Three-toed sloth

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Three-toed sloths [1]
Brown-throated three-toed sloth (Bradypus variegatus)
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Pilosa
Superfamily: Megatherioidea
Family: Bradypodidae
Gray, 1821
Genus: Bradypus
Linnaeus, 1758
Type species
Bradypus tridactylus

5, see text

Three-toed sloth range.jpg
Green: B. pygmaeus, blue: B. tridactylus, red: B. variegatus, yellow: B. torquatus, orange: B. crinitus

The three-toed or three-fingered sloths are arboreal neotropical mammals. [2] They are the only members of the genus Bradypus (meaning "slow-footed") and the family Bradypodidae. The five living species of three-toed sloths are the brown-throated sloth, the maned sloth, the pale-throated sloth, the southern maned sloth, and the pygmy three-toed sloth. In complete contrast to past morphological studies, which tended to place Bradypus as the sister group to all other folivorans, molecular studies place them nested within the sloth superfamily Megatherioidea, making them the only surviving members of that radiation. [2] [3]


Extant species

ImageScientific nameCommon nameDistribution
Bradypus pygmaeus.jpg B. pygmaeus Pygmy three-toed sloth Isla Escudo de Veraguas (Panama)
PisaMammalGallery (6).JPG B. torquatus Maned sloth Atlantic coastal rainforest of southeastern Brazil
Bradypus tridactylus.jpg B. tridactylus Pale-throated sloth Part of northern South America, including Guyana, Suriname, French Guiana, eastern Venezuela and Brazil north of the Amazon River
Bicho-preguica 3.jpg B. variegatus Brown-throated sloth Central America and much of north and central South America, from Honduras through Colombia, Venezuela, Ecuador, eastern Peru, Bolivia and Brazil
Bradypus torquatus 47880997.jpg B. crinitus Southern maned sloth Coastal Brazil in Rio de Janeiro and Espirito Santo. [4]


A study of mitochondrial cytochrome b and 16S rRNA sequences suggests that B. torquatus diverged from B. variegatus and B. tridactylus about 12 million years ago, while the latter two split 5 to 6 million years ago. The diversification of B. variegatus lineages was estimated to have started 4 to 5 million years ago. [5]

Relation to the two-toed sloth

Both types of sloth tend to occupy the same forests; in most areas, a particular species of three-toed sloth and a single species of the somewhat larger and generally faster-moving two-toed sloth will jointly predominate. Although similar in overall appearance, the two genera are placed in different families. Recent phylogenetic analyses support the morphological data from the 1970s and 1980s that the two genera are not closely related and that each adopted their arboreal lifestyles independently. [6] From morphological studies it was unclear from which ground-dwelling sloth taxa the three-toed sloths evolved or whether they retained their arboreality from the last common ancestor of extant sloths. The two-toed sloths were thought on the basis of morphology to nest phylogenetically within one of the divisions of ground-dwelling Caribbean sloths. [7]

Understanding of sloth phylogeny has recently been greatly revised by molecular studies, based on collagen [3] and mitochondrial DNA [2] sequences. These investigations consistently place three-toed sloths within Megatherioidea, close to Megalonyx , megatheriids and nothrotheriids, and two-toed sloths close to mylodontids, while moving the Caribbean sloths to a separate, basal branch of the sloth evolutionary tree. [3] [2] These results provide further strong support for the long-held belief that arboreality arose separately in the two genera via convergent paths.

The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee et al., 2019). [3]



Famously slow-moving, a sloth travels at an average speed of 0.24 km/h (0.15 mph). [8] Three-toed sloths are about the size of a small dog or a large cat, with the head and body having a combined length of around 45 cm (18 in) and a weight of 3.5–4.5 kg (8–10 lb). Unlike the two-toed sloths, they also have short tails of 6–7 cm (2–3 in), and they have three clawed toes on each limb. All sloths have three digits on their hindlimbs; the difference is found in the number of digits on the forelimbs; thus they are sometimes referred to as three-fingered sloths. [9] However, sloths are generally regarded as quadrupeds.


Like the two-toed sloth, three-toed sloths are agile swimmers. They are still slow in trees. [10] The muscles that sloths use to grip and produce a pulling motion are much more prominent than those that produce a pushing motion. [11] This means that they struggle to support their body weight when walking on all four limbs, so traveling on the ground is a dangerous and laborious process. [12]

Three-toed sloths are arboreal (tree-dwelling), with a body adapted to hang by their limbs. Large, curved claws and muscles specifically adapted for strength and stamina help sloths to keep a strong grip on tree branches. [13] The abdominal organs close to their diaphragm (such as their stomach, liver, and kidneys) are attached to their lower ribs (or pelvic girdle in the latter case) by fibrinous adhesions, which prevent the weight of these organs from compressing their lungs when hanging, making inhalation easier. [14]

They live high in the canopy but descend once a week to defecate on the forest floor. During this week-long interval, their feces and urine accumulate to about a third of their total body mass. [14] It takes about a month for a single leaf to pass through its four-chambered stomach and digestive tract. [15] Although they get most of their fluids from the leaves that they eat, they have been observed drinking directly from rivers. [16] Because of their slow metabolism, they do not need to ingest many leaves on a daily basis, but when ambient temperatures are high, the symbiotic microbes and bacteria present in their gut will break down and ferment food at a faster rate. [17] Conversely, when temperatures are lower, sloths will consume less, which is opposite to what has been observed in most other mammals. [15] Only very few species of sloths are found at higher altitudes, and these are found to have thicker coats than those living in lower altitudes. Some of the extinct species of sloths were able to tolerate cooler temperatures, but researchers believe this was probably due to thicker fur, larger size, larger muscle mass, and more access to a constant food supply. [18]

Their long, coarse fur often appears greenish, due not to pigment, but to algae growing on it. Sloths' greenish color and their sluggish habits provide an effective camouflage; hanging quietly, sloths resemble a bundle of leaves.

They move between different trees up to four times a day, although they prefer to keep to a particular type of tree, which varies between individuals, perhaps as a means of allowing multiple sloths to occupy overlapping home ranges without competing with each other. [19]

Three-toed sloths are predominantly diurnal, although they can be active at any hour of the day, while two-toed sloths are nocturnal. [20]



Three-toed sloth crossing a road in Alajuela, Costa Rica Three-toed sloth crossing road in Costa Rica.jpg
Three-toed sloth crossing a road in Alajuela, Costa Rica

Members of this genus tend to live around 25 to 30 years, reaching sexual maturity at three to five years of age. Three-toed sloths do not have a mating season but breed year-round.

Male three-toed sloths are attracted to females in estrus by their screams echoing throughout the canopy. Sloth copulation lasts an average of 25 minutes. [21] Male three-toed sloths are strongly polygamous and exclude competitors from their territory. Males are also able to compete with one another within small habitable territories. [22] Females give birth to a single young after a gestation period of around six months. The offspring cling to their mother's bellies for around nine months. They are weaned around nine months of age when the mother leaves her home territory to her offspring and moves elsewhere. Adults are solitary, and mark their territories using anal scent glands and dung middens. [19]

The home ranges used by wild brown-throated three-toed sloths in Costa Rica include cacao, pasture, riparian forests, peri-urban areas, and living fence-rows. [23] For the first few months after giving birth, mothers remain at just one or two trees and guide their young. At about five to seven months of age, when the young have become more independent, mothers expand their resources and leave their young in new areas. During natal dispersion, three-toed sloths prefer tropical forests, often using riparian forest habitat to disperse while avoiding pastures and shade-grown cacao. [24] The home range for mothers is larger than those of young. After separation, only the mothers use the cacao-growing forest, but both use riparian forests. Different types of trees are used by both mother and young, which indicates that this agricultural matrix provides an important habitat type for these animals. [25]

Dentition and skeleton

Three-toed sloths have no incisor or canine teeth, just a set of peg-shaped cheek teeth that are not clearly divided into premolars and molars, and lack homology with those teeth in other mammals, and thus are referred to as molariforms. The molariform dentition in three-toed sloths is simple and can be characterized as dental formula of: 54-5. [26]

Three-toed sloths are unusual amongst mammals in possessing as many as nine cervical vertebrae, which may be due to mutations in the homeotic genes. [27] All other mammals have seven cervical vertebrae, [28] other than the two-toed sloth and the manatee, which have only six.

Microbial ecology

Three-toed sloths use their gut microbiome to break down the lignocellulose found in the leaves of the cecropia trees that they eat into short chain organic acids, which are then absorbed into the bloodstream to provide energy to the sloth. [29] Firmicutes and Proteobacteria are the main bacterial phyla that dominate the sloth gut microbiome, which is less diverse than in many other herbivores. Not only are Firmicutes bacteria found in feces and digesta, but they are also found externally on the fur of sloths. Some research has found that Firmicutes bacteria in the genera Brevibacterium and Rothia can secrete antibiotic compounds that may provide protection from pathogenic bacteria. [30]

Related Research Articles

<span class="mw-page-title-main">Xenarthra</span> Superorder of mammals including anteaters, sloths, and armadillos

Xenarthra is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Extinct xenarthrans include the glyptodonts, pampatheres and ground sloths. Xenarthrans originated in South America during the late Paleocene about 60 million years ago. They evolved and diversified extensively in South America during the continent's long period of isolation in the early to mid Cenozoic Era. They spread to the Antilles by the early Miocene and, starting about 3 million years ago, spread to Central and North America as part of the Great American Interchange. Nearly all of the formerly abundant megafaunal xenarthrans became extinct at the end of the Pleistocene.

<span class="mw-page-title-main">Mylodontidae</span> Extinct family of ground sloths

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 family of extinct ground sloths, Scelidotheriidae, as well as to the extant arboreal two-toed sloths, family Choloepodidae; together these make up the superfamily Mylodontoidea. Phylogenetic analyses based on morphology uncovered the relationship between Mylodontidae and Scelidotheriidae; in fact, the latter was for a time considered a subfamily of mylodontids. However, molecular sequence comparisons were needed for the correct placement of Choloepodidae. These studies have been carried out using mitochondrial DNA sequences as well as with collagen amino acid sequences. The latter results indicate that Choloepodidae is closer to Mylodontidae than Scelidotheriidae is. The only other living sloth family, Bradypodidae, belongs to a different sloth radiation, Megatherioidea.

<span class="mw-page-title-main">Scelidotheriidae</span> Extinct family of prehistoric ground sloths

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 Choloepodidae. The only other extant family of the suborder Folivora is the distantly related Bradypodidae. Erected as the family Scelidotheriidae by Ameghino in 1889, the taxon was demoted to a subfamily of Mylodontidae by Gaudin in 1995. However, recent collagen sequence data indicates the group is less closely related to Mylodon and Lestodon than Choloepus is, and thus it has been elevated back to full family status by Presslee et al. (2019).

<span class="mw-page-title-main">Pale-throated sloth</span> Species of mammals related to anteaters and armadillos

The pale-throated sloth, occasionally known as the ai, is a species of three-toed sloth that inhabits tropical rainforests in northern South America.

<span class="mw-page-title-main">Maned sloth</span> Species of mammals related to anteaters and armadillos

The maned sloth 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 coastal rainforest of southeastern and northeastern Brazil, located in the states of Espírito Santo, Rio de Janeiro and Bahia. Each of the individuals within the species are genetically distinct with different genetic makeup.The maned sloth is listen under Vulnerable (VU) according to the IUCN Red List and have a decreasing population trend.

<span class="mw-page-title-main">Brown-throated sloth</span> Species of New World mammals related to anteaters and armadillos

The brown-throated sloth is a species of three-toed sloth found in the Neotropical realm of Central and South America.

<span class="mw-page-title-main">Hoffmann's two-toed sloth</span> Species of mammals related to anteaters and armadillos

The Hoffmann's two-toed sloth, 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 rainforests and deciduous forests. The common name commemorates the German naturalist Karl Hoffmann.

<span class="mw-page-title-main">Linnaeus's two-toed sloth</span> Species of mammals related to anteaters and armadillos

Linnaeus's two-toed sloth, 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, Suriname, Guyana, French Guiana, Colombia, Ecuador, Peru, and Brazil north of the Amazon River. There is now evidence suggesting the species' range expands into Bolivia. It's the largest extant sloth species.

<span class="mw-page-title-main">Pilosa</span> Order of mammals

The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes anteaters and sloths. The name comes from the Latin word for "hairy".

<span class="mw-page-title-main">Two-toed sloth</span> Genus of 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, Linnaeus's two-toed sloth and Hoffmann's two-toed sloth, were formerly believed on the basis of morphological studies to be the only surviving members of the sloth family Megalonychidae, but have now been shown by molecular results to be closest to extinct ground sloths of the family Mylodontidae.

<span class="mw-page-title-main">Sloth</span> Group of tree dwelling mammals noted for slowness

Sloths are a Neotropical group of xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their lives hanging upside down in the trees of the tropical rainforests of South America and Central America. Sloths are considered to be most closely related to anteaters, together making up the xenarthran order Pilosa.

<span class="mw-page-title-main">Pygmy three-toed sloth</span> Species of mammals related to anteaters and armadillos

The pygmy three-toed sloth, also known as the monk sloth or dwarf sloth, is a species of sloth in the family Bradypodidae. The species is endemic to Isla Escudo de Veraguas, a small island off the Caribbean coast of Panama. The species was first described by Robert P. Anderson of the University of Kansas and Charles O. Handley Jr., of the Smithsonian Institution in 2001. The pygmy three-toed sloth is significantly smaller than the other three members of its genus, but otherwise resembles the brown-throated three-toed sloth. According to Anderson and Handley Jr., the head-and-body length is between 48 and 53 centimetres, and the body mass ranges from 2.5 to 3.5 kg.

The mammalian order Pilosa, which includes the sloths and anteaters, includes various species from the Caribbean region. Many species of sloths are known from the Greater Antilles, all of which became extinct over the last millennia, but some sloths and anteaters survive on islands closer to the mainland.

<span class="mw-page-title-main">Sloth moth</span> Type of 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, Cryptoses waagei, Cryptoses rufipictus, and Bradypophila garbei.

<span class="mw-page-title-main">Arthropods associated with sloths</span>

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 hundreds of insects. Sloths have a highly specific community of commensal beetles, mites and moths.

<span class="mw-page-title-main">Rebecca Cliffe</span> British zoologist

Rebecca Cliffe is a British zoologist, award winning conservationist, and one of the leading experts on sloth biology and ecology. She is the Founder and Executive Director of The Sloth Conservation Foundation and author of the book Sloths: Life in the Slow Lane.

<span class="mw-page-title-main">The Sloth Conservation Foundation</span> Non-profit organization in Costa Rica

TheSloth Conservation Foundation (SloCo) is a non-profit organisation based in Costa Rica that is dedicated to the protection of sloths living in wild and human-modified habitats through research, education and community-based conservation. SloCo was founded in 2017 by sloth researcher Dr. Rebecca Cliffe.

<span class="mw-page-title-main">Southern maned sloth</span> Species of mammal

The southern maned sloth is a three-toed sloth species.


  1. Gardner, A. L. (2005). "Order Pilosa". 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. 100–101. ISBN   978-0-8018-8221-0. OCLC   62265494.
  2. 1 2 3 4 Delsuc, F.; Kuch, M.; Gibb, G.C.; Karpinski, E.; Hackenberger, D.; Szpak, P.; et al. (2019). "Ancient Mitogenomes reveal the evolutionary history and biogeography of sloths". Current Biology. 29 (12): 2031–2042.e6. doi: 10.1016/j.cub.2019.05.043 . hdl: 11336/136908 . PMID   31178321.
  3. 1 2 3 4 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.
  4. Miranda, Flavia (February 2023). "Taxonomic revision of maned sloths, subgenus Bradypus (Scaeopus), Pilosa, Bradypodidae, with revalidation of Bradypus crinitus Gray, 1850". Journal of Mammalogy. 104 (1): 86–103. doi:10.1093/jmammal/gyac059.
  5. de Moraes-Barros, N.; Silva, J. A. B.; Morgante, J. S. (2011-02-16). "Morphology, molecular phylogeny, and taxonomic inconsistencies in the study of Bradypus sloths (Pilosa: Bradypodidae)". Journal of Mammalogy. 92 (1): 86–100. doi: 10.1644/10-MAMM-A-086.1 .
  6. Hoss, Matthias; Dilling, Amrei; Currant, Andrew; Paabo, Svante (9 Jan 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.
  7. 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, FL / London, UK / New York, NY / Washington, DC: CRC Press. pp. 201–235. doi:10.1201/9781420039481-14. ISBN   978-0-8493-2001-9.
  8. "Speed of Animals".
  9. Judy Avey-Arroyo for Gage, L. J. (2008). Hand-Rearing Wild and Domestic Mammals, Iowa State University Press, 2002, pages 81-89. doi : 10.1002/9780470385005.ch12
  10. Three-toed Sloth, National Geographic.
  11. Olson, Rachel A.; Glenn, Zachary D.; Cliffe, Rebecca N.; Butcher, Michael T. (2018-12-01). "Architectural Properties of Sloth Forelimb Muscles (Pilosa: Bradypodidae)". Journal of Mammalian Evolution. 25 (4): 573–588. doi:10.1007/s10914-017-9411-z. ISSN   1573-7055. S2CID   22238582.
  12. Keating, Ian D. (2018-01-01), Audacious Sloth , retrieved 2020-03-30
  13. Olson, R.A.; Glenn, Z.D.; Cliffe, R.N.; Butcher, M.T. (2018-12-01). "Architectural Properties of Sloth Forelimb Muscles (Pilosa: Bradypodidae)". Journal of Mammalian Evolution. 25 (4): 573–588. doi:10.1007/s10914-017-9411-z. S2CID   22238582.
  14. 1 2 Cliffe, R.N.; Avey-Arroyo, J.A.; Arroyo, F.J.; Holton, M.D.; Wilson, R.P. (2014-04-30). "Mitigating the squash effect: sloths breathe easily upside down". Biology Letters. 10 (4): 20140172. doi:10.1098/rsbl.2014.0172. PMC   4013704 . PMID   24759371.
  15. 1 2 Cliffe, Rebecca N.; Haupt, Ryan J.; Avey-Arroyo, Judy A.; Wilson, Rory P. (2015-04-02). "Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth (Bradypus variegatus)". PeerJ. 3: e875. doi: 10.7717/peerj.875 . ISSN   2167-8359. PMC   4389270 . PMID   25861559.
  16. Cliffe, Rebecca N.; Haupt, Ryan J. (2018). "Hanging out for a drink". Frontiers in Ecology and the Environment. 16 (8): 446. doi: 10.1002/fee.1955 . ISSN   1540-9309.
  17. Cliffe, R.N.; Haupt, R.J.; Avey-Arroyo, J.A.; Wilson, R.P. (2015-04-02). "Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth (Bradypus variegatus)". PeerJ. 3: e875. doi: 10.7717/peerj.875 . PMC   4389270 . PMID   25861559.
  18. Gilmore, D. P.; Da Costa, C. P.; Duarte, D. P. F. (January 2001). "Sloth biology: an update on their physiological ecology, behavior and role as vectors of arthropods and arboviruses". Brazilian Journal of Medical and Biological Research. 34 (1): 9–25. doi: 10.1590/S0100-879X2001000100002 . ISSN   0100-879X. PMID   11151024.
  19. 1 2 Dickman, Christopher R. (1984). Macdonald, D. (ed.). The Encyclopedia of Mammals . New York: Facts on File. pp.  776–779. ISBN   0-87196-871-1.
  20. Eisenberg, John F.; Redford, Kent H. (May 15, 2000). Mammals of the Neotropics, Volume 3: The Central Neotropics: Ecuador, Peru, Bolivia, Brazil. University of Chicago Press. pp. 624 (see pp. 94–95, 97). ISBN   978-0-226-19542-1. OCLC   493329394.
  21. Gilmore, D.P.; et al. (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 . PMID   10657054.
  22. Pauli, J.N. & Peery, M.Z. (2012). "Unexpected strong polygyny in the brown-throated three-toed sloth". PLOS ONE. 7 (12): e51389. Bibcode:2012PLoSO...751389P. doi: 10.1371/journal.pone.0051389 . PMC   3526605 . PMID   23284687.
  23. Garcés-Restrepo, M.F.; Pauli, J.N.; Peery, M.Z. (2018). "Natal dispersal of tree sloths in a human-dominated landscape: Implications for tropical biodiversity conservation". Journal of Applied Ecology. 55 (5): 2253–2262. doi: 10.1111/1365-2664.13138 .
  24. 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. doi: 10.1111/1365-2664.13138 . ISSN   1365-2664.
  25. Ramirez, O.; et al. (2011). "Temporal and spatial resource use by female three-toed sloths and their young in an agricultural landscape in Costa Rica". Revista de Biología Tropical. 59 (4): 1743–1755. PMID   22208090.
  26. Ungar, P.S. (2010). Mammal Teeth: Origin, Evolution, and Diversity. Baltimore, US: Johns Hopkins University Press. pp. 145–6. ISBN   9780801899515.
  27. "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.
  28. 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. 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 2004-11-10.
  29. Dill-McFarland, Kimberly A.; Weimer, Paul J.; Pauli, Jonathan N.; Peery, M. Zachariah; Suen, Garret (2015-09-10). "Diet specialization selects for an unusual and simplified gut microbiota in two- and three-toed sloths". Environmental Microbiology. 18 (5): 1391–1402. doi:10.1111/1462-2920.13022. ISSN   1462-2912. PMID   26271635.
  30. Rojas-Gätjens, Diego; Valverde-Madrigal, Katherine S.; Rojas-Jimenez, Keilor; Pereira, Reinaldo; Avey-Arroyo, Judy; Chavarría, Max (2022-04-08). "Antibiotic-producing Micrococcales govern the microbiome that inhabits the fur of two- and three-toed sloths". doi:10.1101/2022.04.08.486316. S2CID   249096251 . Retrieved 2023-12-01.