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At the Zoo: Slow and Steady Sloths
by Melissa Stewart

In the midst of the Smithsonian National Zoo's Amazonia exhibit, a mammal as large as a raccoon but only half as heavy hangs upside down from a slender tree branch. Hour after hour, the shaggy-coated creature remains silent and still. It is a sloth—the slowest mammal on Earth.

This 40-year-old female is one of three Linné's two-toed sloths (Choloepus didactylus) now living at the Zoo, but since 1950, a total of 18 sloths have called the Zoo home. Although these animals are not currently being bred or studied, past research conducted by Zoo scientists has contributed significantly to our understanding of sloth behavior and taxonomy.

Two-toed sloth at the National Zoo
Sloths are arboreal. Their bodies and behaviors are adapted to an upside-down lifestyle. (Jessie Cohen/NZP)

Along with anteaters and armadillos, sloths are members of the order Xenarthra—a group of mammals that appeared roughly 60 million years ago. The earliest xenarthrans were arboreal herbivores with simple, stubby teeth, small brains, fused pelvises, and sturdy spines.

About 35 million years ago in South America, a few cat-sized xenarthrans began to live on the ground. No longer confined to trees, they increased in size, expanded their range in parts of South, Central, and North America, and diversified into several dozen species of tree and ground sloths.

During intermittent glacial periods between 1.8 million and 12,000 years ago, several giant ground sloth species traveled north from South America into Central and North America, crossing a land bridge that formed at the Isthmus of Panama.

Four oxen-sized species migrated as far north as the southern United States, but they became extinct about 10,000 years ago, leaving behind the five species that now inhabit the tropical forest canopies of Central and South America.

Scientists divide modern sloths into two families based on the number of toes on their front feet. Linné's two-toed sloths and Hoffman's two-toed sloths (Choloepus hoffmanni) are larger than their three-toed cousins. They also have bigger eyes and longer hair, and their front and back legs are more equal in length. Pale-faced three-toed sloths (Bradypus tridactylus), brown-throated three-toed sloths (Bradypus variegates), and maned three-toed sloths (Bradypus torquatus) have small tails and extra neck vertebrae, allowing them to twist their heads farther to the right and left than any other mammal.

Whether they have two toes or three, all sloths spend most of the day asleep, curled up in a tree notch or hanging from a branch with all four legs close together and their heads tucked between their front legs. At feeding time, sloths move slowly and carefully, hand over hand through the treetops, searching for leaves, buds, fruits, and twigs. They even mate and give birth while suspended from their long, curved claws.

Why do sloths have such an unusual lifestyle? It's the strategy they evolved to survive as tree-dwelling plant eaters in a place with many predators.

Built for Hanging Around
Most domestic herbivores, such as cattle, horses, and sheep, graze all day long. Because they feed on nutrient-poor vegetation, they must eat almost constantly. Sloths are also herbivores, and their diets are also low in nutrition; but rather than munching placidly all day long, they have become masters at conserving energy.

As any athlete knows, maintaining muscle requires large quantities of food energy—far more than a sloth's vegetarian diet can provide. Because sloths are relatively inactive and spend most of their time hanging upside down, they can get by with half as much muscle mass as similar-sized mammals. As a result, they don't need to eat as much.

And because muscle is heavy, sloths weigh far less than other mammals their size. This makes it possible for them to climb on thin branches high in the tropical forest canopy, where they can more easily find food and avoid heftier predators.

Although a sloth has relatively less overall muscle mass than other mammals, the muscles in its shoulders, neck, and front legs are quite strong. According to David Kessler, a biologist at the Zoo, it may take as many as five people to catch the sloths in the Small Mammal House. But a sloth's back legs are so weak that it can't walk. To move across the ground, it lies on its stomach and reaches ahead for a toehold. Then it uses its long claws to slowly drag its body forward.

Two-toed sloth
Sloths weigh far less than other mammals their size.

Maintaining a high body temperature also takes energy. Sloths reduce this cost by maintaining a lower average body temperature than other mammals. Dogs, cats, horses, sheep, rabbits, pigs, and cows all have average body temperatures between 100 and 103°F, but a sloth's average body temperature is 93°F.

Just as important, a sloth's body temperature fluctuates with the ambient temperature. Like snakes, frogs, and other "cold-blooded" creatures, a sloth's body temperature is highest on warm, sunny days, and lowest at night and on rainy days. Sometimes a sloth sunbathes in the morning to warm up. Then, during the hottest part of the day, it hides in the shade so it won't overheat. During a 24-hour period, a sloth's body temperature may vary as much as 10°F. If the body temperature of a person, a dog, or a cow varies just 5°F, it can be life threatening.

Slow Food
Because sloths have difficulty moving over land, they spend most of their time in the trees. They can get just about everything they need high above the forest floor—even water, which comes from eating juicy leaves and licking up drops of morning dew. Short bursts of feeding followed by long periods of inactivity make sloths less vulnerable to large raptors such as harpy eagles (Harpia harpyja) and other carnivores such as ocelots (Leopardus pardalis). As long as sloths remain perfectly quiet and still, they're nearly impossible to detect.

At feeding time, a sloth reaches out, grabs an overhead branch with its flexible feet, and tugs until the food is within reach of its long tongue. After pulling the vegetation into its mouth, the sloth clips the leaves with its hard, tough lips and slowly grinds them with large, peg-like teeth.

In the 1970s, John Eisenberg, then head of the National Zoo's Department of Zoological Research, asked scientists at the Smithsonian Tropical Research Institute on Barro Colorado Island, Panama, to take a close look at sloths' feeding habits. The scientists found that two-toed sloths eat leaves, fruits, and flowers from a variety of trees and vines, but three-toed sloths are more specialized eaters, feeding on leaves from just a few tree species. Further studies showed that each three-toed sloth's food preferences are determined by the bacteria living in its gut. These bacteria are passed down from a sloth's mother when she shares food with her young. Because each animal has such a specific diet, three-toed sloths rarely compete with one another for food.

A sloth digests its food just as it does everything else—very slowly. People usually digest their food in about a day, but in sloths, the process may take a month. As soon as a sloth swallows a mouthful of pulverized plants, the bacteria in its stomach and intestines begin breaking down the food. It takes many hours for the bacteria to digest the tough plant material that makes up a sloth's diet. Only then can the sloth absorb the nutrients it needs to live and grow.

During the rainy season, sloths release their waste materials in the midst of afternoon storms. It's the perfect way to avoid attracting the attention of hungry predators. But during the dry season, getting rid of wastes is a bit more difficult. A sloth stores its feces and urine for about a week. Then it slowly descends to the ground, and deposits up to two pounds of dung. That's a lot of waste for an animal that may weigh only ten pounds!

Living Coats
Even the fur of sloths is adapted to their lifestyle. Because sloths hang upside down, their stiff, wiry fur grows differently from the coats of most mammals. Horses' hair, for example, grows from the back downward toward the belly, but sloths' fur grows from the belly toward the back, so rainwater will run off. Below this tough top layer, a dense downy layer of hair provides additional protection from pesky insects.

Each of the long, coarse hairs in a sloth's shaggy coat has a deep groove in which symbiotic algae colonies grow. During rainy periods, the algae give the fur a greenish tinge that helps the sloth blend into its forest habitat and evade predators. From time to time, the sloth will lick some of the tiny organisms from its coat for a quick, nutritious meal.

Algae aren't the only living organisms on sloths' coats. More than 120 moths of the subfamily Chrysauginae have been found in one individual sloth's fur, where the moths probably eat the algae and hide from birds and other predators.

Family Tree
Most individual sloths are solitary creatures, so when a female is ready to mate, she lets out a high-pitched scream in the middle of the night to attract a mate. Within a few hours, any males in the area slowly move in her direction. If two males arrive at the same time, they each grasp a tree branch with their back legs and swing one or both front legs at one another. The males continue their upside-down wrestling match until one gives up and leaves.

During the next few hours, the victorious male mates with the female several times. Then he leaves, and the female is on her own. Most females give birth to one tiny baby about six months after conception, but female Hoffman's two-toed sloths are pregnant for almost a year.

A newborn sloth is about ten inches long and weighs about 12 ounces. For its first five to six weeks of life, the helpless baby clings tightly to the shaggy hair on its mother's belly. It spends all its time nursing and sleeping. But gradually, the baby begins to nibble on leaves, reaching for whatever vegetation is within its grasp. It is during this period that the young sloth learns to prefer certain species of trees.

As the youngster grows, it becomes increasingly independent. In some species, juveniles can fend for themselves when they are just six months old, but in others, young continue to receive maternal care for up to two years.

When a mother sloth senses that her youngster can survive on its own, she leaves it behind and heads off to another part of her home range. Then both sloths will continue to exist as their ancestors have for millions of years—silent and sedentary, living up to their name.

Melissa Stewart is a freelance science writer based in Acton, Massachusetts.

ZooGoer 33(6) 2004. Copyright 2004 Friends of the National Zoo.
All rights reserved.

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