Animals that eat plants in trees are rare, and only about 10 mammal species are specialized tree-dwelling plant-eaters, sloths among them. Sloths mostly eat leaves. Leaves give sloths very little energy, so sloths get few calories from their food. Sloths are slow because their leaf diet gives them little energy and their bodies use energy very slowly. 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.

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. Three-toed sloths have small home ranges and are strict leaf-eaters. There are six species of sloth, two kinds of two-toed sloth and four kinds of three-toed sloth, and they are named for the claws on their front feet. Two-toed sloths are active at night, and three-toed sloths can turn their heads about 270 degrees, almost all the way around. A sloth's body temperature rises and falls with the temperature around it, and sloths warm up by moving into the sun and cool off by moving into the shade.

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. A curled-up sloth can look like a termite nest or a knot in the wood. Sloth hairs have grooves or cracks where algae can take hold and grow, and the green algae help a sloth blend in with the green leaves. About once a week, a sloth climbs down from its tree to go to the bathroom. On the ground, sloths cannot really walk, so they drag themselves along with their claws and front legs. About half of all sloth deaths happen when a sloth is on the ground, where it loses the camouflage of the leaves.

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. The maned three-toed sloth was once listed as Endangered because its range was thought to be very small, but new data found more places where it lives, so it was moved to Vulnerable. 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, 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.

Sloths live alone, except for a mother with her baby or sloths that are mating. 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. 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.

Words to know

• captivity: not wild
• predators: hunters
• camouflage: blending in
• species: kinds