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a Sloth at the Zoo's Amazonia Exhibit
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a Sloth at the Small Mammal House
Two-toed
Sloth Fact Sheet
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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.
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| 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.
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| 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.
