MOUNTAIN GORILLA: There are few sights in the animal kingdom more terrifying than an ill-tempered gorilla. Twice the size of its female counterpart, a full-grown male silverback gorilla typically packs between 450 and 500 pounds of weight into a five-to-six-foot frame. It's called a silverback because of a gray or silver-colored patch of hair that develops on its back when it reaches maturity. The silverback leads the group on daily treks for food — seasonal vegetation, such as bamboo, nettles, celery and thistles — and protects it from outside dangers, like other silverbacks. When confronted, it will stand on its hind legs and beat its well-muscled chest, which can span nearly five feet across. Only the most powerful silverbacks gain breeding rights. Mountain gorillas live in the Virunga Mountains — a chain of volcanoes along the northern border of Rwanda, Uganda and the Democratic Republic of Congo — and Uganda's Bwindi Impenetrable Forest National Park. There are only about 700 mountain gorillas left in the wild, split between the two regions. War, human disease, habitat loss and poaching threatens their survival.