|Brown-throated three-toed sloth (Bradypus variegatus)|
4, see text
|Green: B. variegatus, blue: B. tridactylus, red: B. torquatus|
The three-toed sloths are tree-living mammals from Latin 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. 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.
|Image||Scientific name||Common name||Distribution|
|B. pygmaeus||Pygmy three-toed sloth||Isla Escudos de Veraguas (Panama)|
|B. torquatus||Maned three-toed sloth||Atlantic coastal rainforest of southeastern Brazil|
|B. tridactylus||Pale-throated three-toed sloth||Part of northern South America, including Guyana, Suriname, French Guiana, eastern Venezuela and Brazil north of the Amazon River|
|B. variegatus||Brown-throated three-toed sloth||Central America and much of north and central South America, from Honduras through Colombia, Venezuela, Ecuador, eastern Peru, Bolivia and Brazil|
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.
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. 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.
Our understanding of sloth phylogeny has recently been greatly revised by molecular studies, based on collagen and mitochondrial DNA 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. These results provide further strong support for the long-held belief that arboreality arose separately in the two genera via convergent paths.
Famously slow-moving, a sloth travels at an average speed of 0.24 km/h (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 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. However, sloths are generally regarded as quadrupeds.
Unlike the two-toed sloth, three-toed sloths are agile swimmers. They are still slow in trees. 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 rainforest 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 genus tend to live around 25 to 30 years, reaching sexual maturation at three to five 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 around six 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.
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. 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 the young have become 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, thus are 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, which have only six.
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