swirls around the edge of their forest home.
Newborn gorillas are small, covered with black hair, and weigh about 2.3 kg (5 lbs). They must be cared for at all times. By age two they are able to reach and chew on vines and branches. They develop about twice as fast as human babies.
Young male and female gorillas are classed as juvenile between the ages of about three and six. During this stage, both sexes have thick black hair and black skin. Juveniles of both sexes increase in size and weight at similar rates for the first six years. At age six they are about 1.2 m (4 ft) tall and weigh about 68 kg (150 lbs).
Females mature at about age six and cease to grow taller, although they continue to gain weight slowly until they reach weights of 113-136 kg (250-300 lbs) at ages of ten to eleven years. Males continue to grow both in size and weight past the age of six; they do not reach maturity until they are about ten years old. Between the ages of about six and ten years, males retain the uniformly black hair color of their youth and are called blackbacks.
When male mountain gorillas reach maturity, they develop a patch of grayish or silver-colored hair on their backs. Consequently, mature males are called silverbacks. Males cease to grow in size or weight after maturity, but at typical heights of 1.5-1.8 m (5-6 ft) and weights of 204-227 kg (450-500 lbs), they are impressively large animals. The silverbacks' large size and distinctive coloring make them very easy to recognize in the wild.
The maximum life span of mountain gorillas in the wild is difficult to estimate. The longest-lived gorillas in captivity reached ages of thirty to thirty-five years. No gorilla has been seen in the wild that looked as aged as the oldest captive gorillas, so the life span in the wild is probably somewhat less, perhaps twenty-five to thirty years.
The potential for population growth for undisturbed mountain gorillas is comparable to that for human beings. The gestation period is about nine months. Gorilla mothers with an infant may not have another for up to four years.. There is also no apparent breeding season, since births of baby gorillas occur throughout the year. However, due to mishaps and disease, many baby gorillas die in the first year of life, and nearly half of all gorillas die before reaching adulthood.