Sloth

Sloths
	Hoffman's Two-toed Sloth, Choloepus hoffmanni
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Suborder:	Folivora; Delsuc et al, 2001
Families
	Megalonychidae; Bradypodidae; †Rathymotheriidae; †Scelidotheriidae; †Mylodontidae; †Orophodontidae; †Megatheriidae

Sloths are 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 these two families the Folivora suborder, while some call it Phyllophaga.

Sloths are omnivores. They may eat insects, small lizards and carrion, but their diet consists mostly of buds, tender shoots, and leaves.

Sloths 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 have very 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 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 Celsius or 86 to 93 degrees Fahrenheit), and still lower temperatures when resting. Sloths mainly live in Cecropia trees.

Physiology

Sloth fur also exhibits specialized functions: the outer hairs grow in a direction opposite from that of other mammals. In most mammals, hairs grow toward the extremities, but because sloths spend so much time with their legs above their bodies, their hairs grow away from the extremities in order to provide protection from the elements while the sloth hangs upside down. In moist conditions, the fur hosts two species of symbiotic cyanobacteria, which may provide camouflage. The outer fur coat is usually a thick brown, but occasionally wild sloths appear to have a green tinge to their fur because of the presence of these bacteria. The bacteria provide nutrients to the sloth when licked during grooming. Sloths have short, flat heads, big eyes, a short snout, long legs, and tiny ears. They also exhibit stubby tails, usually 6-7cm long. Altogether, sloths' bodies usually are anywhere between 50 and 60 cm long.

Their claws also 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: 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. The main predators of sloths are the jaguar, the harpy eagle, and humans. The majority of sloth deaths in Costa Rica are from contact with electrical lines and from 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 do not fall down even if shot from below, so that they are not worth shooting in the first place.

Sloths move only when necessary and 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 / 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 1.5 m / 5 feet per minute. They mostly move at 15-30 cm (0.5-1 feet) per minute.

Sloths are among the most somnolent animals, sleeping from 15 to 18 hours each day. They are particularly partial to nesting in the crowns of palm trees where they can camouflage as coconuts. They come to the ground, to urinate and defecate, only about once a week.

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 sometimes prove unwilling to leave the safety of the trees to retrieve them. Females normally bear one baby every year, but sometimes sloths' lack of movement actually keeps females from finding males for longer than one year.

File:Sloth47.JPG

Pale-throated Three-toed Sloth in a Costa Rican rehabilitation center.

The living sloths belong to one of two families, known as the two-toed sloths (Megalonychidae) and the three-toed sloths (Bradypodidae). Both families have 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.

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^{[citation needed]}. Of the six species, only one, the Maned Three-toed Sloth, has a classification of "endangered" at present. The ongoing destruction of South America's forests, however, may soon prove a threat to the others.

Extinct species

Until geologically recent times, large ground-dwelling sloths such as Megatherium lived in South America and parts of North America,^[2] 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 far northern Asia, Australia, and New Zealand, resulted from human activity. However, simultaneous climate change that came with the end of the last Ice Age probably played a role as well.