5 minute read


Sloths are mammals of the Central and South American jungle that spend their lives in trees, eating leaves in a very slow, or "slothful," manner. They belong to order Edentata, which means "without teeth." However, sloths are not actually without teeth. They have molars, or chewing teeth, that have no roots and continue to grow throughout their lives. Anteaters, for which this order was named, actually have no teeth.

The two kinds of sloths belong to two different families of edentates. The three-toed sloths makes up family Bradypodidae. Three-toed sloths make a sound that has been described as "ai-ai," which has given them the name of ai. The two-toed sloths are family Megalonychidae. Actually, though, these animals should be called "two- and three-fingered" sloths because all five species have three toes on each of their hind feet.

The three species of three-toed sloths are smaller than the two-toed. Their head-body length ranges from about 18-24 in (50-60 cm), with a weight of only about 9 lb (4 kg). The two-toed species are larger, with a head-body length up to 28 in (70 cm) and weighing up to 17 lb (8 kg). The famed extinct ground sloth, Mylodon listai, which was about the size of an elephant, belonged to the two-toed family.

Sloths have quite flat faces on very round heads, with round eyes, a round snout, and round nostrils. Even their tiny round ears are hidden in their coarse, dense fur. The hair of the fur, which is usually light brown or gray, is grooved. Within these grooves grow algae, encouraged to A three-toed sloth. Photograph by Bud Leinhausen. Photo Researchers, Inc. Reproduced by permission.
grow by the high humidity of the rain forest, so the animal more often looks green than brown. This coloration keeps the animal camouflaged against predators. The coarse hair of the two-toed sloths is much longer than that of the three-toed, about 6 in (15 cm), compared to 2-3 in (5-7 cm). Both of them have a soft undercoat of denser fur. Because they spend most of their lives upside down, their fur parts on their bellies instead of along their backs.

There is a good reason why the word "sloth" means laziness and slowness. These animals do everything slowly. They live strictly by browsing on leaves in trees. Their entire bodies are adapted for this activity. Their limbs are geared for clinging to tree branches-upside down. Their claws are 3-4 in (8-10 cm) long and curve tightly around branches.

Their stomachs are equipped with several chambers in order to digest plant material that would poison other animals. The chambers also contain bacteria that help digest the tough material in leaves. Their digestive systems work just as slowly as the animals' reputation. It can take a month or more for the huge quantity of leaves they eat to make their way through the system. Then the waste remains in the body except for their very occasional—and painfully slow—trips to the ground, when they defecate at the base of the tree in which they live, perhaps once a week.

In addition, their body metabolisms are geared toward conservation of energy. Instead of depending on their metabolism to keep them warm, as most mammals do, they warm up in the sun and cool down in the shade of the high canopies where they live. Their system of blood-carrying arteries and veins is arranged so that the heat carried by the blood continues to circulate in the body instead of being lost out the fingers and toes. This arrangement is of real benefit to an animal that becomes uncomfortable if the temperature drops below 80°F (26.6°C).

They do not even waste energy getting into position for sleep. They just fall asleep as they are, generally upside down, with the head falling forward onto the chest. They spend at least 20 hours a day sleeping. During those remaining four hours, they eat. They move very slowly, just a gentle hand-over-hand motion, no leaping, no quick turns. They do make progress, however. They go after the leaves on different branches. They even change trees frequently. However, when they reach the ground, all they can do is pull themselves along with their strong front arms. Their muscles will not support their weight.

Female sloths don't change their habits just because they have babies. The young are born after varying gestation periods (almost a year in Hoffman's two-toed sloth, Choloepus hoffmanni, of Nicaragua to Central Brazil). The single baby is born up in the tree, where the mother turns into the infant's nest. She stays upside down and the baby snuggles down to nurse. It continues to nurse for a month, gradually taking in more and more nearby leaves. The mother carries the baby until it is at least six months old. About three months after that, it must head off on its own.

In some parts of Central America, members of the two different families share the same area. When this occurs, there are usually more of the smaller three-toed sloths than the bigger two-toed. The two species are active at different times of the day or night. They also have different tastes in trees, so they don't compete.

Edentates are regarded as the remains of a large group of South American animals that spread throughout that continent many millions of years ago, probably from North America. There were once many more sloths. The ground sloths were known and killed by early natives before becoming extinct. Today, the maned sloth (B. torquatus) of Brazil, is classified by the World Conservation Monitoring Center as endangered. Remaining sloths are isolated to the Atlantic coastal forests of eastern Brazil, with some pockets of individuals surviving elsewhere. The maned sloth is endangered because its coastal habitat has almost entirely been taken over by resort and urban development. Less than 3% now remains. Also, sloths are hunted for food and traditional medicinal purposes, adding to the threat of their extinction.



Hartman, Jane E. Armadillos, Anteaters, and Sloths: How They Live. New York: Holiday House, 1980.

Hoke, John. Discovering the World of the Three-Toed Sloth. New York: Franklin Watts, 1976.

Jean F. Blashfield


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


—To eliminate solid waste from the body.


—The total energy use of the body necessary for maintaining life.

Additional topics

Science EncyclopediaScience & Philosophy: Semiotics to Smelting