The living sloths comprise six species of medium-sized mammals that live in Central and South America belonging to the families Megalonychidae and Bradypodidae, part of the order Pilosa. Most scientists call the sloth suborder Folivora, while some call it Phyllophaga. Both names mean "leaf-eaters"; the first is derived from Latin, the second from ancient Greek. Tribal names include Ritto, Rit and Ridette, mostly forms of the word "sleep", "eat" and "dirty" from Tagaeri tribe of Huaorani.
This article mainly deals with the living tree-dwelling sloths. Until geologically recent times, large ground sloths such as Megatherium lived in South America and parts of North America, but along with many other animals they disappeared immediately after the arrival of humans on the continent. Much evidence suggests that human hunting contributed to the extinction of the American megafauna, like that of far northern Asia, Australia, New Zealand, and Madagascar. Simultaneous climate change that came with the end of the last Ice age may have also played a role in some cases. However, the fact that ground sloths survived on the Antilles long after they had died out on the mainland points towards human activities as the agency of extinction.
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 °C or 86 to 93 °F), and still lower temperatures when resting.
Although unable to survive outside the tropical rainforests of South and Central America, within that environment sloths are outstandingly successful creatures: 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 six living species, only one, the Maned Three-toed Sloth (Bradypus torquatus), has a classification of "endangered" at present. The ongoing destruction of South America's forests, however, may soon prove a threat to other sloth species.
Sloths' claws serve as their only natural defense. A cornered sloth may swipe at its attackers in an effort to scare them away or wound them. Despite sloths' apparent defenselessness, predators do not pose special problems: sloths blend in with the trees and, moving only slowly, do not attract attention. Only during their infrequent visits to ground level do they become vulnerable. The main predators of sloths are the jaguar, the harpy eagle, and humans. The majority of sloth deaths in Costa Rica are due to contact with electrical line and poachers. Despite their adaptation to living in trees, sloths make competent swimmers. Their claws also provide a further unexpected deterrent to human hunters - when hanging upside-down in a tree they are held in place by the claws themselves and often do not fall down even if shot from below.
Sloths move only when necessary and even then very slowly: they have about half as much muscle tissue as other animals of similar weight. They can move at a marginally higher speed if they are in immediate danger from a predator (4.5 m or 15 feet per minute), but they burn large amounts of energy doing so. Their specialized 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 sometimes remain hanging from branches after death. On the ground their maximum speed is 0.54 meters per second (1.2 mph).
It had been thought that sloths were among the most somnolent animals, sleeping from 15 to 18 hours each day. Recently, however, Dr. Neil Rattenborg and his colleagues from the Max Planck Institute for Ornithology in Starnberg, Germany, published a study testing sloth sleep-patterns in the wild; this is the first study of its kind. The study indicated that sloths sleep just under 10 hours a day. They go to the ground to urinate and defecate about once a week. They go to the same spot each time and are vulnerable while doing so. The reason for this risky behaviour is unknown.
Infant sloths normally cling to their mother's fur, but occasionally fall off. Sloths are very sturdily built and rarely die from a fall. In some cases they die from a fall indirectly because the mothers prove unwilling to leave the safety of the trees to retrieve the young. Females normally bear one baby every year, but sometimes sloths' low level of movement actually keeps females from finding males for longer than one year.
Almost all mammals have seven cervical vertebrae or "neck bones" (including those with very short necks, such as elephants or whales, and those with very long necks, such as giraffes). The few exceptions include manatees and two-toed sloths, which each have only six cervical vertebrae, and three-toed sloths with nine cervical vertebrae.
However, their adaptations belie the actual relationships of the living sloth genera, which are more distant from each other than their outward similarity suggests. The two-toed sloths of today are far more closely related to one particular group of ground sloths than to the living three-toed sloths. Whether these ground-dwelling Megalonychidae were descended from tree-climbing ancestors or whether the two-toed sloths are really miniature ground sloths converted (or reverted) to arboreal life cannot presently be determined to satisfaction. The latter possibility seems slightly more likely, given the fact that the small ground sloths Acratocnus and Synocnus which were also able to climb are among the closer relatives of the two-toed sloths, and that these together were related to the huge ground sloths Megalonyx and Megalocnus.
The evolutionary history of the three-toed sloths is not at all well-known. No particularly close relatives, ground-dwelling or not, have yet been identified.
It remains to be said that the ground sloths do not constitute a monophyletic group. Rather, they make up a number of lineages, and as far as is known until the Holocene most sloths were in fact ground-dwellers. The famous Megatherium for example belonged to a lineage of ground sloths that was not very close to the living sloths and their ground-living relatives like the small Synocnus or the massive Megalonyx. Meanwhile, Mylodon, among the last ground sloths to disappear, was only very distantly related to either of these.