There are more than 260 recognised species of monkey, of which around 50% are threatened and 24 critically endangered. New World monkeys differ from Old World monkeys in having flatter noses and (in some cases) prehensile tails, enabling them to use the tail as a "fifth limb" for climbing. Most species of monkeys are arboreal but a few such as baboons are more terrestrial in habit. Most monkeys live in large family groups. Renowned for their intelligence, several species of monkey have learned how to use tools.