Sloths on the flipside

Posted by: Rebecca Whitham, Communications


Now this is a story all about how my life got flipped, turned upside down...

Upside down sloth. Photo by Ryan Hawk/Woodland Park Zoo.

OK, maybe the Fresh Prince wasn’t rapping about sloths, but considering they spend the vast majority of their day turned upside down, he sure could have been.

Snacking while upside down. Don’t try this at home. Photo by Dennis Dow/Woodland Park Zoo.

Sloths eat, sleep, mate and even give birth upside down. If you were to hang upside down from your seat right now (use your imagination and spare yourself the pain of actually doing it, trust me), you’ll feel your head start to pound, gravity tugging at your hair, and honestly, I’m getting dizzy just writing about it.

But none of that happens for the upside-down kings of the rain forest, because sloths are specially adapted to live life from their unique point of view.

So how do sloths function upside down?

Looking good: Sloth fur defies gravity by inverting the typical hair growth pattern. Think about a four-legged terrestrial animal—say a bear. The bear’s hair has a part on the mid-back and the hair grows towards the belly. But for sloths, the part runs down the belly, and the hair grows from the belly towards the back, which not only works with gravity, but also makes it easy for rain to run right off, handy when you live in the rain forests of Central and South America.

Notice the direction the fur is growing, from the belly toward the back. Photo by Ryan Hawk/Woodland Park Zoo.

Taking care of business: There are a few things that sloths can’t do while upside down in the trees. They do come down to the forest floor to urinate and defecate. But they don’t otherwise spend time on the forest floor because their tree-adapted bodies aren’t designed for walking around. Surprisingly, though, they are good swimmers, allowing them to get around the forest in search of new food sources when they aren’t in the trees.

See the hooked claws of the two-toed sloth’s back feet. If you’re wondering why you are counting three toes in this picture of a two-toed sloth, it’s because the two-toed moniker refers to how many toes they have on their front feet. Both two- and three-toed sloths have three toes on the back feet. Photo by Ryan Hawk/Woodland Park Zoo.

Hang on tight: Sloths hang upside down even while sleeping, so they need to be locked in place securely or else the forest floor would be splattered with sleepy sloths every night. To this end, sloths have developed an incredibly tight grip, using their long, sturdy, curved claws to keep safely attached to trees at all hours.

The claws work through tendons so the sloth’s claw grip doesn’t require muscle power to hang on. This is all part of the sloth’s master plan to conserve energy and use as little muscle power as possible—hence the glacial pace at which they move, when they bother to move at all. In fact, sloths are so still they actually can have algae grow on their fur in the wild, which gives them a green tint. That works to their advantage, helping them blend into the leafy treetops of their rain forest canopy home.

Why are they so slow? Their diet is made up primarily of leaves, which is a calorie poor food that doesn't supply much energy and takes a long time to digest. Of course, staying still while slowly digesting low energy food also works to the sloth's advantage, since that incredible stillness helps it stay hidden from predators.

Who would have guessed that this face…

Hello. Photo by Ryan Hawk/Woodland Park Zoo.

…is the face of a finely tuned, rain forest domination machine?

At Woodland Park Zoo, our two-toed sloths sure do spend a lot of time upside down even when interacting with their right-side up zookeepers (in fact, I couldn’t find a single sloth photo in our archives that was anything other than upside down).

A sloth emerges from a nesting box to check out what snacks the zookeeper has to offer. Photo by Dennis Dow/Woodland Park Zoo. 

We carefully maintain the light cycle for these nocturnal animals, giving them some darkness during the day to encourage the activity they’d otherwise save for nighttime. 16-year-old female Nentas and 14-year-old male Mocha are almost always seen up in the trees or nest boxes in their exhibit in the Adaptations building, next door to the zoo’s Komodo dragons. Come by to observe them, and if you’re like me, you’ll find it relaxing to be amidst their stillness.