Everybody knows that sloths are slow. It's true the tropical tree dwellers possess the lowest metabolic rates of any non-hibernating mammals.

But when it comes to their biology, the Central and South American critters are anything but boring.

For instance, sloths are actually three times faster in water than they are on land, says Becky Cliffe, a zoologist and founder of the Costa Rica-based Sloth Conservation Foundation. What’s more, they float. (See ridiculously cute pictures of baby sloths.)

"Thirty percent of their body weight is just digesting, fermenting leaves," says Cliffe. "So they’ve quite a lot of gas in there as well. They're like big balls of air with arms and legs."

Sloth immobility is just one of the many misconceptions Cliffe hopes to dispel with her new book, Sloths: Life In The Slow Lane.

"I really wanted to paint a picture of an animal that is actually perfectly adapted for survival," she says.

Slow Is Not Stupid

How long do you think you could hang upside down in a tree? A minute? Ten? Sloths do it all day, every day.

The six species of sloth have evolved long claws that act like hooks and tendons that draw their digits closed when at rest. Sloths also have a network of blood vessels running through their forearms to keep their muscles cool and reduce energy usage.

Sloths are also shockingly strong, even though their muscle mass is 30 percent less than that of similarly sized animals. That's because their muscles are composed of slow-twitch fibers that provide loads of endurance while not using a lot of energy.

Orphaned Baby Sloths Climb Rocking Chairs Baby sloths learn to climb trees in the wild, but how do they figure it out if they live at a rescue center? Find out how rocking chairs are helping orphaned baby sloths in this video with National Geographic explorer Lucy Cooke.

One thing these muscles cannot do is shiver, so sloths bask in sunlight to raise their body temperatures—sort of like a reptile.

Even their digestion is sluggish—Cliffe says it can take up to 30 days to process a single leaf.

Sadly, their slowness has earned sloths a bum rap for being stupid. (Watch how sloths slowly navigate the world.)

Some people say if "you fire a gun next to a sloth's head, it won't even turn around," says Cliffe.

In fact, sloths benefit by slowly reacting to danger. The tropical tree dweller evolved alongside the harpy eagle, a bird of prey that can detect even the tiniest of movements.

"They're as smart as they need to be, in their own way."

Watching and Waiting

Though sloths seem easy to follow and photograph, it took wildlife photographer Suzi Eszterhas about six years to capture all the shots in the new book. On one expedition, Eszterhas and Cliffe and spent eight weeks letting a mother sloth, nicknamed Apple, and her newborn, Pie, acclimate to their presence.

View Images A brown-throated three-toed sloth peeks through the leaves at the Aviarios Sloth Sancutary in Costa Rica. Photograph by Suzi Eszterhas

"She used to stay like a hundred feet up in the air, and then she slowly started to come down a little bit more often and give us these little glimpses," says Cliffe.

Patience also paid off on another trip to study and photograph pygmy three-toed sloths, a critically endangered species that lives on a remote island off Panama. (See "Pictures: Pygmy Sloth Among 100 Species Most At Risk.")

The animals are smaller than your average housecat and famous for swimming between mangroves, a behavior Cliffe and Eszterhas sat in a boat for five days waiting to see.

"When finally got it on the last day, we were like, 'Yes! Let's go home!'" says Cliffe.

Unfortunately, a tropical storm rolled in, and the pair had to ration water for two days and sleep curled up on the boat. The only good news?

“We were able to pay some fishermen to catch us some lobster and cook it on a fire,” says Cliffe.