The slowest mammals on Earth, and slow on purpose. Sloths live on leaves so poor in energy that everything about them is built to spend as little as possible: a body that hangs without effort, a temperature that drifts with the weather, and a coat that grows its own algae and houses its own moths.

Why so slow?See the numbers
OrderPilosa (with anteaters)
Lives inCentral and South American forest
IUCN statusLeast Concern to Critically Endangered
DietLeaves
SLOTHS AT A GLANCE

Sloths are tree-living mammals of the rainforests of Central and South America. There are seven living species in two groups: five three-toed sloths (Bradypus) and two two-toed sloths (Choloepus). They weigh 2.5 to 8 kg, eat mostly leaves, and spend nearly all their lives hanging in the canopy, coming down about once a week to defecate. Their closest living relatives are anteaters, then armadillos. Until about 11,000 years ago their family also included ground sloths the size of elephants. Most species are common, but the pygmy sloth of Panama is Critically Endangered.

Figures graded and sourced in the fact sheet below. Most field data come from three-toed sloths in Panama and Costa Rica.
KEY DISCOVERIES
1797Jefferson’s clawThomas Jefferson describes giant fossil claws as a lion’s. They belong to a ground sloth, later named Megalonyx jeffersonii.
1973Under 38 metres a dayRadio-collared three-toed sloths in Panama are found to travel under 38 m on 89% of days (Sunquist & Montgomery).
2001A new speciesThe pygmy three-toed sloth is described from one small island off Panama (Anderson & Handley).
2008Not so sleepyBrain recordings show wild sloths sleep about 9.6 hours a day, not the 16 seen in zoos (Rattenborg et al.).
2014Moths and algaeA study links the sloth’s risky trips to the ground with the moths and algae in its fur (Pauli et al.).
2019Family tree redrawnAncient DNA and proteins show the two living kinds of sloth took to the trees separately (Delsuc et al.; Presslee et al.).
ONLY IN THE SLOTH

What makes the sloth unlike any other animal?

Short answer: It runs on less energy than any other mammal that does not hibernate, and its whole body is arranged around hanging upside down.

01The lowest energy budgetThree-toed sloths have the lowest daily energy use measured in any non-hibernating mammal.Pauli et al. 2016
02Extra neck bonesThree-toed sloths have eight or nine neck vertebrae where almost every other mammal has seven.Hautier et al. 2010
03A coat with tenantsGreen algae grow in grooved hairs, and moths found nowhere else live and breed around them.Suutari et al. 2010
04Organs tied downInternal attachments hold the liver and stomach in place so they do not press on the lungs when it hangs.Cliffe et al. 2014
05A body that cools downBody temperature drifts by several degrees through the day, more like a reptile’s than a mammal’s.Cliffe et al. 2018
06Two inventions of the same lifeTwo-toed and three-toed sloths evolved tree-hanging separately, from different ground sloths.Delsuc et al. 2019
FACT SHEET

How slow is a sloth?

Short answer: On the ground, about 2 m a minute. A three-toed sloth typically covers less than 40 m in a whole day.

MeasuredEstimatedQuoted, unverified
WeightMeasured~2.5 to 8 kgTwo-toed4 to 8 kgThree-toed2.5 to 5.5 kgBody length 45 to 75 cm. About a third of the weight can be stomach contents
Ground speedEstimated~2 m a minuteIn treesup to ~4.6 mOn the ground~2 mPer minute, three-toed sloth. Guinness lists it as the slowest mammal
Distance per dayMeasuredUnder 38 mThree-toed<38 mTwo-toed≥38 mRadio-tracked in Panama: 89% of three-toed days under 38 m, 54% of two-toed days at 38 m or more. Sunquist & Montgomery 1973
Daily energy useMeasured162 kJ per kgThree-toed162 kJ/kgTwo-toed234 kJ/kgMeasured in wild sloths in Costa Rica. Pauli et al. 2016
Sleep in the wildMeasured9.6 hours a dayWild9.6 hIn zoos~16 hBrain activity recorded in wild three-toed sloths. Rattenborg et al. 2008
Body temperatureMeasured~30 to 35 °CSloth, range30 to 35 °CHuman37 °CFollows the air temperature; sloths bask to warm up. Cliffe et al. 2018
Digesting a leafEstimated~1 week to 1 monthSlothdays to weeksCow~2 to 3 daysAmong the slowest of any mammal. Estimates vary with method and temperature
Neck bonesMeasured8 to 9 or 5 to 7Three-toed8 to 9Two-toed5 to 7Nearly all other mammals have exactly 7. Hautier et al. 2010
Holds its breath 40 minutesQuoted, unverified?Repeated widely, and sloths can clearly slow their hearts and stay under for some time. We could not trace the 40-minute figure to a published measurement.Popular claim
Speed Race tool →
WHY SO SLOW

Why are sloths so slow?

Short answer: Because leaves are a poor fuel. Slowness is how a sloth balances its budget: it earns very little energy, so it spends very little.

Read: why sloths are so slow →
01Poor foodRainforest leaves are low in calories, hard to digest and often laced with toxins.
02Slow gutA large, chambered stomach ferments leaves for days or weeks to get what it can.
03Small budgetThe energy that comes out is tiny, so the body runs at a very low rate.
04Little muscleSloths carry far less muscle than other mammals their size, which is cheap to maintain.
05Slow movementLess muscle and a low budget mean slow, deliberate motion and long rests.
Slow is also safeHarpy eagles and cats hunt by spotting movement. A still, green-tinted sloth is hard to see.
Cheap heatingInstead of burning food to stay warm, a sloth lets its temperature drop at night and basks in the morning.
Too hot is a problemUnlike most mammals, a sloth’s metabolism slows again when it gets too warm, so heat waves are dangerous.
A full stomachBecause digestion depends on warmth, a cold sloth can starve with a stomach full of leaves.
Pauli et al. 2016; Cliffe et al. 2018.
TWO KINDS

What is the difference between two-toed and three-toed sloths?

Short answer: The number of claws on the front limbs. Both have three toes on the back feet. Despite looking alike, the two groups are only distant relatives.

Three-toed (Bradypus)Two-toed (Choloepus)FingersThreeTwoToesThreeThreeNeck bones8 to 95 to 7Weight2.5 to 5.5 kg4 to 8 kgDietLeaves almost onlyLeaves, fruit, flowers, sometimes eggs or insectsActiveDay and nightMostly at nightTailA short stubNoneTemperPlacidWill bite and slash if corneredPregnancy~6 months~10 to 11.5 months
Because both have three toes, some biologists prefer "two-fingered" and "three-fingered". The lineages split roughly 30 million years ago. Delsuc et al. 2019.
UPSIDE-DOWN BODY

How do sloths hang upside down for so long?

Short answer: With hooks, not grip. Long curved claws and tendons that lock in place let a sloth hang with almost no muscular effort, even while asleep.

Claws as hooksThe claws are extended finger bones, 8 to 10 cm long, sheathed in keratin.
Locking tendonsTendons in the hands and feet hold the grip without effort. Dead sloths have been found still hanging.
Anchored organsFibrous attachments carry the weight of the stomach and liver, cutting energy use by almost 13% whenever the sloth hangs fully inverted.
A head that turns 270°The extra neck bones of three-toed sloths let them look almost straight behind.
Less muscleMuscle is roughly a quarter to a third of body weight, against 40% or more in most mammals.
Built to pullWhat muscle there is suits slow pulling and holding, not pushing. A sloth cannot stand on the ground and must drag itself.
Hair that parts on the bellyThe fur runs from belly to back, the reverse of other mammals, so rain drains off a hanging animal.
A capable swimmerIn water a sloth moves about three times faster than on land, using a long-armed front crawl.
Cliffe et al. 2014; Hautier et al. 2010.
LIFE IN THE FUR

Why are sloths green?

Short answer: Algae. Each hair has grooves or cracks that hold water, and green algae grow in them. The coat also carries fungi, mites, beetles and moths that live nowhere else.

An alga of its ownThe main species, Trichophilus welckeri, has only ever been found on sloths and seems to pass from mother to young.
CamouflageThe green tint helps a motionless sloth vanish among leaves, especially in the wet season.
Sloth mothsSeveral moth species spend their adult lives in the fur. A three-toed sloth can carry dozens.
Fungi with promiseFungi cultured from sloth hair have shown activity against bacteria, parasites and cancer cells in lab tests.
Suutari et al. 2010; Higginbotham et al. 2014 (PLoS ONE).
THE MOTH CYCLE
01DescentThe sloth climbs down to the base of a tree to defecate.
02EggsFemale moths leave the fur and lay their eggs in the fresh dung.
03LarvaeThe caterpillars feed and grow inside the dung.
04ReturnNewly hatched moths fly up into the canopy to find a sloth.
05FertiliserMoths and their waste may add nutrients to the fur, feeding the algae.
Steps 1 to 4 are well documented. Step 5, and whether sloths eat the algae, is a hypothesis. Pauli et al. 2014.
THE WEEKLY DESCENT

Why do sloths come down to the ground to poo?

Short answer: Nobody is sure. About once a week a three-toed sloth climbs down, digs a small hole with its tail and defecates at the base of a tree. It is the most dangerous thing it does, and no explanation is settled.

Feeding the fur gardenPROPOSEDThe claimGoing to the ground lets fur moths breed. More moths means more algae, which the sloth eats.Evidence forSloths with more moths had more nitrogen and more algae in their fur, and algae were found in stomachs.AgainstSloths have rarely been seen eating from their fur, and the amounts of algae are small.Pauli et al. 2014
Leaving messagesPROPOSEDThe claimDung piles at the base of trees act as scent posts for finding mates.Evidence forSloths have a good sense of smell, live alone, and females in heat draw males from a distance.AgainstHard to test in the canopy. No study has shown sloths visiting each other’s latrines.Suggested by field researchers
Feeding the treeWEAKThe claimBurying dung at the roots fertilises the tree the sloth depends on.Evidence forSloths do return to favourite trees.AgainstThe nutrients involved are tiny compared with the risk and the size of a rainforest tree.Montgomery & Sunquist 1978
A big eventA sloth can lose a noticeable share of its body weight in one visit, since it stores a week of waste.
The riskOn the ground a sloth cannot run or defend itself well. Many recorded deaths by predators happen there.
Two-toed slothsThey also come down, but less strictly, and sometimes defecate from the branches.
On a scheduleThree-toed sloths keep to roughly a weekly rhythm, even in captivity.
SEVEN SPECIES

How many species of sloth are there?

Short answer: Seven: five three-toed and two two-toed. The count rose from six in 2022, when the maned sloth of Brazil was split in two.

Read: pygmy three-toed sloth →
Three-toed sloths
Brown-throated slothB. variegatusHonduras to northern ArgentinaThe commonest and most studied sloth.LEAST CONCERN
Pale-throated slothB. tridactylusThe Guianas and northern BrazilYellowish throat; lives north of the Amazon.LEAST CONCERN
Maned slothB. torquatusAtlantic forest, Bahia and SergipeA mane of long black hair on the neck.ENDANGERED
Southern maned slothB. crinitusAtlantic forest, Rio de Janeiro and Espírito SantoSplit from the maned sloth in 2022.ENDANGERED
Pygmy three-toed slothB. pygmaeusOne island off PanamaThe smallest sloth. A 2012 survey counted 79.CRITICALLY ENDANGERED
Two-toed sloths
Hoffmann’s two-toed slothC. hoffmanniNicaragua to Peru and Brazil, in two separate areasThe two-toed sloth most often seen in Costa Rica and Panama.LEAST CONCERN
Linnaeus’s two-toed slothC. didactylusNorthern South America, east of the AndesSlightly larger; the species most often kept in zoos.LEAST CONCERN
The pygmy slothIt lives only on Isla Escudo de Veraguas, an island of about 4.3 square kilometres that separated from mainland Panama roughly 9,000 years ago. It was first found only in the red mangroves at the island’s edge; later surveys found sloths in the interior forest as well, so the true number is uncertain.Anderson & Handley 2001; Smith et al. (IUCN) 2022.
EVOLUTION

What are sloths related to?

Short answer: Anteaters and armadillos, not monkeys or bears. Today’s tree sloths are the small survivors of a family that included ground sloths weighing several tonnes.

Read: giant ground sloths →
Elephants, manatees and kinA separate old branch of placental mammalsArmadillosArmoured insect-eatersAnteatersThe sloths’ closest living relativesTwo-toed slothsNearest to the extinct Mylodon ground slothsMegatherium and MegalonyxGiant ground sloths, extinctThree-toed slothsBradypusXenarthransAnteaters and slothsSloths
Relationships simplified, not to time scale. After Delsuc et al. 2019 and Presslee et al. 2019. Two-toed sloth, anteater and armadillo outlines: PhyloPic, CC0.
MegatheriumA South American ground sloth about 6 m long and around 4 tonnes, as heavy as an elephant.
Recent extinctionMainland ground sloths vanished about 11,000 years ago. On Caribbean islands some survived until roughly 4,400 years ago, soon after people arrived.
ConvergenceBecause the two living groups descend from different ground sloths, hanging upside down in trees evolved twice.
Steadman et al. 2005. Megafauna →
THREATS AND CONSERVATION

Are sloths endangered?

Short answer: Most are not. Four species are Least Concern. The maned sloths of Brazil’s Atlantic forest are threatened, and the pygmy sloth is Critically Endangered.

Endangered register →
Forest lossHIGHA sloth cannot cross open ground safely. Clearing and cutting forest into fragments strands them.
Roads and power linesHIGHSloths crossing between trees are hit by cars or electrocuted on uninsulated cables.
Tourism and the pet tradeMEDIUMWild sloths are taken for selfies and sale. Handling raises their heart rate, and many die in captivity.
Canopy bridgesRope bridges over roads let sloths and monkeys cross without touching the ground.
Insulated cablesCovering power lines and transformers in sloth areas prevents electrocution.
Rescue and releaseCentres in Costa Rica, Panama and Brazil rehabilitate injured and orphaned sloths, with mixed results on release.
Natural predatorsHarpy eagles, jaguars and ocelots. In some forests sloths are the harpy eagle’s main prey.
HeatA metabolism that falters in hot weather makes sloths sensitive to warming.
DogsFree-roaming dogs kill sloths on the ground near towns.
If you visitWatch from a distance. A sloth that is being held is not smiling; that is the shape of its face.
IUCN Red List; Cliffe et al. 2018.
MYTH OR FACT

Six sloth claims, checked

3Myths2Half true1Facts
01MYTH“Sloths sleep 15 to 20 hours a day.”WHERE IT CAME FROMStudies of zoo sloths in the 1980s.
WHAT THE EVIDENCE SAYSWhen brain activity was recorded in wild three-toed sloths, they slept 9.6 hours a day. Captive animals, with no predators and food provided, sleep far more.
WHAT IS TRUE INSTEADAbout as much as a sleepy person.Sources: 02
02MYTH“Sloths grab their own arms, mistake them for branches and fall.”WHERE IT CAME FROMA joke that spread online.
WHAT THE EVIDENCE SAYSThere is no field record of this. Sloths do fall, usually when a branch breaks or in fights, and are built to survive long drops.
WHAT IS TRUE INSTEADAn invented story.Sources: 14
03MYTH“Sloths are a kind of monkey or bear.”WHERE IT CAME FROMThey climb, and "sloth bear" confuses things.
WHAT THE EVIDENCE SAYSSloths are xenarthrans. Their nearest relatives are anteaters, then armadillos. The sloth bear is a true bear named for its long claws.
WHAT IS TRUE INSTEADCousins of anteaters.Sources: 09
04HALF TRUE“Two-toed sloths have two toes.”WHERE IT CAME FROMThe common name.
WHAT THE EVIDENCE SAYSThey have two claws on each front limb and three on each back foot, the same as three-toed sloths.
WHAT IS TRUE INSTEADTwo fingers, three toes.Sources: 11
05HALF TRUE“Sloths only eat cecropia leaves.”WHERE IT CAME FROMSloths are easy to see in open cecropia trees.
WHAT THE EVIDENCE SAYSThree-toed sloths eat leaves from dozens of tree species. Each animal has its own short list of favourites, partly learned from its mother.
WHAT IS TRUE INSTEADMany trees; cecropia is just where we spot them.Sources: 14
06FACT“Sloths are good swimmers.”WHERE IT CAME FROMSeen in flooded forest and mangroves.
WHAT THE EVIDENCE SAYSLong arms make an effective front crawl, and gas in the stomach helps them float. Pygmy sloths swim between mangrove stands.
WHAT IS TRUE INSTEADTrue.Sources: 11
OPEN QUESTIONS

What science still doesn’t know

?Why come down to defecate?Moths, scent-marking and tree-feeding have all been proposed. None is confirmed.
?Do sloths eat their algae?Algae have been found in stomachs, but whether it is a real food source is disputed.
?How long do wild sloths live?Probably 20 to 30 years. Zoo two-toed sloths have passed 50, but wild lifespans are barely known.
?How many pygmy sloths are there?Counts range from under 100 to a few thousand, depending on whether the island’s interior is included.
?How do they find mates?Females call loudly when in heat. How far males travel, and what part scent plays, is unclear.
?Why the extra neck bones?How three-toed sloths broke the seven-bone rule that binds nearly all mammals is still being worked out.
FAQ

Frequently asked questions

Why are sloths so slow?+

Their leaf diet provides very little energy, so they have a very low metabolism, little muscle and slow movements. Moving slowly also helps them avoid being seen by predators.

How fast can a sloth move?+

About 2 m a minute on the ground and up to about 4.6 m a minute in trees. They are faster in water.

What do sloths eat?+

Mostly leaves. Two-toed sloths also eat fruit and flowers, and occasionally insects or eggs.

How long do sloths sleep?+

Wild three-toed sloths sleep about 9.6 hours a day. Zoo sloths sleep around 16.

What is the difference between two-toed and three-toed sloths?+

Two-toed sloths have two claws on each front limb and three-toed sloths have three. Both have three toes on the back feet, and the two groups are only distantly related.

Are sloths endangered?+

Most species are Least Concern. Both maned sloths are Endangered and the pygmy three-toed sloth is Critically Endangered.

COMPARE AND EXPLORE

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HOW WE KNOW

Sources and how we check

This page is written by Strange Animals and checked against primary research. Every figure carries a grade and a numbered source.

Cite this pageStrange Animals. (2026, November 15). Sloth (Bradypus and Choloepus): slowness, anatomy, fur and species. https://strangeanimals.info/mammals/sloths/sloth
  1. 01Pauli, J. N. et al. Arboreal folivores limit their energetic output, all the way to slothfulness. The American Naturalist 188: 196-2042016
  2. 02Rattenborg, N. C. et al. Sleeping outside the box: electroencephalographic measures of sleep in sloths inhabiting a rainforest. Biology Letters 4: 402-4052008
  3. 03Pauli, J. N. et al. A syndrome of mutualism reinforces the lifestyle of a sloth. Proceedings of the Royal Society B 281: 201330062014
  4. 04Cliffe, R. N. et al. Mitigating the squash effect: sloths breathe easily upside down. Biology Letters 10: 201401722014
  5. 05Cliffe, R. N. et al. The metabolic response of the Bradypus sloth to temperature. PeerJ 6: e56002018
  6. 06Hautier, L. et al. Skeletal development in sloths and the evolution of mammalian vertebral patterning. PNAS 107: 18903-189082010
  7. 07Suutari, M. et al. Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri. BMC Evolutionary Biology 10: 862010
  8. 08Sunquist, M. E. and Montgomery, G. G. Activity patterns and rates of movement of two-toed and three-toed sloths. Journal of Mammalogy 54: 946-9541973
  9. 09Delsuc, F. et al. Ancient mitogenomes reveal the evolutionary history and biogeography of sloths. Current Biology 29: 2031-20422019
  10. 10Presslee, S. et al. Palaeoproteomics resolves sloth relationships. Nature Ecology & Evolution 3: 1121-11302019
  11. 11Anderson, R. P. and Handley, C. O. A new species of three-toed sloth (Mammalia: Xenarthra) from Panama, with a review of the genus Bradypus. Proceedings of the Biological Society of Washington 114: 1-332001
  12. 12Miranda, F. R. et al. Taxonomic revision of maned sloths, subgenus Bradypus (Scaeopus), with revalidation of Bradypus crinitus. Journal of Mammalogy 104: 86-1032023
  13. 13Steadman, D. W. et al. Asynchronous extinction of late Quaternary sloths on continents and islands. PNAS 102: 11763-117682005
  14. 14Montgomery, G. G. and Sunquist, M. E. Habitat selection and use by two-toed and three-toed sloths. The Ecology of Arboreal Folivores, Smithsonian Institution Press1978
  15. 15Smith, D. et al. (IUCN) Bradypus pygmaeus. IUCN Red List. Critically Endangered2022
WATCH

Three films worth your time

AnimationTED-EdWhy are sloths so slow?From giant ground sloths to the treetops: the adaptations that let sloths live on so little energy.Pairs with: Why so slow →
ClipBBC EarthSwimming Sloth Searches For MateA pygmy three-toed sloth swims between mangroves on the island of Escudo in search of a mate.Pairs with: Seven species →
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