| Two-toed sloths[1] | |
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| Hoffmann's two-toed sloth (C. hoffmanni) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Pilosa |
| Superfamily: | Mylodontoidea |
| Family: | Choloepodidae Gray, 1871[2] |
| Genus: | Choloepus Illiger, 1811[1] |
| Type species | |
| Bradypus didactylus Linnaeus, 1758 | |
| Species | |
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Choloepus is agenus ofxenarthranmammals from Central and South America within themonotypicfamilyCholoepodidae, consisting oftwo-toed sloths,[3] sometimes also calledtwo-fingered sloths.[4] The two species ofCholoepus (which means "lame foot" inAncient Greek[5]),Linnaeus's two-toed sloth (Choloepus didactylus) andHoffmann's two-toed sloth (Choloepus hoffmanni), were formerly believed on the basis of morphological studies to be the only surviving members of thesloth familyMegalonychidae,[6] but have now been shown by molecular results to be closest to extinctground sloths of the familyMylodontidae.[4][3]
| Common name | Scientific name and subspecies | Range | Size and ecology | IUCN status and estimated population |
|---|---|---|---|---|
| Linnaeus's two-toed sloth
| Choloepus didactylus (Linnaeus, 1758) | Northern South America, found in Venezuela, the Guyanas, Colombia, Ecuador, Peru, and Brazil north of the Amazon River | Size: Habitat: Diet: | LC |
| Hoffmann's two-toed sloth
| Choloepus hoffmanni Peters, 1858 Five subspecies | Central America and northwestern South America | Size: Habitat: Diet: | LC |
A study ofretrovirus andmitochondrial DNA suggests thatC. didactylus andC. hoffmani diverged 6 to 7 million years ago.[7] Furthermore, based oncytochrome c oxidase subunit I sequences, a similar divergence date (c. 7 million years ago) between the two populations ofC. hofmanni separated by theAndes has been reported.[8] Their ancestors evolved with marine vertebrae, the three toed-sloth and the manatee are the only other mammals with similar vertebrae.[9]
Both types of sloth tend to occupy the same forests; in most areas, a particular species of the somewhat smaller and generally slower-movingthree-toed sloth (Bradypus) and a single species of the two-toed type will jointly predominate. Although similar in overall appearance, the relationship between the two genera is not close. Recentphylogenetic analyses[10] support analysis of morphological data from the 1970s and 1980s, indicating the two genera are not closely related and adapted to theirarboreal lifestyles independently. It was unclear from this work from whichground-dwelling sloth taxa the three-toed sloths evolved. Based on the morphological comparisons, it was thought the two-toed sloths nested phylogenetically within one of the divisions ofCaribbean sloths.[11] 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.[12]
Much of the morphological evidence to support the hypothesis of diphyly has been based on the structure of the inner ear.[13] Most morphological studies have concluded thatconvergent evolution is the mechanism that resulted in today's two genera of tree sloths. This means that the extant genera evolved analogous traits, such as locomotion methods, size, habitat, and many other traits independently from one another as opposed to from their last common ancestor. This makes tree sloths “one of the most striking examples of convergent evolution known among mammals”.[12]
Recently obtained molecular data from collagen[3] and mitochondrial DNA[4] 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 tomylodontids and three-toed sloths within Megatherioidea, close toMegalonyx,megatheriids andnothrotheriids. They make the previously recognized familyMegalonychidaepolyphyletic, with both two-toed sloths and the Caribbean sloths being moved out of that family and away fromMegalonyx. Caribbean sloths are placed in a separate,basal branch of the sloth evolutionary tree.[3][4]
The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Pressleeet al., 2019).[3]
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The name "two-toed sloth" was intended to describe an anatomical difference between the generaCholoepus andBradypus, but does so in a potentially misleading way. Members ofCholoepus have two digits on theirforelimbs (the thoracic limbs) and three digits on theirhindlimbs (the pelvic limbs), while members ofBradypus have three digits on all limbs. Although the term "two-fingered" sloth is arguably less misleading, the shorter "two-toed" is much more widely used.[note 1]
Members ofCholoepus are larger than three-toed sloths, having a body length of 58 to 70 centimetres (23 to 28 in), and weighing 4 to 8 kilograms (8.8 to 17.6 lb). Other distinguishing features include a more prominent snout, longer fur, and the absence of atail.[14]
Two-toed sloths spend most of their lives hanging upside down from trees. They cannot walk, so they pull hand-over-hand to move around, which is at an extremely slow rate. Almost all of their movement comes from this suspended upside down position, at a higher degree than even three-toed sloths. As a result, they tend to gravitate towards less vertical portions of trees.[15][16] Being predominantly nocturnal, their fur, which grows greenish algae to blend in, is their main source of protection.[17] Their body temperatures depend at least partially on the ambient temperature; they cannot shiver to keep warm, as other mammals do, because of their unusually low metabolic rates and reduced musculature.[14] Two-toed sloths also differ from three-toed sloths in their climbing behavior, preferring to descend head first.
Two-toed sloths have agestation period of six months to a year, depending on the species. Their ovarian cycle lasts around 31 to 33 days, independently of the seasons but dependent on the species.[18] The mother gives birth to a single young, while hanging upside down. The young are born with claws, and areweaned after about a month, although they will remain with the mother for several more months, and do not reach maturity until the age of three years, in the case of females, or four to five years, in the case of males.[16] During natal dispersion, two-toed sloths prefer tropical forests over other types of habitat, often usingriparian forest buffers to disperse. Although they also occupy shade-grown cacao plantations, they avoid open pastures.[19]
They eat primarilyleaves, but alsoshoots,fruits,nuts,berries,bark, some nativeflowers, and even some smallvertebrates.[20] In addition, when they cannot find food, they have been known to eat thealgae that grow on their fur for nutrients.[21] They have also been observed usingmineral licks.[22][23] They have large, four-chamberedstomachs, which help to ferment the large amount of plant matter they eat.[24] Food can take up to a month to digest due to their slow metabolism.[14] Depending on when in the excretion cycle a sloth is weighed, urine and feces may account for up to 30% of the animal's body weight.[25] They get most of their fluids from water in the leaves that they eat but sloths have also been observed drinking directly from rivers.[citation needed]
Two-toed sloths have a reduced, ever growingdentition, with noincisors or truecanines, which overall lacks homology with the dental formula of other mammals. Their first tooth is very canine-like in shape and is referred to as a caniniform. It is used for tearing small chunks off of their food, as well as for defense against predators.[26] It is separated from the other teeth, or molariforms, by adiastema. The molariforms are used specifically for grinding and are mortar and pestle-like in appearance and function. Thus, they can grind food for easier digestibility, which takes the majority of their energy. Thedental formula of two-toed sloths is:45 (unau)
Two-toed sloths are unusual among mammals in possessing as few as fivecervical vertebrae, which may be due to mutations in thehomeotic genes.[27] All other mammals have seven cervical vertebrae,[28] other than the three-toed sloth and themanatee.
Two-toed sloths generally have similar musculature to that of other mammals. This includes theirzygomaticus muscles, their superficialmasseter, their deep masseter, and theirmedial and lateral pterygoids. Additionally, a specific section of theiranterior temporalis is arranged vertically, to allow them to sharpen their caniniform teeth.[26] They tend to have stronger flexor muscles in their fore- and hindlimbs, as well as their shoulders.[15]
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