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The family Bradypodidae contains just a single genus, Bradypus, which includes four species:
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)