Anything that looks like a miniature, happy Chewbacca just has to be awesome—and sloths do not disappoint. From tooth to poo pellet, the creatures of Order Pilosa, Suborder Folivora are as strange and fascinating as they are adorable.
1. They don't actually sleep all that much
It is true that sloths are very still for most of the day. But that's more about self-defense than laziness. The sloth mission statement can be summed up as, essentially, "Avoid being eaten by eagles." Seriously, it's a problem.
"There's no real defense against something that's willing to dive bomb a tree, flip upside down and grab you off a vine," said Donald E. Moore, Ph.D., associate director of animal care at Smithsonian's National Zoo in Washington, D.C. "You're really better off if they just don't see you."
To that end, sloths have picked up a couple of useful adaptations. First, they're covered in a unique sort of fur that's an ideal breeding ground for algae. Second, they're able to spend most daylight hours immobile and, when they do move, it's usually very, very slowly. The result: From the air, sloths look more like green vegetation than tasty, meaty eagle snack.
So how do we know they aren't sleeping that whole time? Research. In fact, brown-throated three-toed sloths were the first animal species to have its sleeping habits studied in the wild, said Bryson Voirin, a doctoral student with the Max Planck Institute.
In 2008, Voirin was part of a team that found some wild sloths only sleep a little more than nine hours a day. To do the study, the researchers had to catch sloths, then implant electrodes just below the skin of the animals' scalps. A tiny hard drive—epoxied to each sloth's head like a hat—recorded brainwaves associated with sleep and wake cycles. The animals were also outfitted with standard radio collars. For two weeks, the sloths lived as naturally as one can with a hard drive glued to one's head. Then, the researchers picked the sloths back up, took out the electrodes and detached the electronics. The recorded brainwave data offered the sort of insight on when subjects were sleeping and when they were awake that Santa Claus only wishes he could access.
2. They're smart--and fast
Just because sloths usually move slowly, doesn't mean they're physically limited to a snail's pace. Donald Moore has first-hand experience with just how fast these creatures can go, when they really want.
One female took a dislike to me. I'm one of the only sloth biologists who's been bitten," he said. "They can use their big claws and slash out. But what she did was run at me, upside down along a vine, as fast as a cat would run along the floor. She grabbed me and pulled my hand to her mouth and then bit. It all happened very quickly."
Luckily for Moore, most of the sloths he's worked with have liked him. Some even took to grooming him. His experiences demonstrate how sloths really can tell individual humans apart from one another, and have individual personalities themselves. Social interaction isn't the only way sloths show their intelligence.
"There was a researcher in the 1930s and '40s who did vine- and rope-based maze testing with sloths and showed that they have the same level of cognitive function as a cat in a maze on the ground," Moore said.
3. Most of them can't survive in zoos
There are six species of sloths—four species of three-toed sloths and two of the two-toed variety. Of those, only the two-toed species are frequently found in zoos. It comes down to an issue of movement and diet. In the wild, two-toed sloths move more than 40 meters a day through the treetops, said Moore. Three-toed sloths move much less.
The result: Two-toers have a more varied diet—enjoying everything from lettuce, to boiled yams, to grapes. (They really, really love grapes.)
Meanwhile, other sloths eat almost nothing but parts of the Cecropia tree. Such picky eaters aren't easy animals for zoos to take care of, but it has been done. The Dallas World Aquarium, for instance, has the only three-toed sloth on display in the United States. They make it work, Moore said, because that particular three-toed sloth is very comfortable with humans—and because there are Cecropia trees in the parking lot.
"The keepers were able to take them out once a day, before the zoo opened, and let them eat from the trees in planters," he said.
4. They're in a league of their their own
Sloths evolved in South America and, for most of their existence, that continent wasn't connected to any others. They're very old—their family tree, which also includes anteaters and armadillos, diverged from the rest of the mammals some 75-80 million years ago, when South America was still joined to Africa. They're also pretty strange.
"Even the animals they're most closely related to, anteaters and armadillos, are as different from sloths as whales are from bats," Moore said.
Among the many sloth oddities is a very slow metabolism. They have the lowest body temperature of any mammal, Moore said, and they only use the bathroom once a week. That last bit has the added benefit of protecting them from predators, because their regular bathroom break is the only time sloths leave the trees. Even weirder, their digestive system is similar to a cow's, with a specialized, multi-chambered stomach that allows them to fully process leaves.
"When dogs eat leaves, they come out relatively whole. When sloths eat leaves, they come out as little pellets at the end of the week," Moore said.
Their teeth are also on the funky side. Sloths have no incisors, and no canines. Instead, they have what are called caniniform molars—conical teeth that look like triangles in cross section. From his personal experience, Moore can vouch for the fact that every side of that triangle is razor sharp.
Even the way sloths store fat is unique. Most mammals have fat deposits tucked around their bodies. Sloths, however, don't seem to, according to Moore. He's never seen an obese sloth and, in autopsies, the only place he's ever found any sign of a fat deposit is on the pads of their feet.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.