Sloths come from one of the earliest mammalian orders, which originated about 35 million years ago in the Late Eocene in South America. They are most closely related to armadillos and anteaters. Today, only two genera of sloths with five species exist in South America, whereas there used to be over thirty-five of the extinct ground sloths ranging from parts of Antarctica, South America, Central America, Hispaniola, and North America with some even going as far as Alaska. The extinct sloths also varied in body size from about 1 meter (equaling living sloth size) all the way up to elephant sized in the largest genus, Megatherium. The two modern genera of sloths are Bradypus, the three- toed sloths, and Choloepus, the two-toes sloths. The number of toes is only inidicative of the forefoot as the hind feet of both genera have three toes. Though both of the living genera of sloths look similar, live in the trees, and have a suspensory mode of movement, they are not related. Bradypus Is most closely related to the family Megatheriidae and choloepus to the family megalonychidae.
Modern sloths live upside-down in the forests of South America. Despite their long, sharply curved claws, they are herbivores, and mainly eat tree leaves as there teeth are to primitive and weak, due to lack of enamel, to chew anything else. The claws are part of their adaptation to life in the trees and help them remain sleeping and suspended underneath branches for hours. Sloths are generally nocturnal and move around little when awake. When they do move, it is at a slow speed. This slow way of life is necessary to navigate the web of small peripheral tree branches where they feed and is further reflected in their rate of metabolism, only defecate once or twice in a one week period. The defecation usually takes place at the base of the tree where the sloth excavates a spot for it with its short, stubby tail. This event marks one of the rare occasions that sloths will venture to the ground.