Sloth

Sloths are medium-sized mammals belonging to the families Megalonychidae (two-toed sloth) and Bradypodidae (three-toed sloth), classified into six species. They are part of the order Pilosa and are therefore related to anteaters, which sport a similar set of specialized claws. Extant sloths are arboreal (tree dwelling) residents of the jungles of Central and South America, and are known for being slow-moving, and hence named "sloths". Extinct sloth species include many ground sloths.

Sloths make a good habitat for other organisms, and a single sloth may be home to moths, beetles, cockroaches, ciliates, fungi, and algae.

The sloth's taxonomic suborder is Folivora, while some call it Phyllophaga. Both names mean "leaf-eaters"; derived from Latin and Greek respectively. Names for the animals used by tribes in Ecuador include Ritto, Rit and Ridette, mostly forms of the word "sleep", "eat" and "dirty" from Tagaeri tribe of Huaorani.

Sloths are classified as folivores as the bulk of their diet consists of buds, tender shoots, and leaves, mainly of Cecropia trees. Some two-toed sloths have been documented as eating insects, small reptiles, and birds as a small supplement to their diet. Linnaeus's two-toed sloth has recently been documented eating human faeces from open latrines. They have made extraordinary adaptations to an arboreal browsing lifestyle. Leaves, their main food source, provide very little energy or nutrition and do not digest easily. Sloths therefore have large, specialized, 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 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 mammal of their size), and maintain low body temperatures when active (30–34 °C or 86–93 °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. Four of the six living species are presently rated "least concern"; the maned three-toed sloth (Bradypus torquatus), which inhabits Brazil's dwindling Atlantic Forest, is classified as "endangered", while the island-dwelling pygmy three-toed sloth (B. pygmaeus) is critically endangered.

Sloths are members of the superorder Xenarthra, a group of mammals that appeared approximately 60 million years ago, although at least one source puts the date at which sloths and related animals broke off from other placental mammals at about 100 million years ago. Also included among the Xenarthra are anteaters and armadillos. The earliest xenarthrans were arboreal herbivores with sturdy spines, fused pelvises, stubby teeth and small brains.

The living sloths belong to one of two families, known as the Megalonychidae ("two-toed" sloths) and the Bradypodidae (three-toed sloths). All living sloths have in fact three toes; the "two-toed" sloths, however, have only two fingers. Two-toed sloths are generally faster moving than three-toed sloths. Both types tend to occupy the same forests: in most areas, one species of three-toed sloth and one species of the larger two-toed type will jointly predominate.

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 common ancestor of the two genera apparently lived 35–40 million years ago, making the living forms stunning examples of convergent or parallel evolution. 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 Neocnus 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 well known. No particularly close relatives, ground-dwelling or not, have yet been identified.

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 Neocnus or the massive Megalonyx. Meanwhile, Mylodon, among the last ground sloths to disappear, was only very distantly related to either of these.

Until geologically recent times, ground sloths such as Megatherium lived in South America and parts of North America, but along with many other animals they disappeared shortly 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 toward human activities as the agency of extinction.