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Sloths[1]
Temporal range: Early Oligocene to Holocene
Bradypus variegatus, a three-toed sloth
Choloepus hoffmanni, a two-toed sloth
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Superorder: Xenarthra
Order: Pilosa
Suborder: Folivora
Delsuc, Catzeflis, Stanhope, and Douzery, 2001[2]
Families
Red: two-toed sloth, blue: three-toed sloth, purple: both two-toed sloth and three-toed sloth
Synonyms
  • Tardigrada Latham & Davies, 1795
  • Phyllophaga Owen, 1842
Sloth
Three-toed sloth crossing a road in Costa Rica

Sloths are a Neotropical group of xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. 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 anteaters, 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.[3] 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 Caribbean sloths 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. German: Faultier, French: paresseux, Spanish: perezoso, Portuguese: preguiça, Romanian: leneș, Finnish: 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.[3] Sloths are almost helpless on the ground but are able to swim.[4] 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 sloth moths, some species of which exist solely on sloths.[5]

Taxonomy and evolution

[edit]
See also: List of pilosans

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.[6] One study found that xenarthrans broke off from other placental mammals around 100 million years ago.[7] 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[2] 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 (†):[8]

Megatherium americanum (Megatheriidae, London)

Evolution

[edit]
Nothrotheriops shastensis (Nothrotheriidae, La Brea)

The common ancestor of the two existing sloth genera dates to about 28 million years ago,[8] with similarities between the two- and three-toed sloths an example of convergent evolution to an arboreal lifestyle, "one of the most striking examples of convergent evolution known among mammals".[13] 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.[4]

Megalonyx wheatleyi (Megalonychidae) fossil (AMNH) and restoration
Paramylodon harlani (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.[14] 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.[15]

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.[16] 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.[13] Much of the morphological evidence collected to support the hypothesis of diphyly has been based on the structure of the inner ear.[17]

Recently obtained molecular data from collagen[8] and mitochondrial DNA sequences[18] 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.[8][18]

Phylogeny

[edit]

The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data.[8]

Folivora

Megalocnidae (Greater Antilles sloths)

Mylodontoidea
Megatherioidea

Extinctions

[edit]

The marine sloths of South America's Pacific coast became extinct at the end of the Pliocene following the closing of the Central American Seaway; the closing caused a cooling trend in the coastal waters which killed off much of the area's seagrass (and which would have also made thermoregulation difficult for the sloths, with their slow metabolism).[19]

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.[4] 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.[20]

Biology

[edit]
Feeding brown-throated three-toed sloth (Bradypus variegatus), Cahuita National Park, Costa Rica

Morphology and anatomy

[edit]
Size comparison of various ground sloths 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 60 to 80 cm (24 to 31 in) long and, depending on the species, weigh from 3.6 to 7.7 kg (7.9 to 17.0 lb). Two-toed sloths are slightly larger than three-toed sloths.[21] Sloths have long limbs and rounded heads with tiny ears. Three-toed sloths also have stubby tails about 5 to 6 cm (2.0 to 2.4 in) 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.[22]

Physiology

[edit]

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.[23]

Sloths have very low metabolic rates (less than half of that expected for a mammal of their size), and low body temperatures: 30 to 34 °C (86 to 93 °F) when active, and still lower when resting. Sloths are heterothermic, meaning their body temperature may vary according to the environment, normally ranging from 25 to 35 °C (77 to 95 °F), but able to drop to as low as 20 °C (68 °F), inducing torpor.[23]

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 slow twitch (Type I) muscle fibers, with high activity of the anaerobic enzyme CK 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 non-hibernating mammals as well suspensory lifestyle and this also would to explain their slow speed of movement.[24] Forelimb muscle mass makes up only 5.1 percent of total body weight in the brown-throated sloth.[25]

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[26] from predatory jaguars, ocelots,[27] and harpy eagles.[28] Because of the algae, sloth fur is a small ecosystem of its own, hosting many species of commensal and parasitic arthropods.[29] There are a large number of arthropods associated with sloths. These include biting and blood-sucking flies such as mosquitoes and sandflies, triatomine bugs, lice, ticks, and mites. Sloths have a highly specific community of commensal beetles, mites, and moths.[29] The species of sloths recorded to host arthropods include[29] 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.[30]

Activity

[edit]

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.[31] Their specialised hands and feet have long, curved claws to allow them to hang upside down from branches without effort,[32] 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.[23]

Sloths move only when necessary and even then, very slowly. They usually move at an average speed of 4 m (13 ft) per minute but can move at a marginally higher speed of 4.5 m (15 ft) 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 3 m (9.8 ft) per minute. Two-toed sloths are generally better able than three-toed sloths to disperse between clumps of trees on the ground.[33]

Sloths are surprisingly strong swimmers and can reach speeds of 13.5 m (44 ft) per minute.[34] They use their long arms to paddle through the water and can cross rivers and swim between islands.[35] 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.[36]

Wild brown-throated three-toed sloths sleep on average 9.6 hours a day.[37] Two-toed sloths are nocturnal.[38]: 94–95, 97  Three-toed sloths are mostly nocturnal but can be active in the day. They spend 90 percent of their time motionless.[23]

Behavior

[edit]

Sloths are solitary animals that rarely interact with one another except during breeding season,[39] though female sloths do sometimes congregate, more so than do males.[40]

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:

  1. To fertilize trees when feces are deposited at the base of the tree,[41]
  2. To cover feces and avoid predation,[42][43][44]
  3. To serve as chemical communication between individuals,[45]
  4. To pick up trace nutrients in their claws, that are then ingested,[46] and
  5. To favor a mutualistic relationship with populations of fur moths.[44][46]

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.[47]

Diet

[edit]
Hoffman's two-toed sloth (Choloepus hoffmanni) feeding in Manuel Antonio National Park in Costa Rica

Baby sloths learn what to eat by licking the lips of their mother.[48] 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 140 hectares (350 acres). Three-toed sloths, on the other hand, are almost entirely herbivorous (plant eaters), with a limited diet of leaves from only a few trees,[39] 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.[39] As much as two-thirds of a well-fed sloth's body weight consists of the contents of its stomach, and the digestive process 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.[49][50][51] 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.[5] 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.[52]

Reproduction

[edit]

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.[53] 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.[54] Sloths are not particularly sexually dimorphic and several zoos have received sloths of the wrong sex.[55][56]

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.[57] 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.[58]

Distribution

[edit]
Depiction of a pygmy three-toed sloth (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.[38]: 96  Four of the six living species are currently rated "least concern"; the maned three-toed sloth (Bradypus torquatus), which inhabits Brazil's dwindling Atlantic Forest, is classified as "vulnerable",[59] 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.[60]

Human relations

[edit]
Three-toed sloth in the Dallas World Aquarium

The 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.[61]

References

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Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sloth

View on Grokipedia
from Grokipedia
Sloths are arboreal in the suborder Folivora within the superorder , native to the tropical rainforests of Central and , and are renowned for their extremely slow movements as an energy-conserving adaptation to a low-calorie folivorous diet. There are six extant species divided into two families: the two-toed sloths of the genus Choloepus (Hoffmann's and Linnaeus's two-toed sloths) and the three-toed sloths of the genus Bradypus (brown-throated, maned, pale-throated, and pygmy three-toed sloths). These mammals typically measure 2 to 2.5 feet (0.6 to 0.8 meters) in length and weigh between 8 and 17 pounds (3.6 to 7.7 kilograms), with shaggy fur that often hosts symbiotic , providing against predators in the forest canopy. Their limbs end in long, curved claws—two or three on the front feet, depending on the —that enable them to hang upside down from branches for most of their lives, including while eating, sleeping, mating, and giving birth. Sloths possess specialized , such as continuously growing teeth without enamel and, in three-toed , extra neck vertebrae allowing nearly 360-degree head rotation. Sloths exhibit a metabolic rate of only 40-45% of that expected for their body size, enabling survival on a diet primarily consisting of leaves, supplemented occasionally by fruits, tender shoots, insects, or small vertebrates, which they digest over up to 28 days in their multi-chambered stomachs. They move at a maximum speed of about 0.24 kilometers per hour (0.15 miles per hour) on land but are adept swimmers, capable of faster propulsion in water using an overhand stroke. Largely solitary and cathemeral (active sporadically day and night), they sleep approximately 8-10 hours daily and descend to the forest floor just once a week to defecate, a behavior that exposes them to risks from predators like jaguars, ocelots, and harpy eagles. Reproduction in sloths is slow-paced, with gestation periods ranging from 6 to 11.5 months depending on the species, resulting in a single offspring every 1-2 years; mothers carry and nurse their young for 5-12 months while teaching essential arboreal skills. Evolutionarily, sloths trace back to ancient South American lineages, with extinct relatives including massive ground sloths that weighed up to several tons and roamed as far north as North America until the late Pleistocene. Today, all sloth species face threats from habitat destruction due to deforestation, electrocution on power lines, and the illegal pet trade, with the pygmy three-toed sloth classified as critically endangered, the maned three-toed sloth as vulnerable, and the other four species as least concern by the IUCN (as of 2025). Conservation efforts include habitat protection, rescue and rehabilitation centers, and innovations like wildlife rope bridges to mitigate road crossings.

Taxonomy and evolution

Evolutionary history

The evolutionary history of sloths (Folivora) traces back to the late Eocene, with the earliest known fossils attributed to Pseudoglyptodon from southern South America, dating to approximately 37 million years ago (mya). These primitive forms were terrestrial and represented the initial diversification within Xenarthra, diverging from anteaters (Vermilingua) around 57.5 mya during the Paleocene-Eocene transition. The fossil record remains sparse through the Eocene-Oligocene boundary, but early Oligocene finds, such as a small femur from Puerto Rico around 30 mya, indicate initial dispersals into the Greater Antilles, marking the onset of broader biogeographic expansion. During the (34–23 mya), sloths underwent key adaptive shifts driven by the Eocene-Oligocene climatic transition, which involved and the fragmentation of tropical forests in into more isolated humid refugia. This environmental pressure favored the emergence of an arboreal lifestyle among early sloth lineages, allowing exploitation of stable, leafy canopies in persisting tropical forests. Concurrently, the development of a slow evolved as an energy-conserving strategy linked to folivory, enabling efficient of low-nutrient foliage through prolonged gut retention and reduced activity. These adaptations distinguished sloths from faster, more terrestrial xenarthrans and set the stage for their specialization as suspensory folivores. A 2025 analysis indicates that sloth body mass evolved in a multi-peak adaptive landscape, with terrestrial lineages repeatedly increasing in size and arboreal forms converging on smaller sizes, driven mainly by preferences. The epoch (23–5.3 mya) marked a period of significant diversification, with multiple sloth families radiating across and into Central and via the emerging Panamanian . Within this timeframe, the major extant clades diverged: (ancestors of two-toed sloths) split from the lineage leading to Bradypodidae (three-toed sloths) around 20–25 mya, reflecting independent convergences toward arboreality from larger, ground-dwelling forebears. This proliferation included both arboreal and terrestrial forms, with the latter achieving in open habitats, underscoring sloths' versatility amid Miocene climatic fluctuations and forest expansions.

Phylogenetic relationships

Sloths belong to the superorder , which also encompasses anteaters (Vermilingua) and armadillos (). Within , sloths form the clade Folivora and are most closely related to anteaters in the order , with molecular evidence supporting a divergence between and approximately 70 million years ago (Ma). The split between anteaters and sloths occurred around 60 Ma, based on analyses of mitochondrial and nuclear genes such as 12S and 16S rRNA, ND1, VWF, ADRA2B, and BRCA1. Extant sloths are divided into two families: Bradypodidae, comprising three-toed sloths of the Bradypus with four species (B. variegatus, B. tridactylus, B. torquatus, and B. pygmaeus), and , comprising two-toed sloths of the Choloepus with two species (C. hoffmanni and C. didactylus). Genetic studies using complete mitochondrial genomes and nuclear loci confirm that Bradypodidae and diverged around 20 Ma during the , with high congruence between mitochondrial and nuclear datasets supporting this topology. Species boundaries among sloths are reinforced by distinct karyotypes, particularly in Bradypus, where diploid chromosome numbers vary from 50 to 54 across (e.g., 2n=50 in B. torquatus and 2n=54 in B. variegatus), reflecting chromosomal rearrangements that limit interbreeding. Hybridization between sloth or genera is rare and undocumented in the wild, attributed to these cytogenetic differences and ecological separations, with no confirmed hybrids reported for Bradypus. In Choloepus, chromosome numbers range from 50 to 65, further distinguishing the families and supporting genetic isolation.

Extinct species

Among the most notable extinct sloths are the giant ground sloths of the Pleistocene epoch, including Megatherium americanum, which reached lengths of up to 6 meters and weighed approximately 4 tons, making it one of the largest terrestrial mammals in South American history. These massive herbivores, characterized by powerful limbs, enormous claws for foraging and defense, and a robust build adapted for quadrupedal locomotion, roamed open woodlands and grasslands across South America until their extinction around 10,000 years ago at the close of the Pleistocene. Another prominent ground sloth, Mylodon darwinii, was smaller but still formidable, measuring nearly 3 meters in length and weighing between 1,000 and 2,000 kilograms, with thick, shaggy fur and strong hind limbs suited for digging and pulling vegetation. Earlier in the epoch, arboreal forms like Hapalops represented transitional sloths that bridged primitive and more specialized lineages, exhibiting semi-arboreal adaptations such as elongated forelimbs for climbing and a body mass estimated at 20-40 kilograms. Hapalops retained primitive dental features, including short anterior caniniform teeth and an elevated , which facilitated a mediolateral grinding motion for processing tough foliage while suspended in trees. The extinction of these Pleistocene ground sloths, including and , occurred rapidly around 10,000-11,000 years ago, coinciding with the arrival of humans in the and marked climatic shifts at the end of the last . Evidence from archaeological sites, such as human footprints interacting with sloth tracks in dated to approximately 11,000-13,000 years ago, suggests direct human predation, potentially by hunters using projectile points to target these slow-moving . Concurrently, abrupt warming and from retreating glaciers contributed to vegetation changes that reduced food availability, exacerbating the pressures from human overhunting. Recent paleontological discoveries have illuminated the lives of these extinct sloths, including well-preserved mummies of the Shasta ( shastensis) from Rampart Cave in the Grand Canyon, dating to about 11,000 years ago, which include desiccated skin, fur, and coprolites revealing a diet of desert shrubs like and . Additionally, in the Colombian Amazon's Serranía de la Lindosa, radiocarbon-dated to over 12,500 years old, depicts what appear to be giant with characteristic massive claws and short rostrums, providing potential cultural evidence of human encounters with these animals before their disappearance.

Physical characteristics

External morphology

Sloths exhibit a compact body adapted for an arboreal lifestyle, with most measuring 50-70 cm in head-body length and weighing between 3 and 9 kg, though three-toed sloths (genus Bradypus) are generally smaller at 2.25-5.5 kg compared to the larger two-toed sloths (genus Choloepus) at 4-9 kg. The distinction between two-toed and three-toed sloths primarily refers to the number of digits on their forelimbs: two-toed sloths possess two fingers with long, curved claws on the front limbs (and three on the hind limbs), while three-toed sloths have three fingers on all limbs, each tipped with prominent, hook-like claws measuring up to 10 cm in length that facilitate suspension from branches. The fur of sloths is notably coarse and shaggy, consisting of long outer guard hairs that grow in the opposite direction to most mammals—from belly to back—to aid in water runoff and camouflage when hanging upside down. These hairs often feature longitudinal grooves or cracks along their shafts, which harbor symbiotic green algae and cyanobacteria, imparting a greenish tint that blends with the forest canopy for camouflage. Coloration varies from grayish-brown to tan or buff, with the algae enhancing the mottled pattern, though the undercoat is finer and shorter for insulation. Sloths have rounded heads with flattened faces, featuring a short, naked , rudimentary external ears that are small and inconspicuous, and large, forward-facing eyes adapted for low-light conditions in the canopy. Their mouths are wide and lipless, giving a perpetual "" due to the shape of the and teeth arrangement, though the overall facial expression remains stoic. The neck is highly flexible, supported by an unusual number of —ranging from 6 to 7 in two-toed sloths and 8 to 9 in three-toed sloths, compared to the standard 7 in most mammals—which allows for extensive head rotation up to 270 degrees in three-toed species. A vestigial tail, if present, is short and non-prehensile, measuring less than 2.5 cm and often indistinguishable externally, reflecting the sloths' specialized arboreal adaptations where a functional tail is unnecessary.

Internal anatomy

The internal anatomy of sloths is highly specialized for their arboreal, low-energy , featuring adaptations in the skeletal, , digestive, and sensory systems that prioritize over speed or power. These structures support prolonged suspension from branches and slow processing of fibrous vegetation, distinguishing sloths from more active mammals. Sloths possess teeth lacking enamel, which continuously grow throughout their lives to compensate for constant wear from their abrasive, leaf-based diet. Three-toed sloths ( Bradypus) have eight or nine , an unusually high number that enables head rotation of up to 270 degrees, facilitating of surroundings without significant body movement; in contrast, two-toed sloths ( Choloepus) typically have six . The in sloths is notably reduced, comprising only about 25% of body weight—compared to 40–50% in most other mammals of similar size—reflecting their low metabolic demands and reliance on passive suspension rather than active locomotion. Despite this, the forelimbs are disproportionately strong, with powerful flexor muscles accounting for roughly 11% of body mass in two-toed sloths, adapted for maintaining a firm grip on branches over extended periods. The digestive system centers on a large, multi-chambered suited for microbial of tough material. In three-toed sloths, the features four chambers that allow slow breakdown of by , while two-toed sloths have a four-chambered version; both configurations occupy about 25% of body weight when full. Complementing this, the intestines are elongated, promoting extended retention times of up to 30 days through the entire digestive tract for maximal nutrient extraction from low-quality . Sloths exhibit poor eyesight, with shortsightedness (3–4 diopters) and immobile eyes lacking a for focus, limiting in their dim forest habitat. However, their sense of smell is enhanced by two large olfactory bulbs and a well-developed piriform lobe, aiding in food detection and . Hearing is supported by a compact and a 4 mm cochlear height, providing sensitivity to low-frequency sounds despite the small region dedicated to it.

Physiology and behavior

Metabolic processes

Sloths exhibit one of the lowest metabolic rates among non-hibernating mammals, typically ranging from 40% to 74% of the value predicted by allometric scaling for their body mass. This reduced rate is an to their energy-limited folivorous diet and helps conserve resources in their stable tropical environment. Associated with this is a lower core body , averaging 30–34°C, which is 3–7°C below the norm for most eutherian mammals and fluctuates with ambient conditions. A 2024 study highlights how their low may hinder to change-induced rises. Digestion in sloths is highly specialized for processing nutrient-poor leaves through in the multi-chambered , where microbial breakdown occurs slowly to maximize energy extraction from low-quality . Mean digesta retention times are approximately 150 hours for particulates, with the majority (about 73%) spent in the multi-chambered , allowing for prolonged despite the low caloric yield of their diet. This slow process reflects their overall energy-minimizing , enabling survival on minimal intake. Symbiotic in sloth , particularly in three-toed , form a mutualistic relationship that may supply minor nutrients via absorption through the skin and provide by imparting a greenish hue that blends with mossy surroundings. relies on passive insulation from their coarse outer hairs and dense undercoat, combined with behavioral basking to elevate body temperature during cooler periods, as sloths lack robust physiological heat generation mechanisms. To support oxygen delivery despite low metabolic demands, sloths maintain a relatively high count—averaging 4.11 × 10⁶ cells/mm³ in three-toed sloths—paired with a resting of 60–110 beats per minute, which minimizes cardiac energy expenditure while ensuring adequate circulation.

Locomotion and activity

Sloths primarily employ a suspensory locomotion style, hanging upside down from branches using their elongated, curved claws that function as natural hooks to grip without constant muscular effort. This adaptation enables efficient navigation through the forest canopy, where they propel themselves forward by releasing and reattaching their limbs in a deliberate, pendulum-like motion. Their arboreal movement is characterized by a slow pace, typically averaging around 1.2 km/h in trees, which allows for energy-efficient traversal despite the demands of vertical climbing. On the ground, however, sloths are markedly slower, achieving a maximum speed of approximately 0.24 km/h due to their disproportionate limb structure and reduced terrestrial agility. To conserve energy in line with their low metabolic rate, sloths limit daily movement to about 38 meters, focusing efforts on essential tasks like or repositioning within their home range. They spend the majority of their time inactive, with three-toed sloths sleeping approximately 9-10 hours per day in the wild, interspersed with rest periods. This pattern supports their overall strategy of minimizing exertion, as their folivorous diet provides limited caloric intake, necessitating prolonged periods of rest to maintain physiological balance. Activity levels in sloths exhibit seasonal variations, with reduced movement during dry seasons when higher temperatures and lower humidity can constrain their and efficiency. Despite their sluggish pace, sloths demonstrate remarkable climbing efficiency, ascending and descending trees with a steady grip that relies on specialized muscle fibers for sustained suspension rather than speed. When faced with threats, sloths adopt a defensive posture by curling into a tight ball, tucking their limbs and head to protect vulnerable areas and blend with foliage for .

Social structure and behavior

Sloths exhibit a predominantly solitary lifestyle, with individuals maintaining largely asocial interactions outside of brief mother-offspring associations and occasional mating encounters. Adult sloths of both two-toed (Choloepus spp.) and three-toed (Bradypus spp.) species typically occupy home ranges spanning 4 to 15 hectares, depending on habitat availability and species, where overlaps occur minimally and primarily facilitate reproductive opportunities rather than sustained social bonds. For instance, in two-toed sloths, male home ranges often encompass those of multiple females, but direct interactions remain rare due to their low metabolic rates and arboreal habits that limit energy expenditure on social activities. This asociality aligns with their evolutionary adaptations to energy conservation in nutrient-poor environments, reducing competition and conflict while promoting individual survival. Communication among sloths is infrequent and primarily vocal, serving functions such as advertisement or distress signaling rather than complex social coordination. Three-toed sloths produce high-pitched, shrill calls—often described as an "eeh" or scream-like sound in D-sharp range—emitted by females at the canopy tops to attract males during estrus, which can carry over distances in dense forests. In contrast, two-toed sloths are generally more silent but issue low-frequency bleats or hisses when distressed or threatened, with infants vocalizing prolonged bleats lasting 30 to 90 seconds upon separation from their mothers. These vocalizations are low-energy and infrequent, reflecting the sloths' overall reticence, and lack the elaborate repertoires seen in more gregarious mammals. Predation avoidance in sloths relies heavily on passive strategies emphasizing concealment over active evasion or collective defense, given their solitary nature. Their mottled fur, often colonized by and microorganisms, provides effective against the dappled light of forest canopies, blending seamlessly with surrounding foliage to deter visual hunters like harpy eagles and jaguars. Prolonged immobility—sloths can remain motionless for hours—further enhances this , as their minimal movement reduces detection by predators attuned to motion. While rare, defensive responses include slashing with sharp claws or biting if cornered, but group defense is virtually absent due to the lack of stable social units beyond familial pairs. Fur maintenance in sloths involves a combination of self-grooming behaviors and symbiotic relationships that support their low-maintenance lifestyle. Individuals routinely scratch and comb their fur using specialized claws and, to a lesser extent, their tongues, though this autogrooming is often inefficient at dislodging embedded organisms. A key aspect is the mutualistic symbiosis with moths (primarily from the family ), which inhabit the sloth's fur; these insects deposit nitrogen-rich waste that fertilizes algal growth (Trichophilus spp.), providing the sloth with supplemental nutrients consumed during grooming—up to 24.4 mg of volatile fatty acids per gram of fur via fermentation. This ecosystem in the fur not only aids camouflage but also reinforces the sloth's energy-conserving adaptations by minimizing the need for frequent or intensive cleaning. Maternal grooming is limited to licking the infant's head, face, and ano-genital areas to stimulate elimination, underscoring the overall infrequency of inter-individual contact.

Diet and foraging

Sloths are primarily folivorous, relying on leaves as the cornerstone of their diet to sustain their low-energy lifestyle. Three-toed sloths (genus Bradypus) exhibit a highly specialized folivory, consuming nearly 100% leaves in the wild, with a strong preference for young, nutrient-rich foliage from trees such as Cecropia species, which often comprise a major portion of their intake due to their high nitrogen content and low fiber and chemical defenses. In studied populations, feeding records show that approximately 77% of observations involve leaves from Cecropiaceae (including Cecropia), Clethraceae, and Clusiaceae families, with young leaves accounting for over 67% of selections to maximize digestibility. Foraging in three-toed sloths is selective and energy-efficient, focusing on tender, less defended parts accessed primarily in the upper canopy during midday peaks, when they spend about 14% of their time feeding. Daily intake typically ranges from 0.1 to 0.2 kg of fresh leaves, equivalent to roughly 17 g dry per kg of body , reflecting their constrained metabolic needs and constant stomach fullness. Nutritional adaptations include a high tolerance for dietary and alkaloids through microbial , allowing processing of low-protein foliage (often below 10% protein), supplemented occasionally by mineral-rich via geophagy to aid detoxification and provide essential nutrients lacking in leaves. In contrast, two-toed sloths (genus Choloepus) maintain a more varied, omnivorous diet, still dominated by leaves from diverse species like Dipteryx panamensis and Anacardium excelsum, but incorporating fruits, flowers, buds, and stems for broader nutritional intake. They opportunistically consume animal matter, including , small , and even carrion or , which provide protein boosts to offset the low-nutrient base. Foraging occurs nocturnally over larger home ranges, with selective targeting of young shoots and rotation among species to mitigate toxin accumulation from , supported by a diverse that enhances breakdown of varied compounds. Like their three-toed counterparts, they exhibit geophagy for mineral supplementation and tolerate low-protein diets through recycling and microbial aids, though their intake mirrors similar low volumes adjusted for body size.

Reproduction and development

Sloths exhibit a polygynous , in which males defend territories and with multiple females, often excluding rival males from core areas through aggressive interactions and low home range overlap. Males use scent marking, primarily via anal glands rubbed on tree branches, to advertise territories and attract females, establishing meeting points for copulation. by males is rare but has been hypothesized as a selective , with females potentially mating multiply to confuse paternity and reduce risks, though direct observations in the wild are limited. Recent studies (as of 2024) on two-toed sloths reveal detailed hormonal profiles during reproductive cycles. Gestation periods are approximately 5-6 months for three-toed sloths (Bradypus spp.) and 6-12 months for two-toed sloths (Choloepus spp.), sometimes involving delayed implantation. Births typically produce a single offspring, with twins being exceptionally rare across species. Newborn sloths are born fully furred, with eyes open and claws developed, enabling them to instinctively grasp their mother's fur and climb to her chest or back immediately after delivery, often while she hangs from a . Maternal care is the primary form of , with no observed paternal involvement. Mothers carry infants on their bodies for 6-9 months, during which the young nurse and learn behaviors by observing and mimicking. occurs around 1 year of age, after which juveniles remain somewhat dependent but gradually become independent. Sexual maturity is reached at 3-5 years for both sexes, varying slightly by species and sex (earlier in females). Genomic analyses (as of 2025) indicate genetic adaptations contributing to their , estimated at 20-30 years in the wild and up to 40-50 years in captivity. Sloths have a low reproductive rate, producing one offspring every 1-2 years due to their slow and extended periods. Lifespan in the wild is estimated at 20-30 years, though precise data are scarce due to challenges in long-term monitoring; in captivity, individuals can live up to 40-50 years under optimal conditions.

Habitat and distribution

Geographic range

Sloths are endemic to the Neotropical region, inhabiting Central and from southern southward to northern . This distribution encompasses a variety of ecosystems, primarily tropical and subtropical forests, where the two genera—Bradypus (three-toed sloths) and Choloepus (two-toed sloths)—occupy overlapping but distinct niches. Three-toed sloths tend to favor wetter, northern latitudes and dense rainforests, while two-toed sloths exhibit a broader tolerance, extending into more varied and sometimes drier environments across the continent. Among the species, the brown-throated three-toed sloth (Bradypus variegatus) possesses the most extensive range, spanning approximately 13 countries including , , , , , , , , , , , and northern . The pale-throated three-toed sloth (Bradypus tridactylus) is found in northern , including , , , , and northeastern . The (Bradypus pygmaeus) is restricted to Isla Escudo de Veraguas, a small island off the coast of . The maned three-toed sloth (Bradypus torquatus) is endemic to the Atlantic Forest of eastern . In contrast, species of the genus Choloepus are centered in the and ; for instance, (Choloepus didactylus) occurs throughout northern east of the , from to and , primarily in lowland Amazonian forests up to 1,200 meters elevation, while (Choloepus hoffmanni) ranges from and into western , , and , reaching elevations up to 2,400 meters. These distributions reflect the sloths' adaptation to arboreal life in forested habitats, with no presence in , , or other continents outside the . The current Neotropical range of sloths results from their South American origins and northward dispersal during the Great American Biotic Interchange in the to , with the extinction of lineages at the end of the Pleistocene leaving only arboreal forms. In recent decades, has led to significant population declines in some regions; for example, the Atlantic Forest, the primary habitat of the maned sloth (Bradypus torquatus), now covers approximately 12% of its original extent, resulting in isolated and diminished populations.

Habitat preferences

Sloths are predominantly arboreal mammals, spending the majority of their lives in the canopy layers of neotropical rainforests, typically at heights ranging from 20 to 30 meters, where they navigate using lianas and rest among epiphytes for structural support and . This preference for elevated arboreal niches minimizes energy expenditure and predation risk while providing access to foliage. These animals thrive in tropical humid environments characterized by high rainfall exceeding 2000 mm annually, which supports the dense vegetation essential for their lifestyle, and they occur across an altitudinal gradient from up to 2400 m. Within these zones, sloths select microhabitats such as the water-holding tanks of bromeliads for resting, which offer moisture and concealment, though three-toed sloths generally avoid flooded areas to prevent exposure on the ground. In contrast, two-toed sloths exhibit semi-aquatic adaptations, frequently traversing flooded forests by between clusters during seasonal inundations. Sloths show varying suitability across forest types, favoring primary rainforests for their uninterrupted canopy continuity but also utilizing secondary forests and shaded agroecosystems like cacao plantations when primary is limited. However, forest edges created by fragmentation reduce quality due to increased exposure, lower structural complexity, and higher disturbance levels, making such areas less preferable for sustained occupancy.

Conservation and human interactions

Conservation status

Sloths are assessed under the of Threatened Species, with statuses varying across the seven extant species following the 2024 taxonomic split of the maned sloth. The (Bradypus pygmaeus) is classified as Critically Endangered due to its extremely restricted range on Isla Escudo de Veraguas, . The northern maned three-toed sloth (Bradypus torquatus) and southern maned three-toed sloth (Bradypus crinitus) are both listed as Endangered, primarily owing to habitat loss and fragmentation in the Atlantic Forest of . The remaining four species—the brown-throated three-toed sloth (B. variegatus), pale-throated three-toed sloth (B. tridactylus), (Choloepus hoffmanni), and (C. didactylus)—are categorized as Least Concern, reflecting their wider distributions across Central and . Despite the Least Concern designations for most species, all seven sloth species exhibit decreasing population trends, driven by ongoing habitat degradation and loss across their ranges. Precise global population estimates are unavailable due to the challenges of surveying arboreal, low-density species, though the pygmy three-toed sloth numbers between 500 and 2,500 individuals. Local densities vary, with reports of 2.2 to 6.7 individuals per hectare for the brown-throated three-toed sloth in Amazonian forests. Sloth populations are increasingly fragmented, particularly in regions affected by deforestation, with viable groups persisting in protected areas such as Yasuní National Park in Ecuador, where the park's vast rainforest supports multiple sloth species amid surrounding habitat pressures. This fragmentation exacerbates isolation and reduces genetic diversity, as seen in studies of maned sloths showing distinct genomic impacts from habitat loss. Monitoring sloth populations relies on non-invasive techniques, including camera traps to detect presence and estimate densities in forested habitats, and fecal DNA analysis to assess , population structure, and health without disturbing individuals. These methods are essential for tracking trends in remote areas and informing conservation priorities.

Threats and protection efforts

Sloths face significant threats from , primarily driven by in their habitats. Since the 1970s, the Amazon has lost just under 20% of its original (as of 2023), severely fragmenting sloth habitats and reducing available foliage for . poses another major risk, as expanding road networks in Central and force sloths to cross highways while seeking or mates, with studies indicating heightened mortality for arboreal like sloths in fragmented landscapes. The illegal pet trade exacerbates these pressures, with hundreds of young sloths poached annually in countries like for sale to tourists, often resulting in high mortality during capture and transport. Additional risks include , which alters forest composition and increases temperatures beyond sloths' limited thermoregulatory capacity due to their low metabolic rates, potentially rendering large portions of their range uninhabitable by the end of the century. on uninsulated power lines is also a growing concern, particularly in , where sloths use lines as substitutes for tree branches; over half of electrocuted wildlife in affected areas are sloths, with a 70% post-injury from organ failure. These threats are compounded by sloths' low reproductive rates, which limit population recovery. Conservation efforts include regulation under the Convention on International Trade in Endangered Species (CITES), with species like the pygmy three-toed sloth listed in Appendix II to control international trade, and ongoing proposals to include additional two-toed sloths for similar protections. Reforestation initiatives in Costa Rica, such as the Connected Gardens project, aim to restore canopy connectivity in urbanizing areas, planting native trees to link fragmented habitats and support sloth movement. Rescue centers play a vital role, exemplified by the Sloth Sanctuary of Costa Rica, which has rehabilitated and released hundreds of orphaned or injured sloths since 1992 through veterinary care and habitat assessment. Research gaps persist, particularly in , where initial studies in have revealed distinct genetic clusters but call for broader genomic analyses to inform translocation and connectivity strategies amid habitat loss.

Cultural and ecological roles

In human culture, sloths are frequently depicted as symbols of and indolence, a characterization stemming from observations of their deliberate movements and echoed in Western and where they embody and lethargy. This perception draws indirect parallels to biblical admonitions against slothfulness, such as in Proverbs 6:9, which warns against the sluggard's idleness, though the animal itself, native to the Americas, postdates ancient texts. In contemporary contexts, however, sloths have undergone a symbolic transformation, emerging as endearing emblems of tranquility and conservation; in , both two-toed and three-toed sloths were officially designated national symbols of wildlife in 2021, boosting their status as beloved icons of . Ecologically, sloths fulfill vital functions in ecosystems, primarily through facilitated by their infrequent descents to the ground to defecate, which deposits viable seeds from ingested fruits away from parent trees, promoting diversity and regeneration. They contribute to the dispersal of seeds from various species, including cecropia trees that rely on such animal-mediated processes for . Additionally, sloths occupy a key trophic position as prey for apex predators, such as eagles, which snatch them from the canopy using powerful talons, and jaguars, which ambush them on the , thereby sustaining predator populations and maintaining balance. Economically, sloths drive substantial wildlife across , particularly in , where sightings in national parks and reserves attract millions of visitors annually, contributing to the country's sector that accounts for about 13.5% of GDP and generates billions in revenue through lodging, guided tours, and related services. Responsible sloth-focused supports local communities by funding protection and alternative livelihoods, though unregulated practices can stress populations. Conversely, the illegal severely undermines these benefits, with sloths topping the list of trafficked wildlife exported from countries like , leading to high mortality rates during capture and transport—often exceeding 80%—and disrupting wild populations. Sloths hold significant scientific value as model organisms for exploring and metabolic adaptations. Their hosts diverse epibionts, including and moths, forming a mutualistic network where provide camouflage and nutrients, while sloths and moths facilitate algal and fungal dispersal, offering insights into co-evolutionary dynamics in mobile ecosystems. Furthermore, sloths' metabolic rate—among the lowest of any at roughly 40-74% of expected values—enables into energy-efficient physiologies, with applications to understanding low-metabolism states in human conditions like or in chronic diseases.

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