A mountain gorilla (Gorilla gorilla beringei) belonging to the 36-strong Susa group, relaxes in the Volcanoes National Park.The permanent territory of the Susa group lies high on Mount Karisoke. It is the second largest group of mountain gorillas in the world and was the one originally studied by Dian Fossey.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).