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Sloths are a group of arboreal Neotropical xenarthran mammals, constituting the suborder Folivora. Noted for their slowness of movement, they spend most of their lives hanging upside down in the trees of the tropical rainforests of South America and Central America. They are considered to be most closely related to anteaters, together making up the xenarthran order Pilosa.

There are six extant sloth species in two genera – Bradypus (three–toed sloths) and Choloepus (two–toed sloths). Despite this traditional naming, all sloths actually have three toes on each rear limb, although two-toed sloths have only two digits on each forelimb.[3] The two groups of sloths are from different, distantly related families, and are thought to have evolved their morphology via parallel evolution from terrestrial ancestors. Besides the extant species, many species of ground sloths ranging up to the size of elephants (like Megatherium) inhabited both North and South America during the PleistoceneEpoch. However, they became extinct during the Quaternary extinction event around 12,000 years ago, together with most large bodied animals in the New World. The extinction correlates in time with the arrival of humans, but climate change has also been suggested to have contributed. Members of an endemic radiation of Caribbean sloths formerly lived in the Greater Antilles. They included both ground and arboreal forms which became extinct after humans settled the archipelago in the mid-Holocene, around 6,000 years ago.

Sloths are so named because of their very low metabolism and deliberate movements. Sloth, related to slow, literally means "laziness," and their common names in several other languages (e.g. French paresseux) also mean "lazy" or similar. Their slowness permits their low-energy diet of leaves and avoids detection by predatory hawks and cats that hunt by sight.[3] Sloths are almost helpless on the ground, but are able to swim.[4] The shaggy coat has grooved hair that is host to symbiotic green algae which camouflages the animal in the trees and provides it nutrients. The algae also nourishes sloth moths, some species of which exist solely on sloths.[5

Sloths belong to the superorder Xenarthra, a group of placental mammals believed to have evolved in the continent of South America around 60 million years ago.[6] One study found that xenarthrans broke off from other placental mammals around 100 million years ago.[7] Anteaters and armadillos are also included among Xenarthra. The earliest xenarthrans were arboreal herbivores with sturdy vertebral columns, fused pelvises, stubby teeth, and small brains. Sloths are in the taxonomic suborder Folivora[2] of the order Pilosa. These names are from the Latin 'leaf eater' and 'hairy', respectively. Pilosa is one of the smallest of the orders of the mammal class; its only other suborder contains the anteaters.

The Folivora are divided into at least eight families, only two of which have living species; the remainder are entirely extinct ():[8]

  • †Megalocnidae: the Greater Antilles sloths, a basal group that arose about 32 million years ago and became extinct about 5,000 years ago.[8]

  • Superfamily Megatherioidea

  • Bradypodidae, the three-toed sloths, contains four extant species:

  • The brown-throated three-toed sloth is the most common of the extant species of sloth, which inhabits the Neotropical realm[1][9] in the forests of South and Central America.

  • The pale-throated three-toed sloth, which inhabits tropical rainforests in northern South America. It is similar in appearance to, and often confused with, the brown-throated three-toed sloth, which has a much wider distribution. Genetic evidence indicates the two species diverged around 6 million years ago.[10]

  • The maned three-toed sloth, now found only in the Atlantic Forest of southeastern Brazil.

  • The critically endangered pygmy three-toed sloth which is endemic to the small island of Isla Escudo de Veraguas off the coast of Panama.

  • †Megalonychidae: ground sloths that existed for about 35 million years and went extinct about 11,000 years ago. This group was formerly thought to include both the two-toed sloths and the extinct Greater Antilles sloths.

  • †Megatheriidae: ground sloths that existed for about 23 million years and went extinct about 11,000 years ago; this family included the largest sloths.

  • †Nothrotheriidae: ground sloths that lived from approximately 11.6 million to 11,000 years ago. As well as ground sloths, this family included Thalassocnus, a genus of either semiaquatic or fully aquatic sloths.

  • Superfamily Mylodontoidea

  • Choloepodidae, the two-toed sloths, contains two extant species:

  • Linnaeus's two-toed sloth found in Venezuela, the Guianas, Colombia, Ecuador, Peru, and Brazil north of the Amazon River.

  • Hoffmann's two-toed sloth which inhabits tropical forests. It has two separate ranges, split by the Andes. One population is found from eastern Honduras[11] in the north to western Ecuador in the south, and the other in eastern Peru, western Brazil, and northern Bolivia.[12]

  • †Mylodontidae: ground sloths that existed for about 23 million years and went extinct about 11,000 years ago.

  • †Scelidotheriidae: collagen sequence data indicates this group is more distant from Mylodon than Choloepus is, so it has been elevated back to full family status.[8]

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