Sloth


Sloths are a Neotropical group of xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their lives hanging upside down in the trees of the tropical rainforests of South America and Central America. Sloths 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 and Choloepus. Despite this traditional naming, all sloths have three toes on each rear limb – although two-toed sloths have only two digits on each forelimb. 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 inhabited both North and South America during the Pleistocene Epoch. However, they became extinct during the Quaternary extinction event around 12,000 years ago, along with most large animals across the Americas. 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 also formerly lived in the Greater Antilles but 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 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. Sloths are almost helpless on the ground but are able to swim. The shaggy coat has grooved hair that is host to symbiotic green algae which camouflage the animal in the trees and provide it nutrients. The algae also nourish sloth moths, some species of which exist solely on sloths.

Taxonomy and evolution

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. One study found that xenarthrans broke off from other placental mammals around 100 million years ago. 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 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 :
  • †Megalocnidae: the Greater Antilles sloths, a basal group that arose about 32 million years ago and became extinct about 5,000 years ago.
  • 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 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 six million years ago.
  • ** 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 in the north to western Ecuador in the south, and the other in eastern Peru, western Brazil, and northern Bolivia.
  • *†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.
File:Megatherium americanum Skeleton NHM.JPG|thumb|Megatherium americanum

Evolution

The common ancestor of the two existing sloth genera dates to about 28 million years ago, with similarities between the two- and three-toed sloths an example of convergent evolution to an arboreal lifestyle, "one of the most striking examples of convergent evolution known among mammals". The ancient Xenarthra included a significantly greater variety of species, with a wider distribution, than those of today. Ancient sloths were mostly terrestrial, and some reached sizes that rival those of elephants, as was the case for Megatherium.
File:Megalonyx wheatleyi skeleton & restoration.jpg|thumb|Megalonyx wheatleyi fossil and restoration
File:San Diego Paramylodon.jpg|thumb|Paramylodon harlani
Sloths arose in South America during a long period of isolation and eventually spread to a number of the Caribbean islands as well as North America. It is thought that swimming led to oceanic dispersal of pilosans to the Greater Antilles by the Oligocene, and that the megalonychid Pliometanastes and the mylodontid Thinobadistes were able to colonise North America about 9 million years ago, well before the formation of the Isthmus of Panama. The latter development, about 3 million years ago, allowed megatheriids and nothrotheriids to also invade North America as part of the Great American Interchange. Additionally, the nothrotheriid Thalassocnus of the west coast of South America became adapted to a semiaquatic and, eventually, perhaps fully aquatic marine lifestyle. In Peru and Chile, Thalassocnus entered the coastal habitat beginning in the late Miocene. They presumably waded and paddled in the water for short period, but over a span of 4 million years, they eventually evolved into swimming creatures, becoming specialist bottom feeders of seagrasses, similar to the extant sirenians.
Both types of extant tree sloth tend to occupy the same forests; in most areas, a particular species of the somewhat smaller and generally slower-moving three-toed sloth and a single species of the two-toed type will jointly predominate. Based on morphological comparisons, it was thought the two-toed sloths nested phylogenetically within one of the divisions of the extinct Greater Antilles sloths. Though data has been collected on over 33 different species of sloths by analyzing bone structures, many of the relationships between clades on a phylogenetic tree were unclear. Much of the morphological evidence collected to support the hypothesis of diphyly has been based on the structure of the inner ear.
Recently obtained molecular data from collagen and mitochondrial DNA sequences fall in line with the diphyly hypothesis but have overturned some of the other conclusions obtained from morphology. These investigations consistently place two-toed sloths close to mylodontids and three-toed sloths within Megatherioidea, close to Megalonyx, megatheriids and nothrotheriids. They make the previously recognized family Megalonychidae polyphyletic, with both two-toed sloths and Greater Antilles sloths being moved away from Megalonyx. Greater Antilles sloths are now placed in a separate, basal branch of the sloth evolutionary tree.

Phylogeny

The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data.

Extinctions

The marine sloths of South America's Pacific coast became extinct at the end of the Pliocene following the closing of the Central American Seaway; the closing caused a cooling trend in the coastal waters which killed off much of the area's seagrass.
Ground sloths disappeared from both North and South America shortly after the appearance of humans about 11,000 years ago. Evidence suggests human hunting contributed to the extinction of the American megafauna. Ground sloth remains found in both North and South America indicate that they were killed, cooked, and eaten by humans. Climate change that came with the end of the last ice age may have also played a role, although previous similar glacial retreats were not associated with similar extinction rates.
Megalocnus and some other Caribbean sloths survived until about 5,000 years ago, long after ground sloths had died out on the mainland, but then went extinct when humans finally colonized the Greater Antilles.

Biology

Morphology and anatomy

Sloths can be long and, depending on the species, weigh from. Two-toed sloths are slightly larger than three-toed sloths. Sloths have long limbs and rounded heads with tiny ears. Three-toed sloths also have stubby tails about long.
Sloths are unusual among mammals in not having seven cervical vertebrae. Two-toed sloths have five to seven, while three-toed sloths have eight or nine. The other mammals not having seven are the manatees, with six.