The sloth is one of the strangest creatures on the planet. As their name implies, these animals spend a great deal of their time, around 18 hours each day, sleeping or resting.  In fact, moving at top speed, the sloth can only cover about 1 mile of terrain in 4 hours.

The sloth is perfectly adapted to an arboreal, or tree-dwelling, lifestyle.  It spends most of its life suspended upside-down, using curving claws on each foot that hook over and firmly grasp the overhead branches. Eating, sleeping, mating, and birthing is all done upside-down in the world of the sloth. 

photos M. Noonan

The sloth is also quite agile in the trees, it can rotate its feet nearly 180 degrees to reach tree branches.  This is very flexible, especially considering that a ballerina can only rotate her feet about 90 degrees at the most.  On the ground, however, the sloth is very awkward due to its curved feet designed for tree grabbing and the fact that its limbs are too weak to support its own weight.  This makes a sloth on the ground quite vulnerable to predators.  Thus, it makes sense that the sloth is rarely, if ever, found outside of the trees.  

Many tropical biologists never get to see a sloth in the wild.  Now, this doesn't mean that the sloth is exceedingly rare.  Instead, it simply means that, like many rainforest mammals, the sloth is cryptic -- it blends in with its environment very well.  Part of the reason that sloths blend in so well is that their shaggy fur harbors algae and mold that make the sloth green and inconspicuous in the forest.  In fact, hordes of mites, beetles, and caterpillars will graze on the moldy hair of the sloth -- making the sloth a living salad bar! 
Another reason that sloths are so difficult to find in the wild is their slow-moving nature.  This is mainly due to the sloth's unusual diet.  Unlike many other mammals, the sloth feeds entirely on tree leaves.  This makes for a very un-nutritious diet, as most of the calories in the leaves are bound up in hard-to-digest cellulose.  To deal with this, the sloth has a large, multi-chambered stomach that fills one-third of its body to digest plant material.  In addition, the metabolic rate of the sloth is 1/2 that of other mammals their size.  This helps sloths cope with the limited amount of energy that they gain from their diet.  In fact, their metabolism is so slow that food may remain in a sloth's stomach for up to a month. 

photo M. Noonan

One mysterious component of the sloth lifestyle is that, about once a week, when nature calls, the sloth descends to the ground, where it digs a hole and relieves itself into the depression.  As of yet, no one has been able to identify a reason for this strange behavior.  Indeed, sloths are bizarre creatures that are perfectly suited to life in the rainforest.  Using some amazing adaptations -- such as a slow metabolism and hook-like claws -- these unique animals are able to exploit a niche that few other mammals can occupy.
The two-toed sloth has two fingers, but three toes.  It is nocturnal and also tends to be larger in overall body size than the three-toed sloth.
The three-toed sloth has three fingers and three toes.  Unlike the two-toed sloth, it is active during the day (diurnal).  Males can be distinguished from females by a distinctive orange patch of fur with a black stripe that runs down their back.

CAC is a program of the Institute for the Study of Human-Animal Relations at Canisius College in Buffalo, NY.