Why are sloths so slow?

Let's say you're driving around an unfamiliar city one day and you get lost. The odometer racks up more and more miles as you try to find a landmark to reorient yourself. Then, the gas light flickers on, and you start to panic because you have no idea where the nearest gas station is. What do you do to ensure that you don't become stranded? You would have to maximize the return on every drop of fuel burned. For instance, if it's a summer day, you'd turn off the air conditioning and roll up the windows. You'd probably drive at a steady clip and try not to gun the engine.

Like pacing your car when it's low on gas, sloths move slowly to conserve energy as well. In fact, they move slower than any other mammal on Earth. The arboreal animals are known for taking their sweet time getting around, usually trekking no farther than 125 feet (38 meters) in a day [source: Cohn]. On the rare occasions when they're on the ground, sloths crawl at a glacial pace of about 1 foot (30 centimeters) per minute [source: Cohn]. This sluggishness has earned sloths a negative reputation in the human world -- even one of the seven deadly sins shares their name. Yet, a sloth's lethargic nature allows it to make the most of its surrounding resources.

Sloths have little energy at their disposal for darting around at fast speeds because of their herbivorous diet. Native to the Americas, sloths spend most of their lives nestled between or hanging upside down from tree branches in tropical rain forests in Central and South America. Cecropia trees were nicknamed "sloth trees" because people thought sloths preferred their leaves; however, extensive observation has shown that sloths feed on many tree species [source: Lowman et al]. When hunger strikes, sloths simply reach out to the nearest available branch, pull it close and tear the leaves off with their lips. You probably wouldn't move around too quickly either if you had a permanently stocked minifridge within arm's length.

But imagine if you ate nothing but salad greens for a week without taking in any fats or protein. You probably wouldn't have a ton of excess energy to, say, sprint around the block. Likewise, sloths' diets, which may also include fruits, flower buds and branches, don't provide much nutrition. To compensate, sloths have a host of adaptations in addition to their slow pace that allow them to survive on such a lean diet.

Sloths Conserve Energy

Members of the Xenarthra superorder, sloths are related to anteaters and armadillos. There are two types of sloths: two-toed and three-toed. Trying to spot a sloth in a zoo can be like hunting through a "Where's Waldo" illustration. For starters, they prefer to remain tucked away among the tree foliage during the day. And, of course, sloths aren't especially active -- although new research indicates we may have exaggerated their laziness.

A 1983 study found that captive sloths slept 16 hours out of the day, which put them among the heaviest snoozers in the animal kingdom [source: Reebs]. Yet, results of a different study published in 2008 revealed an interesting contrast between captive and wild sloths. According to data from the 2008 study conducted at the Max Planck Institute for Ornithology in Germany, wild sloths sleep far less than their captive cousins, napping only nine or 10 hours each day [source: Reebs]. This probably has to do with food availability and general boredom, since captive sloths' meals are provided for them.

Along with a substantial amount of time spent sleeping, sloths conserve energy by breaking down their food much slower than most animals. While it takes the human body about a day to digest a meal fully, the sloth digestive system spends up to a month doing so. In addition to lowered internal body temperatures, sloths' metabolic rate is about 40 to 45 percent slower than comparably sized mammals [source: Cohn]. That means the sloth's body requires less energy to work, allowing it to thrive on such a sparse diet.

In spite of their slower pace, both internally and externally, sloths continue to function efficiently. For example, sloths have about half the muscle mass as other animals their size, but that doesn't handicap them from a utilitarian perspective. Sure, put a sloth on the ground and it's not strong enough to walk on its hind legs, but that's because it doesn't need much lower body strength in the trees. Rather, the muscles are concentrated in the upper body and forelimbs. On the ground, the sloth uses its forelimbs to drag its body forward, and in the trees, sloths can easily pull themselves up and hang from branches for hours on end.

And even when leaves are scarce, the sloth doesn't have to look too far for a snack. Though their fur contains tinges of green which helps hide them from predators, this camouflage isn't pigment; it's algae. Since sloths live in naturally humid areas, algae sometimes form in the grooves in the individual strands of fur. Sloths can then lick their fur and get some additional nutrition from this algae accessory.

When you put all of these elements together, sloths are kind of like "the Dude" from the cult classic "The Big Lebowski." The Dude moves at his own pace and isn't one to be rushed -- but everything he needs to make himself happy is at arms' length.

Why are sloths so slow?

Sloths Conserve Energy

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