A young mountain gorilla (Gorilla gorilla beringei) in the Volcanoes National Park.The beautiful montane forest ecosystem of the Virunga Volcanoes is the habitat of these rare large mammals, which live in forests and glades between 9,000 and 11,000 feet. With a population of about 600 in the Virunga forests and the Bwindi Impenetable Forest of Uganda, the existence of mountain gorillas is a tenuous one due to the pressures of an ever-expanding population in one of Africa's most densely populated regions (more than 770 people per square mile in Rwanda). The rangeland of mountain gorillas has been reduced disastrously over the past fifty years, yet its low aptitude for dispersal and colonization has meant that it has never survived in captivity.Sexual dimorphism is very marked with male mountain gorillas twice the size of females. A silverback (a mature gorilla of about 15 years old when the hairs on its back turn silver-grey) may weigh up to 400 lbs, with a chest the same dimension as its height (6 feet). Female mountain gorillas are usually excellent mothers, though silverbacks will also participate in tending and playing with the young.