Sloths are a group of
Neotropical
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 ...
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. Ex ...
n mammals constituting the suborder Folivora, including the extant
arboreal
Arboreal locomotion is the Animal locomotion, 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. Th ...
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 Carib ...
. 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
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
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. ...
. 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 with ...
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 b ...
.
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
''Megatherium'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Pleistocene. It is best known for the elephant-sized type species ' ...
'') inhabited both North and South America during the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
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
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
has also been suggested to have contributed. Members of an endemic
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
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
The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...
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'', ''Cry ...
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. Ex ...
, a group of
placental
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsup ...
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s 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 with ...
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 mouthpart ...
s with sturdy
vertebral column
The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...
s, fused
pelvis
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The ...
es, 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 l ...
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 b ...
. These names are from the Latin 'leaf eater' and 'hairy', respectively. Pilosa is one of the smallest of the orders of the
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
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
Megalocnidae is an extinct family of sloths, native to the islands of the Greater Antilles from the Early Oligocene to the Mid-Holocene. They are known from Cuba, Hispaniola and Puerto Rico, but are absent from Jamaica. While they were formerly ...
: 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 sloth
The three-toed or three-fingered sloths are arboreal neotropical mammals . They are the only members of the genus ''Bradypus'' and the family Bradypodidae. The four living species of three-toed sloths are the brown-throated sloth, the maned sloth ...
s, 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 South ...
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
The pygmy three-toed sloth (''Bradypus pygmaeus''), also known as the monk sloth or dwarf sloth, is a sloth endemic to Isla Escudo de Veraguas, a small island off the Caribbean coast of Panama. The species was first described by Robert P. Ander ...
which is endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
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 coastlin ...
off the coast of Panama
Panama ( , ; es, link=no, Panamá ), officially the Republic of Panama ( es, República de Panamá), is a transcontinental country spanning the southern part of North America and the northern part of South America. It is bordered by Cos ...
.
**†Megalonychidae
Megalonychidae is an extinct family of sloths including the extinct ''Megalonyx''. Megalonychids first appeared in the early Oligocene, about 35 million years (Ma) ago, in southern Argentina (Patagonia). There is actually one possible find dating ...
: 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 America ...
: ground sloths that existed for about 23 million years and went extinct about 11,000 years ago; this family included the largest sloths.
**†Nothrotheriidae
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 ...
: ground sloths that lived from approximately 11.6 million to 11,000 years ago. As well as ground sloths, this family included ''Thalassocnus
''Thalassocnus'' is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast. It is monotypic within the subfamily Thalassocninae. The five species—''T. antiquus'', ''T. natans'', ''T. lit ...
'', a genus of either semiaquatic or fully aquatic sloths.
*Superfamily Mylodontoidea
** Choloepodidae, the two-toed sloth
''Choloepus'' is a genus of xenarthran mammals of Central and South America within the monotypic family Choloepodidae, consisting of two-toed sloths, sometimes also called two-fingered sloths. The two species of ''Choloepus'' (which means "lame ...
s, contains two extant species:
*** Linnaeus's two-toed sloth
Linnaeus's two-toed sloth (''Choloepus didactylus''), also known as the southern two-toed sloth, unau, or Linne's two-toed sloth is a species of sloth from South America, found in Venezuela, the Guyanas, Colombia, Ecuador, Peru, and Brazil north ...
found in Venezuela
Venezuela (; ), officially the Bolivarian Republic of Venezuela ( es, link=no, República Bolivariana de Venezuela), is a country on the northern coast of South America, consisting of a continental landmass and many islands and islets in th ...
, 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
Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the Car ...
, Ecuador
Ecuador ( ; ; Quechua: ''Ikwayur''; Shuar: ''Ecuador'' or ''Ekuatur''), officially the Republic of Ecuador ( es, República del Ecuador, which literally translates as "Republic of the Equator"; Quechua: ''Ikwadur Ripuwlika''; Shuar: ''Eku ...
, Peru
, image_flag = Flag of Peru.svg
, image_coat = Escudo nacional del Perú.svg
, other_symbol = Great Seal of the State
, other_symbol_type = Seal (emblem), National seal
, national_motto = "Fi ...
, and Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
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 t ...
.
*** 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
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
. One population is found from eastern Honduras
Honduras, officially the Republic of Honduras, is a country in Central America. The republic of Honduras is bordered to the west by Guatemala, to the southwest by El Salvador, to the southeast by Nicaragua, to the south by the Pacific Oce ...
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
Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa
The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters a ...
: ground sloths that existed for about 23 million years and went extinct about 11,000 years ago.
**†Scelidotheriidae
Scelidotheriidae is a family of extinct ground sloths within the order Pilosa, suborder Folivora and superfamily Mylodontoidea, related to the other extinct mylodontoid family, Mylodontidae, as well as to the living two-toed sloth family Choloe ...
: 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 re ...
'' 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
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
to an arboreal
Arboreal locomotion is the Animal locomotion, 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. Th ...
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
''Megatherium'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Pleistocene. It is best known for the elephant-sized type species ' ...
''.
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 island ...
of pilosans
The order Pilosa is a clade of Xenarthra, 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 taxono ...
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
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
, 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 foun ...
'' 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 fa ...
''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 '' P ...
'' 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 country ...
. 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 lan ...
. 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
''Thalassocnus'' is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast. It is monotypic within the subfamily Thalassocninae. The five species—''T. antiquus'', ''T. natans'', ''T. lit ...
'' of the west coast of South America became adapted to a semiaquatic
In biology, semiaquatic can refer to various types of animals that spend part of their time in water, or plants that naturally grow partially submerged in water. Examples are given below.
Semiaquatic animals
Semiaquatic animals include:
* Verte ...
and, eventually, perhaps fully aquatic marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military
* ...
lifestyle. In Peru
, image_flag = Flag of Peru.svg
, image_coat = Escudo nacional del Perú.svg
, other_symbol = Great Seal of the State
, other_symbol_type = Seal (emblem), National seal
, national_motto = "Fi ...
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
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
. 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 orde ...
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
The three-toed or three-fingered sloths are arboreal neotropical mammals . They are the only members of the genus ''Bradypus'' and the family Bradypodidae. The four living species of three-toed sloths are the brown-throated sloth, the maned 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
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
and mitochondrial DNA
Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...
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 fa ...
s and three-toed sloths within Megatherioidea, close to ''Megalonyx
''Megalonyx'' (Greek, "large claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America during the Pliocene and Pleistocene epochs. It became extinct during the Quaternary extinction event at the end of th ...
'', megatheriid
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 Amer ...
s 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
Megalonychidae is an extinct family of sloths including the extinct ''Megalonyx''. Megalonychids first appeared in the early Oligocene, about 35 million years (Ma) ago, in southern Argentina (Patagonia). There is actually one possible find dating ...
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...
, 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 br ...
; 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 orde ...
(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
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
that came with the end of the last ice age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...
may have also played a role, although previous similar glacial retreats were not associated with similar extinction rates.
''Megalocnus
''Megalocnus'' ("great sloth" in Greek) is a genus of extinct large ground sloths that were native to Cuba during the Pleistocene and Holocene epochs. They were among the largest of the Caribbean sloths (Megalocnidae), with individuals estimate ...
'' 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 mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s 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 ...
. 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
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
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
Heterothermy or heterothermia (from Greek ἕτερος ''heteros'' "other" and θέρμη ''thermē'' "heat") is a physiological term for animals that vary between self-regulating their body temperature, and allowing the surrounding environment to ...
, 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 ...
from predatory jaguar
The jaguar (''Panthera onca'') is a large cat species and the only living member of the genus '' Panthera'' native to the Americas. With a body length of up to and a weight of up to , it is the largest cat species in the Americas and the th ...
s, ocelot
The ocelot (''Leopardus pardalis'') is a medium-sized spotted wild cat that reaches at the shoulders and weighs between on average. It was first described by Carl Linnaeus in 1758. Two subspecies are recognized. It is native to the southwes ...
s, and harpy eagle
The harpy eagle (''Harpia harpyja'') is a neotropical species of eagle. It is also called the American harpy eagle to distinguish it from the Papuan eagle, which is sometimes known as the New Guinea harpy eagle or Papuan harpy eagle. It is the ...
s. 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 c ...
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
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 mosquito
Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
es and sandflies
Sandfly (or sand fly) is a colloquial name for any species or genus of flying, biting, blood-sucking dipteran (fly) encountered in sandy areas. In the United States, ''sandfly'' may refer to certain horse flies that are also known as "greenhea ...
, 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 mite
Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...
s. 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 moth
Moths are a paraphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of w ...
s. 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 nutri ...
, 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 j ...
, 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
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering.
Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particula ...
, leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
and small lizards
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...
, ranging over up to . Three-toed sloths, on the other hand, are almost entirely herbivorous
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 mouthpart ...
(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 Animal locomotion, 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. Th ...
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 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 186 ...
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 parasit ...
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
Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...
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
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
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 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
Panama ( , ; es, link=no, Panamá ), officially the Republic of Panama ( es, República de Panamá), is a transcontinental country spanning the southern part of North America and the northern part of South America. It is bordered by Cos ...
, sloths have been estimated to comprise 70% of the biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
of arboreal mammals. Four of the six living species
In biology, a species is the basic unit of 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 the appropriate s ...
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
The pygmy three-toed sloth (''Bradypus pygmaeus''), also known as the monk sloth or dwarf sloth, is a sloth endemic to Isla Escudo de Veraguas, a small island off the Caribbean coast of Panama. The species was first described by Robert P. Ander ...
(''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
Costa Rica (, ; ; literally "Rich Coast"), officially the Republic of Costa Rica ( es, República de Costa Rica), is a country in the Central American region of North America, bordered by Nicaragua to the north, the Caribbean Sea to the no ...
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
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Clawed herbivores
Folivores
Pilosans
Extant Rupelian first appearances