Sloths live in trees, high up in the rainforest canopy, in the tropical forests of Central and South America, and they spend most of their lives hanging upside down from tree branches, where their long, curved claws hook onto the branches. Sloths are slow because their leaf diet gives them little energy and their bodies use energy very slowly, and the numbers show how slowly: 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. On the ground, sloths cannot really walk, so they drag themselves along with their claws and front legs.

Three features of a sloth's body are tied to hanging upside down. A sloth's esophagus has a loop in it that helps keep swallowed food in the stomach while the sloth hangs upside down. Its organs are anchored to its belly wall, so they do not press on the lungs when the sloth hangs upside down, which lets the sloth breathe more easily and use less energy. And a sloth's hair grows from its belly toward its back, the opposite of most mammals, which helps water run off when the sloth hangs upside down. Sloths also have very little muscle for their size, and 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 have round heads, tiny ears and short, stubby tails, and their long, thick fur makes them look bigger than they are. Nearly all mammals have 7 neck bones. Two-toed sloths have 6 to 9, and three-toed sloths have 8 or 9, which lets three-toed sloths turn their heads about 270 degrees. Sloths lack the cone cells most mammals use to see color. They see poorly in dim light. Sloths are not blind, but their eyesight is poor. Sloth teeth have no enamel and keep growing all through life. They are worn down by chewing leaves, and hard lips help shear the leaves.

A sloth's stomach has four chambers full of bacteria that help break down the tough leaves. 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. A sloth's gut bacteria help it digest leaves, but they need warmth. If a sloth gets too cold, the bacteria can die, and the sloth can starve with a stomach full of leaves. Because leaves stay in a sloth's slow stomach for so long, the stomach is always full, and 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.

About once a week, a sloth climbs down from its tree to go to the bathroom. Sloths are hunted by big birds and big cats, such as harpy eagles, jaguars and ocelots. About half of all sloth deaths happen when a sloth is on the ground, where it loses the camouflage of the leaves. Sloths are good swimmers, and fast swimmers for a sloth: in water they swing their arms and feet more quickly than in the trees. Sloths get water from the leaves they eat, and also by lapping dew off leaves and fruit, but a sloth's big stomach acts like a float when it swims across rivers.

Words to know

• esophagus: food tube
• enamel: tooth coating
• camouflage: blending in
• anchored: held in place