Sloth

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. The Folivora are divided into at least eight families, only two of which have living species; the remainder are entirely extinct (†): in the forests of South and Central America. • The pale-throated three-toed sloth (Bradypus tridactylus), 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 six million years ago. • The maned three-toed sloth (Bradypus torquatus), now found only in the Atlantic Forest of southeastern Brazil. • The critically endangered pygmy three-toed sloth (Bradypus pygmaeus) 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. • Superfamily Mylodontoidea • Choloepodidae, the two-toed sloths, contains two extant species: • Linnaeus's two-toed sloth (Choloepus didactylus) found in Venezuela, the Guianas, Colombia, Ecuador, Peru, and Brazil north of the Amazon River. • Hoffmann's two-toed sloth (Choloepus hoffmanni) 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. The ancient Xenarthra included a significantly 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. '' (Megalonychidae) fossil (AMNH) and restoration '' (Mylodontidae, San Diego) Sloths arose in South America during a 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. They presumably waded and paddled in the water for short period, but over a span of 4 million years, they eventually evolved into swimming creatures, becoming specialist bottom feeders of seagrasses, similar to the extant 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. 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. 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. ==Biology==

(Bradypus variegatus), Cahuita National Park, Costa Rica Morphology and anatomy compared to a human, including Megatherium americanum (A, top left) Eremotherium laurillardi (B, top right), Lestodon armatus (C, middle left) Mylodon darwinii (D, middle right) Glossotherium robustum (E, bottom left) and Catonyx cf. C. cuvieri (F, bottom right) Sloths can be long and, depending on the species, weigh from . Two-toed sloths are slightly larger than three-toed sloths. Sloths have long limbs and rounded heads with tiny ears. Three-toed sloths also have stubby tails about 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. Physiology 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: when active, and still lower when resting. Sloths are heterothermic, meaning their body temperature may vary according to the environment, normally ranging from , but able to drop to as low as , inducing torpor. Forelimb muscle mass makes up only 5.1 percent of total body weight in the brown-throated sloth. 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. The species of sloths recorded to host arthropods include Activity Sloth limbs are adapted for hanging and grasping, not for supporting their weight. Muscle mass makes 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 are surprisingly strong swimmers and can reach speeds of 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 percent of their time motionless. though female sloths do sometimes congregate, more so than do males. Sloths descend about once every eight days to defecate on the ground. The reason and mechanism behind this behaviour have long been debated among scientists. There are at least five hypotheses: • To fertilize trees when feces are deposited at the base of the tree, • To cover feces and avoid predation, • To serve as chemical communication between individuals, • To pick up trace nutrients in their claws, that are then ingested, and • To favor a mutualistic relationship with populations of fur moths. Diet (Choloepus hoffmanni) feeding in Manuel Antonio National Park in Costa RicaBaby sloths learn what to eat by licking the lips of their mother. All sloths eat the leaves of Cecropia. Two-toed sloths are omnivorous, with a diverse diet of insects, carrion, fruits, leaves, and small lizards, ranging over up to . Three-toed sloths, on the other hand, are almost entirely herbivorous (plant eaters), with a limited diet of leaves from only a few trees, 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 they live in the fur and promote growth of algae, which the sloths eat. Reproduction 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 currently 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 and one individual at the Smithsonian Institution's National Zoo reached an age of 49 years before her death. ==Distribution==

(Bradypus pygmaeus) (Thomas Landseer, 1825) 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 constitute 70 percent of the biomass of arboreal mammals. 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 such as sunning themselves. == Human relations ==

in the Dallas World AquariumThe 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 generally make very poor pets, as they have such a specialized ecology. ==References==