Sloths are the six species of medium-sized mammals belonging to the families Megalonychidae (two-toed sloth) and Bradypodidae (three-toed sloth), part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.
They are arboreal (tree dwelling) residents of the jungles of Central and South America.
Sloths are classified as folivores as the bulk of their diet consists mostly 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 feces 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: they can account for as much as half the total energy consumption and two-thirds of the total terrestrial mammalian biomass in some areas. 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.
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 recorded sloth deaths in Costa Rica are due to contact with electrical lines 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.
Pale-throated Three-toed Sloth (Bradypus tridactylus) in a Costa Rican rehabilitation center
Sloths move only when necessary and even then very slowly: they have about a quarter 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 m or 13 feet per minute for the three-toed sloth), 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 the maximum speed of the three-toed sloth is 2 m or 6.5 feet per minute.
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.
Three-toed Sloth in the Dallas World Aquarium
They go to the ground to urinate and defecate about once a week, digging a hole and covering it afterwards. They go to the same spot each time and are vulnerable to predation while doing so. The reason for this risky behavior is unknown, although some believe that this is to avoid making noise while defecating from up high that would attract predators.
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.
After all, as babies, they are really cute