A sloth's fur is home to more than the sloth: sloth hairs have grooves or cracks where algae can take hold and grow. Green algae grow in the fur and help the sloth blend in with the green leaves, and the fur is also home to moths, beetles and other tiny animals. A sloth's fur is long and thick, so sloths look bigger than they are. A curled-up sloth can look like a termite nest or a knot in the wood, and moving slowly helps sloths go unnoticed by hunters that find prey by sight. 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 they found that three-toed sloths had more moths, more nitrogen and more algae in their fur than two-toed sloths.

Algae from sloth fur may be extra food for sloths: 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. Not everyone agrees that sloths eat the algae in their fur, however: a sloth expert who has watched wild sloths for more than 1,000 hours said no one in his group had seen it. 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. A three-toed sloth also climbs down from its tree about once a week to go to the bathroom, and the trip is risky and costs energy, and scientists have long wondered why sloths do it.

Sloths belong to the order Pilosa, along with anteaters, and together with armadillos they form the group Xenarthra. 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, and anteaters have none. Sloth teeth have no enamel and keep growing all through life, and they are worn down by chewing leaves, and hard lips help shear the leaves. Sloths are related to anteaters and armadillos, not to monkeys. 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, and the two groups split from each other on the family tree about 40 million years ago. 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.

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. Sloths have round heads, tiny ears and short, stubby tails, and 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, and the smile is just the shape and coloring of the face. Sloths today are about 2 to 2.5 feet (0.6 to 0.8 meters) long, and only about 10 mammal species are specialized tree-dwelling plant-eaters, sloths among them. Ancient sloths ranged in size from a medium dog to an elephant, and giant ground sloths lived in the Americas long ago and are now extinct. The biggest giant ground sloths stood about 12 feet (3.5 meters) tall and weighed several tons.

Words to know

• algae: green growth
• enamel: tooth coating
• Edentata: toothless
• nutrients: food stuff