Baby mountain gorillas (Gorilla gorilla beringei) play in the Volcanoes National Park. They belong to 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).