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.