Sloths move slowly. Sloths are slow because their leaf diet gives them little energy and their bodies use energy very slowly, and studies put the way they use energy at roughly 40 to 74 percent of what is expected for an animal of the same weight, which is among the lowest rates of any mammal. Sloths also have very little muscle for their size, and researchers report that they have the lowest muscle mass relative to body weight of any mammal, yet they have surprising strength and resist tiredness. Most mammals can keep their bodies at a steady warm temperature, but sloths cannot: a sloth's body temperature changes with the weather and with the time of day, which saves energy. Sloths cannot shiver to warm up, because they have so little muscle, and they warm up by moving into the sun and cool off by moving into the shade.

Sloths mostly eat leaves, and leaves give sloths very little energy, so sloths get few calories from their food. Sloths eat leaves from more than 90 kinds of trees, but one sloth usually sticks to about six to twelve kinds, and a three-toed sloth eats about 73.5 grams (2.5 ounces) of dry leaves a day, only a third of what a howler monkey eats. A sloth's stomach has four chambers full of bacteria that help break down the tough leaves, and because leaves stay in a sloth's slow stomach for so long, the stomach is always full, so a sloth cannot simply eat more to get more energy. A leaf can take anywhere from about 157 hours to 50 days to pass through a sloth, which is the slowest digestion of any mammal, and a sloth's gut bacteria need warmth, and if a sloth gets too cold, the bacteria can die, and the sloth can starve with a stomach full of leaves.

About once a week, a three-toed sloth climbs down from its tree to go to the bathroom, and when it does it can pass up to about one-third of its body weight. The trip is risky and costs energy, and scientists have long wondered why sloths do it, and about half of all sloth deaths happen when a sloth is on the ground, where it loses the camouflage of the leaves. On the ground sloths cannot really walk, and they drag themselves along with their claws and front legs, and one study estimated about 274 meters (900 feet) per hour in trees and 16 meters (53 feet) per hour on the ground for two-toed sloths. Why a sloth leaves the safety of the trees to go to the bathroom is not fully explained: one idea is that the droppings fertilize its favorite tree, and another links it to the moth and algae garden in its fur.

A sloth's fur is home to moths, beetles and other tiny animals, and sloth hairs have grooves or cracks where algae can take hold and grow. In 2014, scientists led by Jonathan Pauli reported that moths live in sloth fur, lay eggs in sloth droppings, and may help algae grow on the sloth, and three-toed sloths had more moths, more nitrogen and more algae in their fur than two-toed sloths. Pauli's team proposed linked partnerships among moths, sloths and algae: moths bring nutrients to sloth fur, the nutrients feed algae, and sloths eat the algae to add to a limited diet. In lab tests the algae were highly digestible and up to five times richer in quick-energy fats (lipids) than a sloth's usual leaves, but not everyone agrees that sloths eat the algae in their fur: a sloth expert who has watched wild sloths for more than 1,000 hours said no one in his group had seen it, and scientists still debate how sloths and fur algae relate: some studies say the algae neither help nor harm the sloth, while other research suggests both benefit, and how algae nutrients could get into a sloth is not known.

Animals that eat plants in trees are rare, and only about 10 mammal species are specialized tree-dwelling plant-eaters, sloths among them. Sloths probably were ground-dwellers first, and their ground-living ancestors were already close to blind before modern sloths moved into the trees fairly recently in evolutionary time. Sloths lack the cone cells most mammals use to see color, and they see poorly in dim light and are almost blind in bright daylight, which is one reason they move carefully in the trees. Two-toed sloths and three-toed sloths split from each other on the family tree about 40 million years ago, and they now live quite differently: two-toed sloths roam over bigger areas and eat a wider range of foods, including fruit and some animal matter, while three-toed sloths have small home ranges and are strict leaf-eaters.

Sloths belong to the order Pilosa, along with anteaters, and together with armadillos they form the group Xenarthra, and sloths are related to anteaters and armadillos, not to monkeys. Sloths and anteaters were once grouped as Edentata, meaning toothless, yet sloths have few teeth: only 10 on the top and 8 on the bottom, while anteaters have none. Two-toed and three-toed sloths used to be placed in one family, but studies showed so many differences that two-toed sloths now have their own family, Megalonychidae. Ancient sloths ranged in size from a medium dog to an elephant, and the biggest giant ground sloths stood about 12 feet (3.5 meters) tall and weighed several tons. Giant ground sloths went extinct on the mainland by about 13,000 years ago, and a few lived on in the Caribbean, and some sources link their disappearance to hunting by early humans.

You may have heard that sloths sleep almost all day. In 2008, scientists recorded sleep in a wild animal for the first time, using small brain-wave (EEG) recorders worn by three-toed sloths in Panama, and the sloths slept 9.63 hours a day, about 6 hours less than sloths in captivity. Three adult female sloths wore the recorders for several days, and about 20 percent of their sleep was REM sleep, close to the share in humans. A wild brown-throated sloth in an EEG study slept about 9 hours and was awake the other 15 hours of the day, so sloths are not asleep most of the time. Even awake, a sloth rarely goes further than about 30 meters in a day. Scientists are not sure why wild sloths sleep less than captive ones: the older captive study included young sloths, which sleep more, and captive sloths may be more relaxed with no predators and steady food.

As of November 2026, no one has tracked a wild sloth from birth to death, so scientists do not know how long wild sloths live, and the oldest sloth on record, a two-toed sloth at a zoo, was about 49 years old. As of November 2026, no one knows how many sloths there are: population estimates are missing for most species because sloths are hard to survey, though the IUCN lists the trend as decreasing for all six species. As of November 2026, the IUCN Red List puts four sloth species at Least Concern, one (the maned sloth) at Vulnerable, and one (the pygmy three-toed sloth) at Critically Endangered. As of November 2026, the pygmy three-toed sloth, found only on a 4.3 square kilometer island off Panama, is Critically Endangered, and only about 100 individuals are believed to remain. As of November 2026, the maned three-toed sloth, once listed as Endangered because its range was thought to be very small, is Vulnerable: new data found more places where it lives.

Moving slowly helps sloths go unnoticed by hunters that find prey by sight, and sloths are hunted by big birds and big cats, such as harpy eagles, jaguars and ocelots. Sloths live in trees, high up in the rainforest canopy, in the tropical forests of Central and South America, and sloths live alone, except for a mother with her baby or sloths that are mating. A baby sloth crawls onto its mother's belly right after it is born and clings to her. As of November 2026, sloths depend on forests, and many forests where they live are being cut down for farms, ranches, towns and logging, and sloths cannot jump from branch to branch the way monkeys do, so they need trees whose branches touch, and cutting down forests splits sloth habitat into small pockets. As of November 2026, sloths cannot cross the gaps, so each small group has fewer partners to breed with.

Words to know

• ancestors: earlier relatives
• lipids: fats
• EEG: brain recorder
• bacteria: tiny living things