Sloths
Sloths
Sloths are mammals of Central and South American forests that spend their lives in trees, eating leaves in a very slow, or “slothful,” manner. They belong to order Xenarthra, a name that refers to the additional bony articulations between the lumbar vertebrae, called xenarthrous processes, which animals in this order possess. The order was previously called the Edentata, which means “without teeth.” However, sloths are not actually toothless. They have molars, or chewing teeth, that have no roots and continue to grow throughout their lives. The anteaters, for which this order was formerly named, actually have no teeth.
The two kinds of sloths belong to two different families of edentates. The three-toed sloths make 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 classified in the family Megalonychidae. Actually, though, these animals should be called “two-fingered” and “three-fingered” sloths because all five species have three toes on each of their hind feet.
The four 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 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. Algae grow within these grooves so that 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 upside down to tree branches. 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 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 tree 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).
Sloths 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.
Xenarthrans 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 native people before becoming extinct.
KEY TERMS
Defecate —To eliminate solid waste from the body.
Metabolism —The total energy use of the body necessary for maintaining life.
Today, the maned sloth (Bradypus torquatus ) of Brazil is classified as endangered by the IUCN, and the newly described pygmy three-toed sloth (Bradypus pyg-maeus ) of Panama is listed as critically endangered. The maned sloth is endangered because its coastal forest 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. The pygmy three-toed sloth is critically endangered due to hunting pressure coupled with a very restricted range (one small island).
Resources
BOOKS
Eisenberg, J.F., and K.H. Redford. Mammals of the Neotropics. Chicago: University of Chicago Press, 1999.
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.
Nowak, Ronald M. Walker’s Mammals of the World. 6th ed. Baltimore: The Johns Hopkins University Press, 1999.
PERIODICALS
Anderson, R.P., and C.O. Handley, Jr. “A New Species of Three-toed Sloth (Mammalia: Xenartha) from Panama, With A Review of the Genus Bradypus.” Proceedings of the Biological Society of Washington 114 (2001): 1–33.
Jean F. Blashfield