|Bradypus variegatus, a three-toed sloth|
|Choloepus hoffmanni, a two-toed sloth|
Delsuc, Catzeflis, Stanhope, and Douzery, 2001
Sloths are a group of arboreal Neotropical xenarthran mammals, constituting the suborder Folivora. Noted for their slowness of movement, they spend most of their lives hanging upside down in the trees of the tropical rainforests of South America and Central America. They are considered to be most closely related to anteaters, together making up the xenarthran order Pilosa.
There are six extant sloth species in two genera – Bradypus (three–toed sloths) and Choloepus (two–toed sloths). Despite this traditional naming, all sloths actually 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 from terrestrial ancestors. Besides the extant species, many species of ground sloths ranging up to the size of elephants (like Megatherium) inhabited both North and South America during the Pleistocene Epoch. However, they became extinct during the Quaternary extinction event around 12,000 years ago, together with most large bodied animals in the New World. The extinction correlates in time with the arrival of humans, but climate change has also been suggested to have contributed. Members of an endemic radiation of Caribbean sloths formerly lived in the Greater Antilles. They included both ground and arboreal forms which 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) also mean "lazy" or similar. Their slowness permits their low-energy diet of leaves and avoids detection by predatory hawks and cats that hunt by sight. Sloths are almost helpless on the ground, but are able to swim. The shaggy coat has grooved hair that is host to symbiotic green algae which camouflages the animal in the trees and provides it nutrients. The algae also nourishes sloth moths, some species of which exist solely on sloths.
Taxonomy and evolution
Sloths belong to the superorder Xenarthra, a group of placental mammals believed to have evolved in the continent of South America around 60 million years ago. One study found that xenarthrans broke off from other placental mammals around 100 million years ago. Anteaters and armadillos are also included among Xenarthra. The earliest xenarthrans were arboreal herbivores with sturdy vertebral columns, fused pelvises, stubby teeth, and small brains. Sloths are in the taxonomic suborder Folivora of the order Pilosa. These names are from the Latin 'leaf eater' and 'hairy', respectively. Pilosa is one of the smallest of the orders of the mammal class; its only other suborder contains the anteaters.
- †Megalocnidae: the Greater Antilles sloths, a basal group that arose about 32 million years ago and became extinct about 5,000 years ago.
- Superfamily Megatherioidea
- Bradypodidae, the three-toed sloths, contains four extant species:
- The brown-throated three-toed sloth is the most common of the extant species of sloth, which inhabits the Neotropical realm in the forests of South and Central America.
- The pale-throated three-toed sloth, 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, now found only in the Atlantic Forest of southeastern Brazil.
- The critically endangered pygmy three-toed sloth which is endemic to the small island of Isla Escudo de Veraguas off the coast of Panama.
- †Megalonychidae: ground sloths that existed for about 35 million years and went extinct about 11,000 years ago. This group was formerly thought to include both the two-toed sloths and the extinct Greater Antilles sloths.
- †Megatheriidae: ground sloths that existed for about 23 million years and went extinct about 11,000 years ago; this family included the largest sloths.
- †Nothrotheriidae: ground sloths that lived from approximately 11.6 million to 11,000 years ago. As well as ground sloths, this family included Thalassocnus, a genus of either semiaquatic or fully aquatic sloths.
- Bradypodidae, the three-toed sloths, contains four extant species:
- Superfamily Mylodontoidea
- Choloepodidae, the two-toed sloths, contains two extant species:
- Linnaeus's two-toed sloth found in Venezuela, the Guianas, Colombia, Ecuador, Peru, and Brazil north of the Amazon River.
- Hoffmann's two-toed sloth which inhabits tropical forests. It has two separate ranges, split by the Andes. One population is found from eastern Honduras in the north to western Ecuador in the south, and the other in eastern Peru, western Brazil, and northern Bolivia.
- †Mylodontidae: ground sloths that existed for about 23 million years and went extinct about 11,000 years ago.
- †Scelidotheriidae: collagen sequence data indicates this group is more distant from Mylodon than Choloepus is, so it has been elevated back to full family status.
- Choloepodidae, the two-toed sloths, contains two extant species:
The common ancestor of the two existing sloth genera dates to about 28 million years ago, with similarities between the two- and three- toed sloths an example of convergent evolution to an arboreal lifestyle, "one of the most striking examples of convergent evolution known among mammals". The ancient Xenarthra included a much greater variety of species, with a wider distribution, than those of today. Ancient sloths were mostly terrestrial, and some reached sizes that rival those of elephants, as was the case for Megatherium.
Sloths arose in South America during its 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 of pilosans to the Greater Antilles by the Oligocene, and that the megalonychid Pliometanastes and the mylodontid Thinobadistes were able to colonise North America about 9 million years ago, well before the formation of the Isthmus of Panama. The latter development, about 3 million years ago, allowed megatheriids and nothrotheriids to also invade North America as part of the Great American Interchange. Additionally, the nothrotheriid Thalassocnus of the west coast of South America became adapted to a semiaquatic and, eventually, perhaps fully aquatic marine lifestyle. In Peru and Chile, Thalassocnus entered the coastal habitat beginning in the late Miocene. Initially they just stood in the water, but over a span of 4 million years they eventually evolved into swimming creatures, becoming specialist bottom feeders of seagrasses, similar to extant marine sirenians.
Both types of extant tree sloth tend to occupy the same forests; in most areas, a particular species of the somewhat smaller and generally slower-moving three-toed sloth (Bradypus) and a single species of the two-toed type will jointly predominate. Based on morphological comparisons, it was thought the two-toed sloths nested phylogenetically within one of the divisions of the extinct Greater Antilles sloths. Though data has been collected on over 33 different species of sloths by analyzing bone structures, many of the relationships between clades on a phylogenetic tree were unclear. Much of the morphological evidence collected to support the hypothesis of diphyly has been based on the structure of the inner ear.
Recently obtained molecular data from collagen and mitochondrial DNA sequences fall in line with the diphyly (convergent evolution) hypothesis, but have overturned some of the other conclusions obtained from morphology. These investigations consistently place two-toed sloths close to 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 Greater Antilles sloths being moved away from Megalonyx. Greater Antilles sloths are now placed in a separate, basal branch of the sloth evolutionary tree.
The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee et al., 2019).
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; this caused a cooling trend in the coastal waters which killed off much of the area's seagrass (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. Ground sloth remains found in both North and South America indicate that they were killed, cooked, and eaten by humans. Climate change that came with the end of the last ice age may have also played a role (although previous similar glacial retreats were not associated with similar extinction rates).
Megalocnus and some other Caribbean sloths survived until about 5,000 years ago, long after ground sloths had died out on the mainland, but then went extinct when humans finally colonized the Greater Antilles.
Morphology and anatomy
Sloths can be 60 to 80 cm (24 to 31 in) long and, depending on the species, weigh from 3.6 to 7.7 kg (7.9 to 17.0 lb). Two-toed sloths are slightly larger. Sloths have long limbs and rounded heads with tiny ears. Three-toed sloths also have stubby tails about 5 to 6 cm (2.0 to 2.4 in) long.
Sloths are unusual among mammals in not having seven cervical vertebrae. Two-toed sloths have five to seven, while three-toed sloths have eight or nine. The other mammals not having seven are the manatees, with six.
Sloths have colour vision, but have poor visual acuity. They also have poor hearing. Thus, they rely on their sense of smell and touch to find food.
Sloths have very low metabolic rates (less than half of that expected for a mammal of their size), and low body temperatures: 30 to 34 °C (86 to 93 °F) when active, and still lower when resting. Sloths are heterothermic, meaning their body temperature may vary according to the environment, normally ranging from 25 to 35 °C (77 to 95 °F), but able to drop to as low as 20 °C (68 °F), inducing torpor.
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 from predatory jaguars, ocelots, and harpy eagles. Because of the algae, sloth fur is a small ecosystem of its own, hosting many species of commensal and parasitic arthropods. There are a large number of arthropods associated with sloths. These include biting and blood-sucking flies such as mosquitoes and sandflies, triatomine bugs, lice, ticks and mites. Sloths have a highly specific community of commensal beetles, mites and moths. The species of sloths recorded to host arthropods include the pale-throated three-toed sloth, the brown-throated three-toed sloth, and Linnaeus's two-toed sloth. Sloths benefit from their relationship with moths because the moths are responsible for fertilizing algae on the sloth, which provides them with nutrients.
Their limbs are adapted for hanging and grasping, not for supporting their weight. Muscles 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 4 metres (13 ft) per minute, but can move at a marginally higher speed of 4.5 metres (15 ft) 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 3 metres (9.8 ft) 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 13.5 metres (44 ft) 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. Three-toed sloths are mostly nocturnal, but can be active in the day. They spend 90 per cent of their time motionless.
Two-toed sloths are omnivorous, with a diverse diet of insects, carrion, fruits, leaves and small lizards, ranging over up to 140 hectares (350 acres). Three-toed sloths, on the other hand, are almost entirely herbivorous (plant eaters), with a limited diet of leaves from only a few trees, and no other mammal digests its food as slowly.
They have made adaptations to arboreal 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 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 relationship with sloths- as these moths live in the fur and promote algae growth, 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.
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 unknown. Litters are of one newborn only, after six months' gestation for three-toed, and 12 months' for two-toed. Newborns stay with their mother for about five months. In some cases, young sloths die from a fall indirectly because the mothers prove unwilling to leave the safety of the trees to retrieve the young. Females normally bear one baby every year, but sometimes sloths' low level of movement actually keeps females from finding males for longer than one year. Sloths are not particularly sexually dimorphic and several zoos have received sloths of the wrong sex.
The average lifespan of two-toed sloths in the wild is currently unknown due to a lack of full-lifespan studies in a natural environment. Median life expectancy in human care is about 16 years, with one individual at the Smithsonian Institution's National Zoo reaching an age of 49 years before her death.
Although habitat is limited to the tropical rainforests of Central and South America, in that environment sloths are successful. On Barro Colorado Island in Panama, sloths have been estimated to comprise 70% of the biomass of arboreal mammals. Four of the six living species are presently rated "least concern"; the maned three-toed sloth (Bradypus torquatus), which inhabits Brazil's dwindling Atlantic Forest, is classified as "vulnerable", while the island-dwelling pygmy three-toed sloth (B. pygmaeus) is critically endangered. Sloths' lower metabolism confines them to the tropics and they adopt thermoregulation behaviors of cold-blooded animals like sunning themselves.
The majority of recorded sloth deaths in Costa Rica are due to contact with electrical lines and poachers. 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 founder and director of the Green Heritage Fund Suriname, Monique Pool, has helped rescue and release more than 600 sloths, anteaters, armadillos, and porcupines.
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.
- Gardner, A. (2005). 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.
- Delsuc, Frédéric; Catzeflis, François M.; Stanhope, Michael J.; Douzery, Emmanuel J. P. (7 August 2001). "The evolution of armadillos, anteaters and sloths depicted by nuclear and mitochondrial phylogenies: implications for the status of the enigmatic fossil Eurotamandua". Proceedings of the Royal Society of London B: Biological Sciences. 268 (1476): 1605–1615. doi:10.1098/rspb.2001.1702. ISSN 0962-8452. PMC 1088784. PMID 11487408.
- "Overview". The Sloth Conservation Foundation. Retrieved 29 November 2017.
- The Land and Wildlife of South America. Time Inc. 1964. pp. 15, 54.
- Bennington-Castro, Joseph. "The Strange Symbiosis Between Sloths and Moths". Gizmodo. Retrieved 1 December 2017.
- O'Leary, Maureen A.; Bloch, Jonathan I.; Flynn, John J.; Gaudin, Timothy J.; Giallombardo, Andres; Giannini, Norberto P.; Goldberg, Suzann L.; Kraatz, Brian P.; Luo, Zhe-Xi (8 February 2013). "The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals". Science. 339 (6120): 662–667. doi:10.1126/science.1229237. hdl:11336/7302. ISSN 0036-8075. PMID 23393258. S2CID 206544776.
- Svartman, Marta; Stone, Gary; Stanyon, Roscoe (21 July 2006). "The Ancestral Eutherian Karyotype Is Present in Xenarthra". PLOS Genetics. 2 (7): e109. doi:10.1371/journal.pgen.0020109. ISSN 1553-7404. PMC 1513266. PMID 16848642.
- Presslee, S.; Slater, G. J.; Pujos, F.; Forasiepi, A. M.; Fischer, R.; Molloy, K.; Mackie, M.; Olsen, J. V.; Kramarz, A.; Taglioretti, M.; Scaglia, F.; Lezcano, M.; Lanata, J. L.; Southon, J.; Feranec, R.; Bloch, J.; Hajduk, A.; Martin, F. M.; Gismondi, R. S.; Reguero, M.; de Muizon, C.; Greenwood, A.; Chait, B. T.; Penkman, K.; Collins, M.; MacPhee, R.D.E. (2019). "Palaeoproteomics resolves sloth relationships" (PDF). Nature Ecology & Evolution. 3 (7): 1121–1130. doi:10.1038/s41559-019-0909-z. PMID 31171860. S2CID 174813630.
- Gardner, Alfred L. (2007). "Suborder Folivora". In Gardner, Alfred L. (ed.). Mammals of South America, Volume 1: Marsupials, Xenarthrans, Shrews, and Bats. Chicago: University of Chicago Press. pp. 157–168 (p. 161). ISBN 978-0-226-28240-4.
- Moraes-Barros, M.C.; et al. (2011). "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.
- Plese, T.; Chiarello, A. (2014). "Choloepus hoffmanni". IUCN Red List of Threatened Species. 2014: e.T4778A47439751. doi:10.2305/IUCN.UK.2014-1.RLTS.T4778A47439751.en.
- Hayssen, V. (2011). "Choloepus hoffmanni (Pilosa: Megalonychidae)". Mammalian Species. 43 (1): 37–55. doi:10.1644/873.1.
- Gaudin, T.J. (1 February 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. ISSN 0024-4082.
- Muizon, C. de; McDonald, H. G.; Salas, R.; Urbina, M. (June 2004). "The evolution of feeding adaptations of the aquatic sloth Thalassocnus". Journal of Vertebrate Paleontology. 24 (2): 398–410. doi:10.1671/2429b. JSTOR 4524727. S2CID 83859607.
- Amson, E.; Muizon, C. de; Laurin, M.; Argot, C.; Buffrénil, V. de (2014). "Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru". Proceedings of the Royal Society B: Biological Sciences. 281 (1782): 20140192. doi:10.1098/rspb.2014.0192. PMC 3973278. PMID 24621950.
- 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: 184. doi:10.1186/s12862-014-0184-1. PMC 4243956. PMID 25319928.
- 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. doi:10.1016/j.cub.2019.05.043. PMID 31178321.
- Amson, E.; Argot, C.; McDonald, H. G.; de Muizon, C. (2015). "Osteology and functional morphology of the axial postcranium of the marine sloth Thalassocnus (Mammalia, Tardigrada) with paleobiological implications". Journal of Mammalian Evolution. 22 (4): 473–518. doi:10.1007/s10914-014-9280-7. S2CID 16700349.
- Steadman, D. W.; Martin, P. S.; MacPhee, R. D. E.; Jull, A. J. T.; McDonald, H. G.; Woods, C. A.; Iturralde-Vinent, M.; Hodgins, G. W. L. (16 August 2005). "Asynchronous extinction of late Quaternary sloths on continents and islands". Proc. Natl. Acad. Sci. USA. 102 (33): 11763–11768. doi:10.1073/pnas.0502777102. PMC 1187974. PMID 16085711.
- "Sloth". National Geographic. Retrieved 1 December 2017.
- "Sticking their necks out for evolution: Why sloths and manatees have unusually long (or short) necks". ScienceDaily. Retrieved 26 April 2019.
- "Sloth". Encyclopedia Britannica. Retrieved 1 December 2017.
- Suutari, Milla; Majaneva, Markus; Fewer, David P.; Voirin, Bryson; Aiello, Annette; Friedl, Thomas; Chiarello, Adriano G.; Blomster, Jaanika (1 January 2010). "Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri(Chlorophyta, Ulvophyceae)". BMC Evolutionary Biology. 10: 86. doi:10.1186/1471-2148-10-86. ISSN 1471-2148. PMC 2858742. PMID 20353556.
- Moreno, Ricardo S.; Kays, Roland W.; Samudio, Rafael (24 August 2006). "Competitive Release in Diets of Ocelot (Leopardus pardalis) and Puma (Puma concolor) after Jaguar (Panthera onca) Decline". Journal of Mammalogy. 87 (4): 808–816. doi:10.1644/05-MAMM-A-360R2.1. ISSN 0022-2372.
- Aguiar-Silva, F. Helena; Sanaiotti, Tânia M.; Luz, Benjamim B. (1 March 2014). "Food Habits of the Harpy Eagle, a Top Predator from the Amazonian Rainforest Canopy". Journal of Raptor Research. 48 (1): 24–35. doi:10.3356/JRR-13-00017.1. ISSN 0892-1016. S2CID 86270583.
- Gilmore, D. P.; Da Costa, C. P.; Duarte, D. P. F. (1 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.
- Gilmore, D. P.; Da Costa, C. P.; Duarte, D. P. F. (2001). "Sloth biology: an update on their physiological ecology, behavior and role as vectors of arthropods and arboviruses" (PDF). Brazilian Journal of Medical and Biological Research. 34 (1): 9–25. doi:10.1590/S0100-879X2001000100002. ISSN 1678-4510. PMID 11151024.
- Ed Yong (21 January 2014). "Can Moths Explain Why Sloths Poo on the Ground?". Phenomena.
- "What Does It Mean to Be a Sloth?". natureinstitute.org. Retrieved 29 June 2017.
- Mendel, Frank C. (1 January 1985). "Use of Hands and Feet of Three-Toed Sloths (Bradypus variegatus) during Climbing and Terrestrial Locomotion". Journal of Mammalogy. 66 (2): 359–366. doi:10.2307/1381249. JSTOR 1381249.
- 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.
- Goffart, M. (1971). "Function and Form in the sloth". International Series of Monographs in Pure and Applied Biology. 34: 94–95.
- BBC (4 November 2016), Swimming sloth - Planet Earth II: Islands Preview - BBC One, retrieved 17 April 2017
- Britton, S. W. (1 January 1941). "Form and Function in the Sloth". The Quarterly Review of Biology. 16 (1): 13–34. doi:10.1086/394620. JSTOR 2808832. S2CID 85162387.
- Briggs, Helen (13 May 2008). "Article "Sloth's Lazy Image 'A Myth'"". BBC News. Retrieved 21 May 2010.
- Eisenberg, John F.; Redford, Kent H. (15 May 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.
- Alina Bradford (26 November 2018). "Sloths: The World's Slowest Mammals". Live Science.
- "Sloth". Animal Corner.
- Venema, Vibeke (4 April 2014). "The woman who got 'slothified'". BBC News. Retrieved 1 December 2017.
- Title:A syndrome of mutualism reinforces the lifestyle of a sloth Authors:Jonathan N. Pauli, Jorge E. Mendoza, Shawn A. Steffan, Cayelan C.Carey, Paul J. Weimer and M. Zachariah Peery Journal:Proceedings of the Royal Society B
- Montgomery, Sy. "Community Ecology of the Sloth". Cecropia: Supplemental Information. Encyclopædia Britannica. Retrieved 6 September 2009.
- Soares, C. A.; Carneiro, R. S. (1 May 2002). "Social behavior between mothers × young of sloths Bradypus variegatus SCHINZ, 1825 (Xenarthra: Bradypodidae)". Brazilian Journal of Biology. 62 (2): 249–252. doi:10.1590/S1519-69842002000200008. ISSN 1519-6984. PMID 12489397.
- Pauli, Jonathan N.; Peery, M. Zachariah (19 December 2012). "Unexpected Strong Polygyny in the Brown-Throated Three-Toed Sloth". PLOS ONE. 7 (12): e51389. doi:10.1371/journal.pone.0051389. ISSN 1932-6203. PMC 3526605. PMID 23284687.
- "Manly secret of non-mating sloth at London Zoo". BBC News. BBC. 19 August 2010. Retrieved 30 April 2015.
- "Same-sex sloths dash Drusillas breeding plan". BBC News. BBC. 5 December 2013. Retrieved 30 April 2015.
- "About the Sloth". Sloth Conservation Foundation. Retrieved 31 October 2019.
- "Southern two-toed sloth". Smithsonian's National Zoo. 25 April 2016. Retrieved 30 October 2019.
- Eisenberg, John F.; Redford, Kent H. (15 May 2000). Mammals of the Neotropics, Volume 3: The Central Neotropics: Ecuador, Peru, Bolivia, Brazil. University of Chicago Press. pp. 624 (see p. 96). ISBN 978-0-226-19542-1. OCLC 493329394.
- Chiarello, A. & Moraes-Barros, N. (2014). "Bradypus torquatus". IUCN Red List of Threatened Species. 2014: e.T3036A47436575. doi:10.2305/IUCN.UK.2014-1.RLTS.T3036A47436575.en.
- Dowling, Stephen (29 August 2019). "Why do sloths move so slowly?". BBC Future. BBC News. Retrieved 2 September 2019.
- "Sloths: Hottest-Selling Animal in Colombia's Illegal Pet Trade". ABC News. 29 May 2013. Retrieved 2 December 2017.
- "When sloths are in trouble, she's the one to call". CNN. Retrieved 1 December 2017.
- "The Sloth Institute website".
- Sevcenko, Melanie (17 April 2013). "Sloth sanctuary nurtures animals back to health". Deutsche Welle. Retrieved 18 April 2013.
- Schelling, Ameena (19 May 2016). "Famous Sloth Sanctuary Is A Nightmare For Animals, Ex-Workers Say". The Dodo. Retrieved 20 May 2016.
|Wikisource has the text of the 1920 Encyclopedia Americana article Sloth.|