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Cladus: Eukaryota
Supergroup: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Superclassis: Tetrapoda
Classis: Mammalia
Subclassis: Theria
Infraclassis: Placentalia
Ordo: Pilosa
Subordo: Folivora
Familiae: Bradypodidae - Megalonychidae - †Megatheriidae - †Mylodontidae - †Scelidotheriidae - †Orophodontidae


Folivora Delsuc, Catzeflis, Stanhope & Douzery, 2001


* Phyllophaga Owen, 1842


* Mammal Species of the World : A Taxonomic and Geographic Reference, 2-volume set, 3rd edition, 2005 ISBN 0801882214

Vernacular names
Català: Peresós
Česky: Lenochodi
English: Sloths
Español: Perezosos
Hrvatski: Ljenivci
‪Norsk (bokmål)‬: Dovendyr
中文: 樹懶


Sloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae, part of the order Pilosa. They are arboreal residents of the rainforests of Central and South America. The sloth's taxonomic suborder is Folivora, while some call it Phyllophaga. Both names mean "leaf-eaters"; the first is derived from Latin, the second from Greek. 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, in Brazil sloths are commonly called "Bicho-preguiça" ("lazy animal") because of slow movements related to their very low metabolism.


Until geologically recent times, ground sloths such as Megatherium[2] 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.[2] 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.[2] 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.


Sloths are considered to be 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. 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 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 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 °C (86 °F) to 34 °C (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.

Sloth furs exhibit 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 most conditions, the fur hosts two species of symbiotic cyanobacteria, which provide camouflage.[3][4] Because of the cyanobacteria, sloth fur is a small ecosystem of its own, hosting many species of non-parasitic insects. Sloths have short, flat heads; big eyes; a short snout; long legs; and tiny ears. They also have stubby tails, usually 6–7 cm long. Altogether, sloths' bodies usually are anywhere between 50 and 60 cm long.

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 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.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 (4.5 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.[5] 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 m/s (1.8 ft/s) (1.9 km/h (1.2 mph)).[6]
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.[7]

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.[8] Consistent with this, they reportedly relieve themselves from their branches during storms in rainy season.[9] Another possible explanation is that the middens provide the sloths with one of their few methods of finding one another for breeding purposes, since their sense of smell is far more acute than their eyesight or hearing.[10] It has also been pointed out that individual sloths tend to spend the bulk of their time feeding on a single "modal" tree; by burying their excreta near the trunk of that tree, they may help nourish it.[11]

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.[12]

Classification of the sloth

Sloths are members of the superorder Xenarthra, a group of mammals that appeared approximately 60 million years ago,[13] 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.[14] 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.[13]

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.[15] 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. Sloths are known to have very long necks.


1. ^ Gardner, Alfred (2005-11-16). Wilson, D. E., and Reeder, D. M.. ed. Mammal Species of the World (3rd ed.). Baltimore: Johns Hopkins University Press. pp. 100-101. ISBN 0-801-88221-4.
2. ^ a b c Wildfacts "Megatherium". BBC Science & Nature. Wildfacts. Retrieved 2007-01-09.
3. ^ "Rainforest Canopy—Animals". Retrieved 2009-12-30.
4. ^ "The Hidden Lives of Sloths". Retrieved 2009-12-30.
5. ^ Mendel, 1979; 1981a; 1981b; 1985
6. ^
7. ^ BBC News Article "Sloth's Lazy Image 'A Myth'"
8. ^ David Attenborough, Life of Mammals, BBC
9. ^ Stewart, Melissa (November 2004). "Slow and Steady Sloths". Smithsonian Zoogoer. Smithsonian Institution. Retrieved 2009-09-14.
10. ^ David Attenborough, David Attenborough's Life Stories, BBC Radio 4; 8:50 am 7 June 2009
11. ^ Montgomery, Sy. "Community Ecology of the Sloth". Cecropia: Supplemental Information. Encyclopedia Britannica. Retrieved 2009-09-06.
12. ^ Narita, Yuichi; Kuratani, Shigeru (2005), "Evolution of the Vertebral Formulae in Mammals: A Perspective on Developmental Constraints", Journal of Experimental Zoology (Mol Dev Evol) 304 (2): 91–106, doi:10.1002/jez.b.21029
13. ^ a b
14. ^ Carl Zimmer (11 January 2010). "Hunting Fossil Viruses in Human DNA". The New York Times. Retrieved 17 January 2010.
15. ^ Gaudin, Timothy J. (2004-02-03). "Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence". Zoological Journal of the Linnean Society (Linnean Society of London) 140 (2): 255–305. doi:10.1111/j.1096-3642.2003.00100.x. Retrieved 2008-12-16.

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