Native to Central and South American rain forest, the sloth is the slowest animal on the planet, so immobile that algae[1] grow on its brown furry back. Sloths have a short and flat head, big round eyes, a snout that is short in length, long tree-climbing legs, a stubby tail and small ears and though some have called them ugly, they have the cutest face that looks like they are always smiling[2].
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, but even when they are awake you won’t see them move much. 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. Sometimes a sloth will die hanging upside down from a tree limb and remain so after death, until they decompose[3] or are forcibly removed.
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[4].
Most domestic herbivores, such as cattle, horses, and sheep, graze all day long[5]. 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[6].
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[7] predators.
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[8] 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.
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 and other carnivores such as ocelots[9]. 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[10] the leaves with its hard, tough lips and slowly grinds them with large, peg-like teeth.
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[11]. 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.
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[12].
The greenish cast of the sloth’s shaggy fur is caused by symbiotic algae that grow in grooves running along the length of each of the mammal’s outer hairs[13]. The sloth hosts the algae, and the algae return the favor by providing nutrients that the sloth either ingests[14] by licking its fur or absorbs through its skin. The grayish-green color makes the sloth look just like a clump of dry leaves hanging from a branch—the perfect jungle camouflage[15]. And the longer a sloth lives, the greener it gets.
Slow and lazy are two different things. The grace, balance, and its hospitality toward the other creatures that depend on it—are traits we could aspire to emulate[16].