1) the Brown-throated Three-toed Sloth (B. variegatus), which is found over much of Central and South America
2) the Pale-throated Three-toed Sloth (B. tridactylus), which is found in northeastern South America
3) the Maned Three-toed Sloth (B. torquatus), which is limited to the remaining fragments of the Atlantic coastal forest in Brazil
4) the Pygmy Three-toed Sloth (B. pygmaeus), described only in 2001, which is found exclusively in Red Mangroves on the several square kilometer Isla Escudo de Veraguas of Bocas del Toro, around 17 km off the Caribbean coast of Panama (Anderson and Handley 2001, 2002; Hayssen 2008)
With the exception of the Pygmy Three-toed Sloth, all three-toed sloths are restricted to New World tropical rainforests. de Moraes-Barros et al. (2011) address the confusion between the Brown-throated and Pale-throated Three-toed Sloths, especially in north-central Brazil (mainly actual Brown-throated specimens misidentified as Pale-throateds).
The two species of two-toed sloths (Hoffmann's Two-toed Sloth, Choloepus hoffmanni, and Southern Two-toed Sloth, C. didactylus) are now placed in the family Megalonychidae (with the extinct giant ground sloths) rather than with the three-toed sloths in the Bradypodidae as they used to be.
Sloth feet have no free toes, but do have two or three long, curved claws that form a hook by which the animals can hang passively from a branch or clasp objects against the palm. Sloths have long limbs, short bodies, and stumpy tails. Their heads can rotate over 90 degrees. Sloths feed mainly on forest canopy leaves, which they digest by bacterial fermentation in a many-chambered stomach. Consistent with their reputation, sloths move slowly—and not much. The single young spends 6 to 9 months clinging to its mother, usually to her chest. Although sloths are silent and inconspicuous, they are preyed on extensively by eagles and, in some places, Jaguars.
The long, coarse hairs of sloths have either deep grooves running the length of each hair (two-toed sloths, Choloepus) or irregular transverse cracks that increase in number and size with age (three-toed sloths, Bradypus). A wide variety of organisms have been reported to occur both in the grooves and cracks of sloth hairs (including cyanobacteria and diatoms) and among their hairs (including moths, beetles, cockroaches, and nematode roundworms). However, the greenish color of the hair, which is most evident in three-toed sloths, is due to green algae, which have generally been identified as Trichophilus welckeri. It is often assumed that the association between three-toed sloths and the algae embedded in their hairs is a mutualistic one, with the algae obtaining shelter in the cracks of the hair while providing green camouflage for the sloth (it has also has been proposed, however, that the relationship may be a commensal one, with the alga offering no benefit to the sloth but simply taking advantage of an available habitat).
Analyses by Suutari et al. (2010) provided support for the hypothesis that there is a specific symbiosis of some sort between sloths and Trichophilus green algae. Suutari et al. identified three different patterns of algae occurrence in the hair of five sloth species: 1) The green alga in the fur of Brown-throated Three-toed Sloths and Pygmy Three-toed Sloths was a unique species and no other green algae were found in their fur. Microscopic features of the algae on the hair were consistent with the genus Trichophilus. 2) Maned Three-toed Sloths were found to host a variety of algae belonging to genera known to be terrestrial,such as Trentepholia and Myrmecia. 3) Hoffmann's Two-toed Sloths and Pale-throated Three-toed Sloths showed both patterns, hosting terrestrial green algae from their surroundings as well as Trichophilus. Molecular phylogenetic analyses of the Trichophilus green algae revealed that those found on Brown-throated Three-toed Sloths, Pygmy Three-toed Sloths and Pale-throated Three-toed Sloths belong to a separate lineage from those occurring on Hoffmann's Two-toed Sloths. Given that Brown-throated Three-toed Sloths and Hoffmann's Two-toed Sloths co-occur, this finding suggests evolved specialized associations between Trichophilus species and different sloth species. The three or more Trichophilus green alga species that have been found only in sloth hair are presumably passed directly from mother to offspring.
In addition to primary producers (algae), sloth fur also harbors heterotrophic organisms (ciliates, apicomplexans, and dinoflagellates) as well as decomposers (fungi), suggesting that sloth fur supports a versatile microscopic ecosystem. Several pyralid moths are known to be closely associated with sloths (Waage and Montgomery 1976; Bradley 1982).
(Emmons 1990; Suutari et al. 2010 and references therein)
- Anderson, R.P. and C.O. Handley, JR. 2001. A new species of three-toed sloth (Mammalia: Xenarthra) from Panama , with a review of the genus Bradypus. Proceedings of the Biological Society of Washington 114:1–33.
- Anderson, R.P. and C.O, Handley. 2002. Dwarfism in insular sloths: Biogeography, selection, and evolutionary rate. Evolution 56(5): 1045-1058.
- Bradley, J.D. 1982. Two new species of moths (Lepidoptera, Pyralidae, Chrysauginae) associated with the Three-toed Sloth (Bradypus spp.) in South America. Acta Amazonica 12(3): 649-656.
- de Moraes-Barros, N., J.A.B. Silva, and J.S. Morgante. 2011. Morphology, molecular phylogeny, and taxonomic inconsistencies in the study of Bradypus sloths (Pilosa: Bradypodidae). Journal of Mammalogy 92(1): 86-100.
- Emmons, L.H. 1990. Neotropical Rainforest Mammals: A Field Guide. University of Chicago Press, Chicago.
- Hayssen, V. 2008. Bradypus pygmaeus (Pilosa: Bradypodidae). Mammalian Species No. 812: 1-4.
- Suutari, M., M. Majaneva, D.P. Fewer, et al. 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
- Waage, J.K. and G.G. Montgomery. 1976. Cryptoses choloepi: A coprophagous moth that lives on a sloth. Science 193(4248): 157-158.
Molecular Biology and Genetics
Statistics of barcoding coverage
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The three-toed sloths are tree-living mammals from South and Central America. 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, the pale-throated sloth, and the pygmy three-toed sloth.
Relation to the two-toed sloth
Although similar to the somewhat larger and generally faster-moving two-toed sloths, 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. It is unclear from which ground-dwelling sloth taxa the three-toed sloths evolved; the two-toed sloths appear to nest phylogenetically within one of the divisions of Caribbean megalonychids, and thus probably either descended from them or are part of a clade that invaded the Caribbean multiple times. Both types of sloths tend to occupy the same forests; in most areas, a particular single species of three-toed sloths and a single species of the larger two-toed type will jointly predominate. Famously slow-moving, the sloth travels at an average speed of 0.24 kilometres per hour (0.15 mph).
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. They are frequently referred to as three-toed sloths, but all sloths have three toes; the difference is found in the number of fingers, meaning they are now more appropriately referred to as three-fingered sloths. This idea was first implemented by Judy Avey-Arroyo, cofounder of the world's only sloth sanctuary, but is now recognized in numerous publications as the correct terminology for these animals.
Although they are quite slow in trees, three-toed sloths are agile swimmers. The offspring cling to their mother's bellies for around nine months. They cannot walk on all four limbs, so they must use their front arms and claws to drag themselves across the rain forest floor.
Three-toed sloths are arboreal (tree-dwelling), with a body adapted to hang by their limbs. They live high in the canopy, but descend once a week to defecate on the forest floor. Their long, coarse fur often appears greenish, not due 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. Large curved claws help sloths to keep a strong grip on tree branches.
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.
Members of this species[which?] tend to live around 25 to 30 years, reaching sexual maturation at two years of age.
Three-toed sloths do not have a mating season and breed year round. Females give birth to a single young after a gestation period of around six months. They are weaned at about 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.
Male three-toed sloths are attracted to females in estrus by their screams echoing throughout the canopy. Sloth copulation lasts an average of twenty-five minutes. 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.
The home ranges used by wild brown-throated three-toed sloths in Costa Rica include cacao, pasture, riparian forests and living fence-rows. 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 young became more independent, mothers expand their resources and leave their young in new areas. The home range for mothers are larger than those of young. After separation, only the mothers use the cacao agro forest, but both use riparian forest. 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.
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, are thus referred to as molariforms. The molariform dentition in three-toed sloths is simple and can be characterized as dental formula of: 5
Three-toed sloths are unusual amongst the mammals in possessing as many as nine cervical vertebrae, which may be due to mutations in the homeotic genes. All other mammals have seven cervical vertebrae, other than the two-toed sloth and the manatee.
- B. pygmaeus – pygmy three-toed sloth
- B. torquatus – maned three-toed sloth
- B. tridactylus – pale-throated three-toed sloth
- B. variegatus – brown-throated three-toed sloth
- Gardner, A. L. (2005). "Order Pilosa". In Wilson, D. E.; Reeder, D. M. Mammal Species of the World (3rd ed.). Johns Hopkins University Press. pp. 100–101. ISBN 978-0-8018-8221-0. OCLC 62265494.
- 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. doi:10.1006/mpev.2000.0860. PMID 11161746. Retrieved 2009-12-28.
- 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. Biogeography of the West Indies: Patterns and Perspectives. CRC Press. pp. 201–235. ISBN 978-0-8493-2001-9
- 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
- National Geographic
- Dickman, Christopher R. (1984). Macdonald, D., ed. The Encyclopedia of Mammals. New York: Facts on File. pp. 776–779. ISBN 0-87196-871-1.
- 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.
- Pauli, J.N. & Peery, M.Z. (2012). "Unexpected strong polygyny in the brown-throated three-toed sloth". PLoS One 7 (12): e51389. doi:10.1371/journal.pone.0051389.
- 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 Biologia Tropical 59 (4): 1743–1755.
- "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". 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.