|Brown-throated three-toed sloth (Bradypus variegatus)|
|Green: B. variegatus, Blue: B. tridactylus, Red: B. torquatus|
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 9 months or so. They cannot walk on all four limbs, so they must use their front arms and claws to drag themselves across the rain forest floor. The three-toed sloth, unlike most other mammals, does not fully maintain a constant body temperature, and this limits it to warm environments.
The three-toed sloth is arboreal (tree-dwelling), with a body adapted to hang by its limbs. It lives high in the canopy, but descends once a week to defecate on the forest floor. Its long, coarse, grayish-brown fur often appears greenish, not due to pigment, but to algae growing on it. The sloth’s greenish color and its sluggish habits provide an effective camouflage: hanging quietly, the sloth resembles a bundle of leaves. Large curved claws help the sloth 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.
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 9 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.
Members of this species tend to live around 25 to 30 years, reaching sexual maturation at two years of age. They 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 9 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.
Species of Bradypus
- 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
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- "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.
- Smithsonian National Zoological Park: At the Zoo - Slow and Steady Sloths
- "Three-toed sloth." Passport to Knowledge. 21 Feb. 2009
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