The Lazy Life of a Sloth

“HURRY and get your camera!” I called to my sister as I spotted a green sloth on the jungle path in front of me. We laughed when we realized that there was no reason to hurry​—the sloth is one of the world’s slowest animals.

To learn more about this mammal, I visited Zoo Ave in La Garita de Alajuela, Costa Rica. More than just a zoo, it is a center for the rescue, rehabilitation, and return to the wild of native animals. There I met biologist Shirley Ramírez, who was Zoo Ave’s enthusiastic research director. She took me to meet the zoo’s resident sloth, Pelota, which in Spanish means “ball.” Sloths, in fact, may curl up into a tight ball to sleep. Pelota is a two-toed sloth the size of a small dog, with a fluffy coat of fur, a snub nose, and big, watery brown eyes.

My research told me that sloths are solitary animals and that they give birth to a single offspring about once a year. The baby sloth clings to its mother for the first four to six weeks until weaned but may ride on her belly for five to eight months afterward. During that time the mother feeds her baby tender, easily digestible leaves from her lips. Later, the baby reaches out to grab its own leaves without ever releasing its grip on its mother. During their time together, the mother sloth also familiarizes her little one with the small home range in which it will live.

Two-Toed, Three-Toed, Green, and Cold

I learned that the sloth I had seen in the jungle was a three-toed sloth. It had a black mask around its eyes, a stubby tail, a coat of wiry hair, forelimbs that are much longer than its hind limbs, and a yellow-gold patch between its shoulders. This kind of sloth has nine vertebras in its neck, allowing it to rotate its head 270 degrees in search of its favorite leaves. But why did it appear to be green? Shirley answers, “That greenish tinge is algae that grows on the sloth’s coat.”

In contrast with their longer-armed, three-toed cousins, the two-toed species have forelimbs that are about the same length as their hind limbs. Their hair is long, brownish-gold, and soft to the touch.

The sloth spends its days sunning itself high up in the forest canopy. Its body temperature can fluctuate from an ambient 75 degrees Fahrenheit [24 degrees Celsius] at night to 91 degrees Fahrenheit [33 degrees Celsius] during the day​—a temperature range greater than that of any other mammal. The sloth has so little muscle mass that it cannot shiver to stay warm. That is why it often sleeps curled up in a ball, to conserve heat. Its undercoat of short, fine hairs helps to insulate it. And yes, the sloth can sleep for 20 hours a day!

Lingering Over Lunch

Since digestion requires body heat for bacterial activity and fermentation, the sloth’s low body temperature gives it an incredibly low metabolic rate. Leaves may take up to a month to go through the stages of digestion in a sloth’s multichambered stomach before passing into the small intestine. During a rainy season with many successive cool days,  sloths can die of starvation with a stomach full of food. “For sloths,” explains Shirley, “the sun’s warmth is indispensable to digestion.”

Shirley adds: “As a zookeeper who handles the animals and cleans their cages, I have to say that my favorite thing about sloths is that they only defecate and urinate once a week! When they do, they go down to the ground, dig a hole, and bury their excrement. It is the only thing they do on the ground.”

Designed for Life Upside Down

Almost everything else sloths do​—eat, sleep, mate, and give birth—​they do suspended in trees. The little mammals were ingeniously designed by their Creator for life in an upside-down world. The animal hangs by its fingers and toes, which are equipped with three-inch [7 cm]-long claws that hook and lock over branches and vines. To prevent the sloth’s skin from getting soaked by tropical downpours, even its hair grows upside down! Its hair parts on the belly and grows down around the back​—just the opposite of the hair of other land-based animals—​so the rain runs right off. Though on the ground the sloth is awkward and clumsy, when in the branches above, it is the epitome of leisurely grace. Surprisingly, the sloth is also an excellent swimmer!

What else did I learn about this quiet denizen of the treetops? Two things stand out. First, the sloth has an amazing ability to survive injuries and even doses of poison that would be lethal to other mammals. Severe wounds heal quickly, and they rarely become infected. Hence, a better understanding of the sloth’s immune defenses would be helpful to medical research. And second, people who are constantly rushing and under stress might find it beneficial to imitate, at least to some extent, the sloth’s relaxed pace and easygoing nature.​—Contributed.

 [Box/​Pictures on page 15]


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. The sloth hosts the algae, and the algae return the favor by providing nutrients that the sloth either ingests 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! And the longer a sloth lives, the greener it gets!

[Credit Lines]

Top right: © Michael and Patricia Fogden; bottom: © Jan Ševčík