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Sloths are a group of Neotropical
xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Exti ...
n mammals constituting the suborder Folivora, including the extant
arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...
tree sloths and extinct terrestrial
ground sloths Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. The term is used to refer to all extinct sloths because of the large size of the earliest forms discovered, compared to existing tree sloths. The Caribbea ...
. Noted for their slowness of movement, tree sloths spend most of their lives hanging upside down in the trees of the
tropical rainforest Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equatori ...
s of South America and
Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. C ...
. Sloths are considered to be most closely related to
anteater Anteater is a common name for the four extant mammal species of the suborder Vermilingua (meaning "worm tongue") commonly known for eating ants and termites. The individual species have other names in English and other languages. Together wit ...
s, together making up the xenarthran order
Pilosa The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters and sloths (which includes the extinct ground sloths). The name comes from the Latin word for "hairy". Origins and taxonomy The bi ...
. There are six extant sloth species in two genera – '' Bradypus'' (three–toed sloths) and '' Choloepus'' (two–toed sloths). 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 Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.Zhang, J. and Kumar, S. 1997Detection of convergent and paral ...
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 Pleistocene Epoch. However, they became extinct during the
Quaternary extinction event The Quaternary period (from 2.588 ± 0.005 million years ago to the present) has seen the extinctions of numerous predominantly megafaunal species, which have resulted in a collapse in faunal density and diversity and the extinction of key ecolog ...
around 12,000 years ago, along 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 also formerly lived in the
Greater Antilles The Greater Antilles ( es, Grandes Antillas or Antillas Mayores; french: Grandes Antilles; ht, Gwo Zantiy; jam, Grieta hAntiliiz) is a grouping of the larger islands in the Caribbean Sea, including Cuba, Hispaniola, Puerto Rico, Jamaica, and ...
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 (e.g. french: paresseux, es, perezoso) 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 moth A sloth moth is a coprophagous moth which has evolved to exclusively inhabit the fur of sloths and to use sloth dung as a substrate for the early stages of reproduction. Sloth moths include '' Bradypodicola hahneli'', ''Cryptoses choloepi'', '' C ...
s, some species of which exist solely on sloths.


Taxonomy and evolution

Sloths belong to the superorder
Xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Exti ...
, 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.
Anteater Anteater is a common name for the four extant mammal species of the suborder Vermilingua (meaning "worm tongue") commonly known for eating ants and termites. The individual species have other names in English and other languages. Together wit ...
s and
armadillo Armadillos (meaning "little armored ones" in Spanish) are New World placental mammals in the order Cingulata. The Chlamyphoridae and Dasypodidae are the only surviving families in the order, which is part of the superorder Xenarthra, along wi ...
s are also included among Xenarthra. The earliest xenarthrans were arboreal
herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthp ...
s with sturdy vertebral columns, fused pelvises, stubby teeth, and small brains. Sloths are in the taxonomic suborder
Folivora Sloths are a group of Neotropical 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 li ...
of the order
Pilosa The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters and sloths (which includes the extinct ground sloths). The name comes from the Latin word for "hairy". Origins and taxonomy The bi ...
. 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 (
A dagger, obelisk, or obelus is a typographical mark that usually indicates a footnote if an asterisk has already been used. The symbol is also used to indicate death (of people) or extinction (of species). It is one of the modern descendan ...
): *† 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 The brown-throated sloth (''Bradypus variegatus'') is a species of three-toed sloth found in the Neotropical realm of Central and South America. It is the most common of the four species of three-toed sloth, and is found in the forests of Sout ...
is the most common of the extant species of sloth, which inhabits the
Neotropical realm The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone. Definition In bioge ...
in the forests of South and Central America. *** The
pale-throated three-toed sloth The pale-throated sloth (''Bradypus tridactylus''), occasionally known as the ai (), is a species of three-toed sloth that inhabits tropical rainforests in northern South America. It is similar in appearance to, and often confused with, the brow ...
, 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. *** The
maned three-toed sloth The maned sloth (''Bradypus torquatus'') is a three-toed sloth that is native to South America. It is one of four species of three-toed sloths belonging to the suborder Xenarthra and are placental mammals. They are endemic to the Atlantic coasta ...
, now found only in the
Atlantic Forest The Atlantic Forest ( pt, Mata Atlântica) is a South American forest that extends along the Atlantic coast of Brazil from Rio Grande do Norte state in the northeast to Rio Grande do Sul state in the south and inland as far as Paraguay and th ...
of southeastern Brazil. *** The critically endangered pygmy three-toed sloth which is endemic to the small island of
Isla Escudo de Veraguas Isla Escudo de Veraguas is a small (4.3 km2) isolated Caribbean island of the Republic of Panama. Despite its name, it is not part of the province of Veraguas, but rather Bocas del Toro. Although located only 17 km from the coastline i ...
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 Megatheriidae is a family of extinct ground sloths that lived from approximately 23 mya—11,000 years ago. Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South Americ ...
: 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 The Guianas, sometimes called by the Spanish loan-word ''Guayanas'' (''Las Guayanas''), is a region in north-eastern South America which includes the following three territories: * French Guiana, an overseas department and region of France * ...
, Colombia, Ecuador, Peru, and Brazil north of the
Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of ...
. ***
Hoffmann's two-toed sloth Hoffmann's two-toed sloth (''Choloepus hoffmanni''), also known as the northern two-toed sloth is a species of sloth from Central and South America. It is a solitary, largely nocturnal and arboreal animal, found in mature and secondary rainf ...
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 , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...
. **† 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 ''Mylodon'' is a genus of extinct ground sloth belonging to the family Mylodontidae, known from the region of Patagonia in Chile and Argentina in southern South America. With a total length of 3 to 4 m, it is one of the best-known and largest rep ...
'' than '' Choloepus'' is, so it has been elevated back to full family status.


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 Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...
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''. 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 Oceanic dispersal is a type of biological dispersal that occurs when terrestrial organisms transfer from one land mass to another by way of a sea crossing. Island hopping is the crossing of an ocean by a series of shorter journeys between isla ...
of pilosans to the
Greater Antilles The Greater Antilles ( es, Grandes Antillas or Antillas Mayores; french: Grandes Antilles; ht, Gwo Zantiy; jam, Grieta hAntiliiz) is a grouping of the larger islands in the Caribbean Sea, including Cuba, Hispaniola, Puerto Rico, Jamaica, and ...
by the Oligocene, and that the megalonychid ''
Pliometanastes ''Pliometanastes'' is an extinct genus of ground sloths of the family Megalonychidae endemic to North America during the Late Miocene epoch through very early Pliocene epoch (Hemphillian in the NALMA classification). Its fossils have been found ...
'' and the
mylodontid Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa, living from around 23 million years ago (Mya) to 11,000 years ago. This family is most closely related to another ...
''
Thinobadistes ''Thinobadistes'' is an extinct genus of ground sloth of the family Mylodontidae endemic to North America during the Miocene-Pliocene epochs (Hemphillian). It lived from 10.3 to 4.9 mya, existing for approximately . ''Thinobadistes'' and '' Pli ...
'' were able to colonise North America about 9 million years ago, well before the formation of the
Isthmus of Panama The Isthmus of Panama ( es, Istmo de Panamá), also historically known as the Isthmus of Darien (), is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the countr ...
. The latter development, about 3 million years ago, allowed megatheriids and nothrotheriids to also invade North America as part of the
Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which la ...
. Additionally, the
nothrotheriid Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, they ...
'' Thalassocnus'' of the west coast of South America became adapted to a semiaquatic and, eventually, perhaps fully aquatic marine lifestyle. In Peru and
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
, ''Thalassocnus'' entered the coastal habitat beginning in the late Miocene. Initially they just stood in the water, but over a span of 4 million years they eventually evolved into swimming creatures, becoming specialist bottom feeders of
seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the ...
es, similar to extant marine
sirenia The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...
ns. 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 (''Bradypus'') 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 (convergent evolution) hypothesis, but have overturned some of the other conclusions obtained from morphology. These investigations consistently place two-toed sloths close to
mylodontid Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa, living from around 23 million years ago (Mya) to 11,000 years ago. This family is most closely related to another ...
s and three-toed sloths within Megatherioidea, close to '' Megalonyx'', megatheriids and
nothrotheriid Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, they ...
s. 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 Central American Seaway (also known as the Panamanic Seaway, Inter-American Seaway and Proto-Caribbean Seaway) was a body of water that once separated North America from South America. It formed during the Jurassic (200–154 Ma) during the ...
; the closing caused a cooling trend in the coastal waters which killed off much of the area's
seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the ...
(and which would have also made thermoregulation difficult for the sloths, with their slow metabolism). 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 In terrestrial zoology, the megafauna (from Greek μέγας ''megas'' "large" and New Latin ''fauna'' "animal life") comprises the large or giant animals of an area, habitat, or geological period, extinct and/or extant. The most common threshold ...
. 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 In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sa ...
. Two-toed sloths have five to seven, while three-toed sloths have eight or nine. The other mammals not having seven are the
manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...
s, with six.


Physiology

Sloths have colour vision, but have poor visual acuity. They also have poor hearing. Thus, they rely on their sense of smell and touch to find food. Sloths have very low metabolic rates (less than half of that expected for a mammal of their size), and low body temperatures: when active, and still lower when resting. Sloths are heterothermic, meaning their body temperature may vary according to the environment, normally ranging from , but able to drop to as low as , inducing
torpor Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability. The term "torpor" can refer to the time ...
. The outer hairs of sloth fur 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 limbs above their bodies, their hairs grow away from the extremities to provide protection from the elements while they hang upside down. In most conditions, the fur hosts symbiotic algae, which provide
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...
from predatory jaguars, ocelots, and harpy eagles. Because of the algae, sloth fur is a small ecosystem of its own, hosting many species of
commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
and
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, o ...
s. There are a large number of
arthropods associated with sloths A large number of arthropods are associated with sloths. These include biting and blood-sucking flies such as mosquitoes and sandflies, triatomine bugs, lice, ticks and mites. The sloth’s fur forms a micro-ecozone inhabited by green algae and h ...
. These include biting and blood-sucking flies such as mosquitoes and sandflies, triatomine bugs,
lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...
,
tick Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...
s and mites. Sloths have a highly specific community of commensal
beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
s, mites and moths. The species of sloths recorded to host arthropods include the pale-throated three-toed sloth, the brown-throated three-toed sloth, and Linnaeus's two-toed sloth. Sloths benefit from their relationship with moths because the moths are responsible for fertilizing algae on the sloth, which provides them with nutrients.


Activity

Their limbs are adapted for hanging and grasping, not for supporting their weight. Muscle mass make up only 25 to 30 percent of their total body weight. Most other mammals have a muscle mass that makes up 40 to 45 percent of their total body weight. Their specialised hands and feet have long, curved claws to allow them to hang upside down from branches without effort, and are used to drag themselves along the ground, since they cannot walk. On three-toed sloths, the arms are 50 percent longer than the legs. Sloths move only when necessary and even then very slowly. They usually move at an average speed of per minute, but can move at a marginally higher speed of per minute if they are in immediate danger from a predator. While they sometimes sit on top of branches, they usually eat, sleep, and even give birth hanging from branches. They sometimes remain hanging from branches even after death. On the ground, the maximum speed of sloths is per minute. Two-toed sloths are generally better able than three-toed sloths to disperse between clumps of trees on the ground. Sloths are surprisingly strong swimmers and can reach speeds of per minute. They use their long arms to paddle through the water and can cross rivers and swim between islands. Sloths can reduce their already slow metabolism even further and slow their heart rate to less than a third of normal, allowing them to hold their breath underwater for up to 40 minutes. Wild brown-throated three-toed sloths sleep on average 9.6 hours a day. Two-toed sloths are
nocturnal Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have highly developed sens ...
. Three-toed sloths are mostly nocturnal, but can be active in the day. They spend 90 per cent of their time motionless.


Behavior

Sloths are solitary animals that rarely interact with one another except during breeding season, though female sloths do sometimes congregate, more so than do males. Sloths descend about once every eight days to defecate on the ground. The reason and mechanism behind this behavior have long been debated among scientists. There are at least five hypotheses: 1) fertilize trees when feces are deposited at the base of the tree; 2) cover feces and avoid predation;Pauli, J. N., Mendoza, J. E., Steffan, S. A., Carey, C. C., Weimer, P. J., & Peery, M. Z. (2014). A syndrome of mutualism reinforces the lifestyle of a sloth. ''Proceedings of the Royal Society B: Biological Sciences'', ''281''(1778), 20133006. DOI: 10.1098/rspb.2013.3006 3) chemical communication between individuals; 4) pick up trace nutrients in their claws, that are then ingested;Voirin, B., Kays, R., Wikelski, M., & Lowman, M. (2013). Why Do Sloths Poop on the Ground? In M. Lowman, S. Devy, & T. Ganesh (eds). ''Treetops at Risk''(pp. 195-199). Springer, New York, NY. and 5) favor a mutualistic relationship with populations of fur moths. More recently, a new hypothesis has emerged, which presents evidence against the previous ones and proposes that all current sloths are descendants from species that defecated on the ground, and there simply has not been enough selective pressure to abandon this behavior, since cases of predation during defecation are actually very rare.


Diet

Baby sloths learn what to eat by licking the lips of their mother. All sloths eat the leaves of the
cecropia ''Cecropia'' is a Neotropical genus consisting of 61 recognized species with a highly distinctive lineage of dioecious trees. The genus consists of pioneer trees in the more or less humid parts of the Neotropics, with the majority of the speci ...
''.'' Two-toed sloths are
omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...
, with a diverse diet of
insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of ...
,
carrion Carrion () is the decaying flesh of dead animals, including human flesh. Overview Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters (or scavengers) include crows, vultures, c ...
, fruits,
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...
and small lizards, ranging over up to . Three-toed sloths, on the other hand, are almost entirely herbivorous (plant eaters), with a limited diet of leaves from only a few trees, and no other mammal digests its food as slowly. They have made adaptations to
arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...
browsing. Leaves, their main food source, provide very little energy or nutrients, and do not digest easily, so sloths have large, slow-acting, multi-chambered
stomachs The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach is ...
in which
symbiotic bacteria Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, rhizobia living in root nodules of legumes provide nitrogen fixing activity for these plants. Symbiosis was first defined by Marko de Bary in 1869 ...
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. Three-toed sloths 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. Considering the large energy expenditure and dangers involved in the journey to the ground, this behaviour has been described as a mystery. Recent research shows that moths, which live in the sloth's fur, lay eggs in the sloth's feces. When they hatch, the larvae feed on the feces, and when mature fly up onto the sloth above. These moths may have a
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasi ...
relationship with sloths, as they live in the fur and promote growth of algae, which the sloths eat. 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 also help nourish it.


Reproduction

The pale- and brown-throated three-toed sloths mate seasonally, while the maned three-toed sloth breeds at any time of the year. The reproduction of pygmy three-toed sloths is currently unknown. Litters are of one newborn only, after six months' gestation for three-toed, and 12 months' for two-toed. Newborns stay with their mother for about five months. In some cases, young sloths 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. Sloths are not particularly sexually dimorphic and several zoos have received sloths of the wrong sex. The average lifespan of two-toed sloths in the wild is currently unknown due to a lack of full-lifespan studies in a natural environment. Median life expectancy in human care is about 16 years, with one individual at the
Smithsonian Institution's The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded o ...
National Zoo reaching an age of 49 years before her death.


Distribution

Although habitat is limited to the tropical rainforests of Central and South America, in that environment sloths are successful. On
Barro Colorado Island Barro Colorado Island is located in the man-made Gatun Lake in the middle of the Panama Canal. The island was formed when the waters of the Chagres River were dammed to form the lake in 1913. When the waters rose, they covered a significant part ...
in Panama, sloths have been estimated to comprise 70% of the biomass of arboreal mammals. Four of the six living
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
are presently rated "least concern"; the
maned three-toed sloth The maned sloth (''Bradypus torquatus'') is a three-toed sloth that is native to South America. It is one of four species of three-toed sloths belonging to the suborder Xenarthra and are placental mammals. They are endemic to the Atlantic coasta ...
(''Bradypus torquatus''), which inhabits Brazil's dwindling
Atlantic Forest The Atlantic Forest ( pt, Mata Atlântica) is a South American forest that extends along the Atlantic coast of Brazil from Rio Grande do Norte state in the northeast to Rio Grande do Sul state in the south and inland as far as Paraguay and th ...
, is classified as "vulnerable", while the island-dwelling pygmy three-toed sloth (''B. pygmaeus'') is critically endangered. Sloths' lower metabolism confines them to the tropics and they adopt thermoregulation behaviors of cold-blooded animals such as sunning themselves.


Human relations

The majority of recorded sloth deaths in Costa Rica are due to contact with electrical lines and
poachers Poaching has been defined as the illegal hunting or capturing of wild animals, usually associated with land use rights. Poaching was once performed by impoverished peasants for subsistence purposes and to supplement meager diets. It was set a ...
. Their claws also provide another, 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 are victims of animal trafficking where they are sold as pets. However, they make very poor pets, as they have such a specialized ecology. The Sloth Institute Costa Rica is known for caring, rehabilitating and releasing sloths back into the wild. Also in Costa Rica, the Aviarios Sloth Sanctuary cares for sloths. It has rehabilitated and released about 130 individuals back into the wild. However, a report in May 2016 featured two former veterinarians from the facility who were intensely critical of the sanctuary's efforts, accusing it of mistreating the animals.


References


External links

* * * {{Authority control Clawed herbivores Folivores Pilosans Extant Rupelian first appearances