Physical Description

Sloths are tropical tree-dwelling herbivores about the size and shape of a small dog. They have flattened faces with a short snout. Their forelegs are longer than the rear; all four limbs are long and thin and made for hanging. Sloths' rear legs end in small, thin pads with three digits; their front digits number either two (2-toed sloths) or three (3-toed sloths). Sloths possess curved claws 3 to 4 inches (8 to 10 centimeters) in length; these are used for gathering food or for defense. When they are born, sloths already have permanent, enamel-less, hallowed-out molars used for grinding food. These teeth grow continuously throughout a sloth's life. However, sloths do not possess front teeth or incisors; instead, they use their hard, roughened lips to grasp and tear vegetation. Sloths appear to have relatively poor hearing and vision, but like most Xenarthrans, their keen sense of smell allows them to locate appropriate trees for food or to find mates.

Sloths are covered with two layers of hair: a finer, shorter undercoat and a longer, thicker outer layer. Sloths live most of their life hanging upside-down in wet, tropical areas; because of this, sloths' hair parts at the belly rather than the spine. The moist air is perfect for the formation of tiny one-celled algae species called Trichophilius and Cyanoderma. These species grow in miniscule grooves that line sloths' hair. Other creatures can also be found living in sloth hair, including butterflies, beetles, and moths. The snout moth specifically feeds off of the algae on sloth hair, and lays its eggs both in a sloth's hair and on its dung. These creatures live in a peaceful, symbiotic relationship with the sloth.

Body Temperature and Metabolism

As with the other Xenarthrans, sloths possess a low and variable body temperature: 86 to 93 degrees Fahrenheit (30 to 34 degrees Celsius). Although it is a warm-blooded mammal, a sloth's body temperature depends on its environment and cools when the sloth is inactive, asleep, or during the nighttime hours. When awake, sloths consciously regulate their body temperature by moving in and out of sunny areas in the rainforest canopy. When they become too hot, sloths may lay with their bellies exposed, pant, or even seek a water source to cool themselves.

More than any other Xenarthran, sloths take every precaution to conserve every ounce of food energy they consume. All bodily movements are reserved only for gathering food and natural processes. When cold, sloths gather themselves into a ball and do not shiver; special blood vessel systems called "retes" help keep warmth centered around the body's vital organs. Muscle mass, which generally uses a large degree of energy, accounts for only 25% of a sloth's body weight - less than half of most other mammals. Food may take up to a month to be fully digested.

Scientists measure an animal's metabolism by how quickly it converts food into working energy. A sloth's metabolic rate is 36% to 66% that of a mammal of the same size and weight. Food is digested in a manner similar to a cow's: sloths possess hardy multi-chambered stomachs which can resist toxic levels of tannin in leaves. Food and liquid within a sloth's stomach may contribute to as much as 30% of its total weight. Once digestion is complete, bodily wastes pass into a short intestinal tract. Defecation and urination occur as seldom as once a week; sloths appear to keep habitual defecation areas at the bases of trees. These areas also serve as breeding grounds for the snout moth.

Slow, Steady, and Stealthy

The common name of "sloth" is derived from the Old English word meaning "lazy" and "sluggish". Although this term may be misleading when applied to a sloth, one cannot deny that they live very quiet lives. A sloth's average climbing speed is only 6 to 8 feet (1.8 to 2.4 meters) a minute. This may increase to as fast as 100 feet (30 meters) a minute if threatened or if a mother sloth and her child are separated. Sloths are very cautious creatures and rarely leave the trees they call home; when on the ground, they cannot support their own weight and must drag themselves forward using their forelimbs and claws. Sloths sometimes may also enter water to cross a river or for cooling; when swimming, they effectively perform a sort of back or breaststroke.

Sloths are built to last. Studies show that sloths live 10 to 12 years in the wild and up to 31 years in captivity. They conserve as much energy as possible and sleep the majority of any given 24-hour period. When asleep, sloths may curl into a ball to retain heat; in this position they look much like a termite's nest or a knot of vegetation. Their white and brown hair tinged with green serves as an effective camouflage within the rainforest canopy. Although sloths have difficulty supporting weight in an upright position, they are well-suited for hanging and cannot be easily dislodged when suspended from a tree branch. Some sloths even remain hanging after death and must be forcibly removed.

High up in the rainforest canopy, sloths are well-protected from natural predators. These include the jaguar and ocelot (leopard-like cats), caimans (alligator-like reptiles), large snakes such as the anaconda, and birds of prey such as the harpy eagle. Unfortunately, humans are also a natural predator and hunt sloths for meat and for pelts, although this practice is diminishing. Sloths have very tough hides and possess amazing recuperative abilities: they can survive physical injuries which would certainly kill another mammal of the same size. All of these factors have allowed sloths to flourish in their natural habitat - but as we will see, this natural habitat is rapidly vanishing, endangering sloths and every other creature that depends on its habitat for survival.

Family Life

Sloths are generally quiet, solitary animals; adults only pair for the purposes of mating. Breeding appears to occur year-round, although some reports show a mating season of March to April for the 3-toed sloth. Other studies show births of certain 2-toed sloths to occur after the rainy season between July and September. Gestation (the length of time between impregnation and birth) is about a year for Hoffman's 2-toed sloth and around 5 to 6 months for all other sloth species.

Sloth mothers give birth hanging upside-down and catch their single offspring as it emerges from the womb. Birth time is approximately 15 to 30 minutes. Sloths are usually born without embryonic membranes and are placed on the mother's chest where it may nurse and hide from predators in the mother's long hair. Baby sloths cease nursing after one month and share their mother's favorite foods; researchers have found that baby sloths "inherit" a preference to specific plants from the mother. The passing on of a specialized diet minimizes competition between sloths within the same area of the rainforest.

After 9 months, a young sloth is usually prepared to venture forth on its own. 2-toed sloths have been seen to drive their young away should they attempt to return to the mother. Female 2-toed sloths reach sexual maturity in 3 years; males do so in 4 to 5 years.

Learn More!

• Learn more about the two groups of sloths: 2-toed sloths (Choloepus) and 3-toed sloths (Bradypus).
• Sloths used to be as large as an elephant! These ancient ground sloths are now extinct, but they are still a fascinating topic of study.
• Want the facts in brief? Check out our quick Fact Sheet.
• Explore the Amazonian tropical rainforest, home of the sloth and other Xenarthrans.
• What do others have to say about sloths? Find out on our Links Page.