Sloths move slowly, and the reason is not laziness. Sloths are slow because their leaf diet gives them little energy and their bodies use energy very slowly. Studies put the way sloths 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. Researchers report that sloths have the lowest muscle mass relative to body weight of any mammal, yet they have surprising strength and resist tiredness.

Sloths mostly eat leaves, and leaves give sloths very little energy. Sloths eat many fewer leaves than other leaf-eaters of their size: 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. 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.

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, so they warm up by moving into the sun and cool off by moving into the shade. Sloth fur is long and thick, so sloths look bigger than they are. Three-toed sloths can turn their heads about 270 degrees, almost all the way around.

Sloths lack the cone cells most mammals use to see color. They see poorly in dim light and are almost blind in bright daylight, which is one reason they move carefully in the trees. Moving slowly also helps sloths go unnoticed by hunters that find prey by sight. 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, and the green algae growing in a sloth's fur help it blend in with the green leaves.

You may have heard that sloths sleep almost all day. In a study of wild three-toed sloths in Panama, the sloths slept about 9.6 hours a day, much less than sloths in captivity. Two-toed sloths are active at night. About once a week, a sloth climbs down from its tree to go to the bathroom, and it can pass up to about one-third of its body weight. Sloths get water from the leaves they eat, and also by lapping dew off leaves and fruit.

You may have heard that sloths are a kind of monkey. In fact, sloths are related to anteaters and armadillos, not to monkeys. There are six species of sloth: two kinds of two-toed sloth and four kinds of three-toed sloth, named for the claws on their front feet. Three-toed sloths have face markings that make them look like they are always smiling. Sloths cannot change their facial expressions the way monkeys, cats and dogs can. The smile is just the shape and coloring of the face.

Words to know

• digestion: breaking down food
• bacteria: tiny living things
• shiver: shake
• species: kinds