A male mountain gorilla (Gorilla gorilla beringei), known as a 'silverback' eats the pith of a plant stem in the Volcanoes National Park.14-year-old 'Nyagakangaga' is the third-ranking male of the 36-strong Susa group whose permanent territory lies high on Mount Karisoke. Susa 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).