Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Megaclassis: Osteichthyes
Cladus: Sarcopterygii
Cladus: Rhipidistia
Cladus: Tetrapodomorpha
Cladus: Eotetrapodiformes
Cladus: Elpistostegalia
Superclassis: Tetrapoda
Cladus: Reptiliomorpha
Cladus: Amniota
Cladus: Synapsida
Cladus: Eupelycosauria
Cladus: Sphenacodontia
Cladus: Sphenacodontoidea
Cladus: Therapsida
Cladus: Theriodontia
Cladus: Cynodontia
Cladus: Eucynodontia
Cladus: Probainognathia
Cladus: Prozostrodontia
Cladus Mammaliaformes
Classis: Mammalia
Subclassis: Trechnotheria
Infraclassis: Zatheria
Supercohors: Theria
Cohors: Eutheria
Infraclassis: Placentalia
Superordo: Xenarthra
Ordo: Pilosa
Subordo: Folivora
Familia: Megalonychidae
Genus: Choloepus
Species (2): C. didactylus - C. hoffmanni
Name
Choloepus Illiger, 1811: 108
Type species: Bradypus didactylus Linnaeus, 1758, by subsequent designation by Gray (1827: 275).
Synonymy
Unaüs Rafinesque, 1815: 57
Type species: Bradypus unau Link, 1795, by subsequent designation (Palmer 1904: 700) [corrected spelling is Unaues Rafinesque, 1815 according ICZN 1999: Art. 32.5.
Choelopus Lichtenstein, 1818: 20 [incorrect subsequent spelling of Choloepus Illiger].
Unaus Gray, 1821: 305
Type species: Bradypus didactylus Linnaeus, 1758, by monotypy.
Cholaepus Schinz, 1821: 328 [incorrect subsequent spelling of Choloepus Illiger].
Cholaepus Gray, 1825: 343 [incorrect subsequent spelling of Choloepus Illiger].
Chaelopus Gray, 1827: 275 [incorrect subsequent spelling of Choloepus Illiger].
Choelopus Sclater, 1873: 253 [incorrect subsequent spelling of Choloepus Illiger].
Cholopus Agassiz, 1847: 83 [unjustified emendation of Choloepus Illiger].
Choelopus Sanborn, 1953: 5 [incorrect subsequent spelling of Choloepus Illiger].
References
Primary references
Illiger, J.K.W. 1811. Prodromus systematis mammalium et avium additis terminis zoographicis utriusque classis, eorumque versione germanica. C. Salfield: Berolini. xviii + 302 pp. BHL Reference page.
Rafinesque, C.S. 1815. Analyse de la nature, ou tableau de l’univers et des corps organisés. Palerme: l’Imprimerie de Jean Barravecchia, 224 pp.
Gray, J.E. 1821. On the Natural Arrangement of Vertebrose Animals. London Medical Repository 15: 296–310. HathiTrust. Reference page.
Gray, J.E. 1827. A synopsis of the species of the class Mammalia. Pp. 1–296 In Griffith, E. (ed.). The animal kingdom arranged in conformity with its organization, by the Baron Cuvier, with additional descriptions of all the species hitherto named, and of many not before noticed. Volume the Fitfh. Geo. B. Whittaker: London. 392 pp. BHL Reference page.
Palmer, T.S. 1904. Index generum mammalium: A list of the genera and families of mammals. N. Amer. Fauna 23: 1–984.
Vernacular names
English: Two-toed Sloths
日本語: フタユビナマケモノ属
Choloepus is a genus of xenarthran mammals from Central and South America within the monotypic family Choloepodidae, consisting of two-toed sloths,[3] sometimes also called two-fingered sloths.[4] The two species of Choloepus (which means "lame foot"[5]), Linnaeus's two-toed sloth (Choloepus didactylus) and Hoffmann's two-toed sloth (Choloepus hoffmanni), were formerly believed on the basis of morphological studies to be the only surviving members of the sloth family Megalonychidae,[6] but have now been shown by molecular results to be closest to extinct ground sloths of the family Mylodontidae.[4][3]
Extant species
Genus Choloepus – Illiger, 1811 – two species 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
Map of range Size:
Habitat:
Diet: LC
Hoffmann's two-toed sloth
Choloepus hoffmanni
Peters, 1858
Five subspecies
C. h. hoffmanni, Peters, 1858 – Honduras, Nicaragua, Costa Rica, Panama
C. h. agustinus, Allen, 1913 – Venezuela, western Colombia, northern Ecuador
C. h. capitalis, Allen, 1913 – western Ecuador
C. h. juruanus, Lönnberg, 1942 – Brazil, Bolivia, extreme eastern Peru
C. h. pallescens, Lönnberg, 1928 – Peru
Central America and northwestern South America
Map of range Size:
Habitat:
Diet: LC
Evolution
A study of retrovirus and mitochondrial DNA suggests that C. didactylus and C. hoffmani diverged 6 to 7 million years ago.[7] Furthermore, based on cytochrome c oxidase subunit I sequences, a similar divergence date (c. 7 million years ago) between the two populations of C. hofmanni separated by the Andes 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]
Relation to the three-toed sloth
Both types of 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. Although similar in overall appearance, the relationship between the two genera is not close. Recent phylogenetic analyses[10] support analysis of morphological data from the 1970s and 1980s, indicating the two genera are not closely related and adapted to their arboreal lifestyles independently. It was unclear from this work from which ground-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 of Caribbean 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 that convergent 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 to mylodontids and three-toed sloths within Megatherioidea, close to Megalonyx, megatheriids and nothrotheriids. They make the previously recognized family Megalonychidae polyphyletic, with both two-toed sloths and the Caribbean sloths being moved out of that family and away from Megalonyx. 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 Presslee et al., 2019).[3]
| Folivora |
|
Characteristics
Display of two "fingers" on forelimbs and three toes on hindlimbs
The name "two-toed sloth" was intended to describe an anatomical difference between the genera Choloepus and Bradypus, but does so in a potentially misleading way. Members of Choloepus have two digits on their forelimbs (the thoracic limbs) and three digits on their hindlimbs (the pelvic limbs), while members of Bradypus 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 of Choloepus 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 a tail.[14]
Behaviour
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.
Lifecycle
Young C. hoffmanni being raised in a wildlife rescue center in the Osa Peninsula, Costa Rica
Two-toed sloths have a gestation 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 are weaned 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 using riparian forest buffers to disperse. Although they also occupy shade-grown cacao plantations, they avoid open pastures.[19]
Feeding
They eat primarily leaves, but also shoots, fruits, nuts, berries, bark, some native flowers, and even some small vertebrates.[20] In addition, when they cannot find food, they have been known to eat the algae that grow on their fur for nutrients.[21] They have also been observed using mineral licks.[22][23] They have large, four-chambered stomachs, 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]
Dentition and skeleton
Two-toed sloths have a reduced, ever growing dentition, with no incisors or true canines, 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 a diastema. 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. The dental formula of two-toed sloths is: 45 (unau)
Two-toed sloths are unusual among mammals in possessing as few as five cervical vertebrae, which may be due to mutations in the homeotic genes.[27] All other mammals have seven cervical vertebrae,[28] other than the three-toed sloth and the manatee.
Musculature
Two-toed sloths generally have similar musculature to that of other mammals. This includes their zygomaticus muscles, their superficial masseter, their deep masseter, and their medial and lateral pterygoids. Additionally, a specific section of their anterior 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]
Notes
Given that sloths are regarded as quadrupeds, whether their forelimb digits should be described as fingers is debatable.
References
Gardner, A. L. (2005). "Genus Choloepus". In Wilson, D. E.; Reeder, D. M. (eds.). Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. pp. 101–102. ISBN 978-0-8018-8221-0. OCLC 62265494.
Anderson, S. (1997). "Mammals of Bolivia, Taxonomy and Distribution". Bulletin of the AMNH (231): 168.
Presslee, S.; Slater, G. J.; Pujos, F.; Forasiepi, A. M.; Fischer, R.; Molloy, K.; et al. (2019). "Palaeoproteomics resolves sloth relationships" (PDF). Nature Ecology & Evolution. 3 (7): 1121–1130. Bibcode:2019NatEE...3.1121P. doi:10.1038/s41559-019-0909-z. PMID 31171860. S2CID 174813630. Archived (PDF) from the original on 12 September 2020. Retrieved 18 September 2020.
Delsuc, F.; Kuch, M.; Gibb, G. C.; Karpinski, E.; Hackenberger, D.; Szpak, P.; Martínez, J. G.; Mead, J. I.; McDonald, H. G.; MacPhee, R.D.E.; Billet, G.; Hautier, L.; Poinar, H. N. (2019). "Ancient Mitogenomes Reveal the Evolutionary History and Biogeography of Sloths". Current Biology. 29 (12): 2031–2042.e6. Bibcode:2019CBio...29E2031D. doi:10.1016/j.cub.2019.05.043. hdl:11336/136908. PMID 31178321.
"Sloth-World.org". Archived from the original on 2 February 2009.
Myers, Phil (2001). "Family Megalonychidae: two-toed sloths". Animal Diversity Web. University of Michigan. Archived from the original on 11 March 2017. Retrieved 11 March 2017.
Slater, G. J.; Cui, P.; Forasiepi, A. M.; Lenz, D.; Tsangaras, K.; Voirin, B.; de Moraes-Barros, N.; MacPhee, R. D. E.; Greenwood, A. D. (14 February 2016). "Evolutionary Relationships among Extinct and Extant Sloths: The Evidence of Mitogenomes and Retroviruses". Genome Biology and Evolution. 8 (3): 607–621. doi:10.1093/gbe/evw023. PMC 4824031. PMID 26878870.
Moraes-Barros, N.; Arteaga, M. C. (1 June 2015). "Genetic diversity in Xenarthra and its relevance to patterns of neotropical biodiversity". Journal of Mammalogy. 96 (4): 690–702. doi:10.1093/jmammal/gyv077.
Muizon, C. de; McDonald, H. G. (May 1995). "An aquatic sloth from the Pliocene of Peru". Nature. 375 (6528): 224–227. Bibcode:1995Natur.375..224M. doi:10.1038/375224a0. ISSN 1476-4687. S2CID 4369283.
Hoss, Matthias; Dilling, Amrei; Currant, Andrew; Paabo, Svante (9 January 1996). "Molecular phylogeny of the extinct ground sloth Mylodon darwinii". Proceedings of the National Academy of Sciences. 93 (1): 181–185. Bibcode:1996PNAS...93..181H. doi:10.1073/pnas.93.1.181. PMC 40202. PMID 8552600.
White, J.L.; MacPhee, R.D.E. (2001). "The sloths of the West Indies: a systematic and phylogenetic review". In Woods, C.A.; Sergile, F.E. (eds.). Biogeography of the West Indies: Patterns and Perspectives. Boca Raton, London, New York, and Washington, D.C.: CRC Press. pp. 201–235. doi:10.1201/9781420039481-14. ISBN 978-0-8493-2001-9.
Gaudin, Timothy (2004). "Phylogenetic Relationships among Sloths (Mammalia, Xenarthra, Tardigrada): The Craniodental Evidence". Zoological Journal of the Linnean Society. 140 (2): 255–305. doi:10.1111/j.1096-3642.2003.00100.x.
Raj Pant, Sara; Goswami, Anjali; Finarelli, John A (2014). "Complex body size trends in the evolution of sloths (Xenarthra: Pilosa)". BMC Evolutionary Biology. 14 (1): 184. Bibcode:2014BMCEE..14..184R. doi:10.1186/s12862-014-0184-1. PMC 4243956. PMID 25319928.
Dickman, Christopher R. (1984). Macdonald, D. (ed.). The Encyclopedia of Mammals. New York: Facts on File. pp. 776–779. ISBN 0-87196-871-1.
Granatosky, Michael C.; Karantanis, Nikolaos E.; Rychlik, Leszek; Youlatos, Dionisios (December 2018). "A suspensory way of life: Integrating locomotion, postures, limb movements, and forces in two-toed sloths Choloepus didactylus (Megalonychidae, Folivora, Pilosa): GRANATOSKY et al". Journal of Experimental Zoology Part A: Ecological and Integrative Physiology. 329 (10): 570–588. doi:10.1002/jez.2221. PMID 30129260. S2CID 52050040.
Gilmore, D. P.; Da-Costa, C. P.; Duarte, D. P. F. (February 2000). "An update on the physiology of two- and three-toed sloths". Brazilian Journal of Medical and Biological Research. 33 (2): 129–146. doi:10.1590/S0100-879X2000000200001. ISSN 0100-879X. PMID 10657054.
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Troll, S.; Gottschalk, J.; Seeburger, J.; Ziemssen, E.; Häfner, M.; Thielebein, J.; Einspanier, A. (1 August 2013). "Characterization of the ovarian cycle in the two-toed sloths (Choloepus didactylus): An innovative, reliable, and noninvasive method using fecal hormone analyses". Theriogenology. 80 (3): 275–283. doi:10.1016/j.theriogenology.2013.04.007. ISSN 0093-691X. PMID 23743067.
Garcés-Restrepo, Mario F.; Pauli, Jonathan N.; Peery, M. Zachariah (2018). "Natal dispersal of tree sloths in a human-dominated landscape: Implications for tropical biodiversity conservation". Journal of Applied Ecology. 55 (5): 2253–2262. Bibcode:2018JApEc..55.2253G. doi:10.1111/1365-2664.13138. ISSN 1365-2664.
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Blake, John G.; Mosquera, Diego; Guerra, Jaime; Loiselle, Bette A.; Romo, David; Swing, Kelly (June 2011). "Mineral Licks as Diversity Hotspots in Lowland Forest of Eastern Ecuador". Diversity. 3 (2): 217–234. doi:10.3390/d3020217. ISSN 1424-2818.
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Naples, Virginia L. (January 1985). "Form and function of the masticatory musculature in the tree sloths, Bradypus and Choloepus". Journal of Morphology. 183 (1): 25–50. doi:10.1002/jmor.1051830104. ISSN 0362-2525. PMID 3973924. S2CID 36168956.
"Sticking Their Necks out for Evolution: Why Sloths and Manatees Have Unusually Long (or Short) Necks". May 6th 2011. Science Daily. Retrieved 25 July 2013.
Frietson Galis (1999). "Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes and Cancer" (PDF). Journal of Experimental Zoology. 285 (1): 19–26. Bibcode:1999JEZ...285...19G. doi:10.1002/(SICI)1097-010X(19990415)285:1<19::AID-JEZ3>3.0.CO;2-Z. PMID 10327647. Archived from the original (PDF) on 10 November 2004.
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