Sloths have made extraordinary adaptations to an aboreal browsing lifestyle. Leaves provide very little energy or nutrition and do not digest easily: sloths have very large, specialised, slow-acting stomachs with multiple compartments in which symbiotic bacteria break down the tough leaves. As much as two-thirds of a well-fed sloth's body-weight consists of the contents of its stomach, and the digestive process can take as long as a month or more to complete. Even so, leaves provide little energy, and sloths deal with this by a range of economy measures: they have very low metabolic rates (less than half of that expected for a creature of their size), and maintain low body temperatures when active (30 to 34 degrees), and still lower temperatures when resting.
Sloths move only when necessary and then very slowly: they have about half as much muscle tissue as other animals of similar weight. Their specialised hands and feet have long, curved claws to allow them to hang upside-down from branches without effort; while they sometimes sit on top of branches, they usually eat, sleep and even give birth hanging from limbs. They come to the ground, to urinate and defecate, only about once a week.
Sloth fur too exhibits specialised functions: the outer hairs grow in the opposite direction to that of other mammals (so as to provide protection from the elements despite living legs-uppermost), and in moist conditions host two species of symbiotic blue-green algae, which provide camouflage and possibly extra nutrition, either licked directly from the fur or absorbed through the skin.
Despite sloths' apparent defencelessness, predators do not pose special problems: in the trees sloths have good camouflage and, moving only slowly, do not attract attention. Only during their infrequent visits to ground level do they become vulnerable. Despite their adaptation to living in trees, sloths make competent swimmers.
Infant sloths normally cling to their mother's fur; those that fall off die in some cases, because the mothers sometimes prove unwilling to leave the safety of the tree to retrieve them.
Until geologically recent times, large ground-dwelling sloths of the Megatherium type lived in North America, but along with many other species they became extinct immediately after the arrival of humans on the continent. Much evidence suggests that the extinction of the American megafauna, like that of Australia, far northern Asia, and New Zealand, resulted from human activity. Nevertheless, scientific debate on the matter continues.
The living sloths belong to one of two families, known as the two-toed and three-toed sloths. Both families have three toes: the "two-toed" sloths, however, have only two fingers. Both types tend to occupy the same forests: in most areas, a particular single species of three-toed sloth and a single species of the larger two-toed type will jointly pre-dominate.
Although unable to survive outside the tropical rainforests of South and Central America, within that environment sloths are outstandingly successful creaures: they can account for as much as half the total energy consumption and two-thirds of the total terrestrial mammalian biomass in some areas. Of the five species, only one, the Maned Two-toed Sloth, has a classification of "endangered" at present. The ongoing destruction of South America's forests, however, may soon require revision of other sloths' "endangered" status.