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Sloths are a Neotropical group of constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial . Noted for their slowness of movement, tree sloths spend most of their lives hanging upside down in the trees of the tropical rainforests of South America and Central America. Sloths are considered to be most closely related to , together making up the xenarthran order Pilosa.
There are six extant sloth species in two genera – Bradypus (three-toed sloths) and Choloepus (two-toed sloths). Despite this traditional naming, all sloths have three toes on each rear limb – although two-toed sloths have only two digits on each forelimb. The two groups of sloths are from different, distantly related families, and are thought to have evolved their morphology via parallel evolution from terrestrial ancestors. Besides the extant species, many species of ground sloths ranging up to the size of elephants (like Megatherium) inhabited both North and South America during the Pleistocene Epoch. However, they became extinct during the Quaternary extinction event around 12,000 years ago, along with most large animals across the Americas. The extinction correlates in time with the arrival of humans, but climate change has also been suggested to have contributed. Members of an endemic radiation of Megalocnidae also formerly lived in the Greater Antilles but became extinct after humans settled the archipelago in the mid-Holocene, around 6,000 years ago.
Sloths are so named because of their very low metabolism and deliberate movements. Sloth, related to slow, literally means "laziness", and their common names in several other languages (e.g. , , , , Romanian: leneș, Finnish language: laiskiainen) also mean "lazy" or similar. Their slowness permits their low-energy diet of leaves and avoids detection by predatory hawks and cats that hunt by sight. Sloths are almost helpless on the ground but are able to swim. The shaggy coat has grooved hair that is host to symbiotic green algae which camouflage the animal in the trees and provide it nutrients. The algae also nourish , some species of which exist solely on sloths.
The Folivora are divided into at least eight families, only two of which have living species; the remainder are entirely extinct (Extinction):
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 Megalocnidae. 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. Much of the morphological evidence collected to support the hypothesis of diphyly has been based on the structure of the inner ear.
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 and three-toed sloths within Megatherioidea, close to Megalonyx, and . 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. Climate change that came with the end of the last ice age may have also played a role, although previous similar glacial retreats were not associated with similar extinction rates.
Megalocnus and some other Caribbean sloths survived until about 5,000 years ago, long after ground sloths had died out on the mainland, but then went extinct when humans finally colonized the Greater Antilles.
Sloths are unusual among 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 , with six.
Sloths have very low Metabolism 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.
Brown-throathed and Hoffman's two-toed sloths use their forelimbs as their principal means of propulsion and their skeletal muscle have very high proportions of oxidative Skeletal muscle (Type I) muscle fibers, with high activity of the anaerobic enzyme Creatine kinase compared to their other metabolic enzymes despite this. CK activity is low compared with other animals. Their muscle fibers proportion would be an adaptation to consume energy more slowly and their principally anaerobic muscle metabolism would be to use energy production sources faster and cheaper, such as ATP production by CK pathway. This would be a product of their lower field metabolic rate than other nonhibernating mammals as well suspensory lifestyle and this also would to explain their slow speed of movement. Forelimb muscle mass makes up only 5.1 % 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 , , and . Because of the algae, sloth fur is a small ecosystem of its own, hosting many species of Commensalism and Parasitism . There are a large number of arthropods associated with sloths. These include biting and blood-sucking flies such as and Sandfly, Triatominae, Louse, and . Sloths have a highly specific community of commensal , mites and . The species of sloths recorded to host arthropods include the pale-throated three-toed sloth, the brown-throated three-toed sloth, and Linnaeus's two-toed sloth. Sloths benefit from their relationship with moths because the moths are responsible for fertilizing algae on the sloth, which provides them with nutrients.
Sloths move only when necessary and even then, very slowly. They usually move at an average speed of per minute but can move at a marginally higher speed of per minute if they are in immediate danger from a predator. While they sometimes sit on top of branches, they usually eat, sleep, and even give birth hanging from branches. They sometimes remain hanging from branches even after death. On the ground, the maximum speed of sloths is per minute. Two-toed sloths are generally better able than three-toed sloths to disperse between clumps of trees on the ground.
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 per cent of their time motionless.
Sloths descend about once every eight days to defecate on the ground. The reason and mechanism behind this behavior have long been debated among scientists. There are at least five hypotheses:
More recently, a new hypothesis has emerged, which presents evidence against the previous ones and proposes that all current sloths are descendants from species that defecated on the ground, and there simply has not been enough selective pressure to abandon this behavior, since cases of predation during defecation are actually very rare.
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, and no other mammal digests its food as slowly.
They have made adaptations to arboreal browsing. Leaves, their main food source, provide very little energy or nutrients, and do not digest easily, so sloths have large, slow-acting, multi-chambered stomachs in which symbiotic bacteria break down the tough leaves. As much as two-thirds of a well-fed sloth's body weight consists of the contents of its stomach, and the digestion can take a month or more to complete.
Three-toed sloths go to the ground to urinate and defecate about once a week, digging a hole and covering it afterwards. They go to the same spot each time and are vulnerable to predation while doing so. Considering the large energy expenditure and dangers involved in the journey to the ground, this behaviour has been described as a mystery. 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 symbiosis relationship with sloths, as they live in the fur and promote growth of algae, which the sloths eat. Individual sloths tend to spend the bulk of their time feeding on a single "modal" tree; by burying their excreta near the trunk of that tree, they may also help nourish it.
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.
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.
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.
Distribution
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% of the biomass of arboreal mammals. Four of the six living species are currently rated "least concern"; the Maned sloth ( Bradypus torquatus), which inhabits Brazil's dwindling Atlantic Forest, is classified as "vulnerable", 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 Aquarium]]The majority of recorded sloth deaths in Costa Rica are due to contact with electrical lines and poaching. 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.
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