Living the Sin: The Strange Lives of Three-Toed Sloths

Special thanks to our good friend Kyle Empringham over at TheStarfish for suggesting this week’s topic. Kyle asked “What’s the deal with sloths?” Fortunately for us, vague questions often lead to interesting answers.

Very few people would envy the lives of three-toed sloths. Native to the rainforests of Costa Rica, these animals are famous for moving incredibly slowly (they climb at a maximum speed of 8 feet per minute), and sleeping up to 20 hours per day. What most people don't realize is that sloths are unique among mammals in a lot of ways that might explain their apparently laziness.

As a general rule of biology, large mammals don’t live their whole lives in trees. Sure, trees offer great protection from predators and give you a pretty solid view of what is going on in your neck of the woods, but the food stinks. Living on a diet of leaves alone usually limits mammals to about the size of a squirrel. Most animals you can think of that make their homes in forest canopies rely on insects, fruits, or other means of added nutrition to keep them going. Not three-toed sloths, though. The reason these furry tree-dwellers move so slowly is because their metabolism is the slowest of any mammal. Leaves offer them so little in the way of nutrition that they can’t afford to waste a single calorie.

Surprisingly, their dietary deficiencies may also help explain one of the weirdest things sloths do. For years, biologists have puzzled over why sloths risk getting chomped on by predators once every week when they leave the trees to poo. They would be easy enough targets if they used different locations as their toilets, but for some reason sloths go back to the same spot time and time again. It is the only thing in their lives that they willingly leave the trees for.

Surprisingly, a recent study from biologist Jonathan Pauli at the University of Wisconsin suggests that the reason for this peculiar bathroom behaviour might lie in the sloths’ disgustingly unkempt fur. One of the odd side effects of their moving so slowly is that algae actually grows in the fur of three-toed sloths. The algae makes them appear green during the wet season and brown during the dry, so the arrangement actually provides them with some added camouflage. The interesting part about what Pauli had found is that the ecosystems in their fur also help supplement a slothful diet. Studying the stomach contents of sloths revealed that they eat the algae when grooming, which provides additional fats and calories that they need to make up for eating all those empty leaf calories. 

That is all well and good but how does it explain sloths’ preferred pooing spots? As it turns out, algae isn’t the only thing living in a sloths' fur. Their follicle forests are also home to moths. Up to 120 individual moths have been found living in the fur of one sloth. By observing the sloths for weeks on end, Pauli developed a theory that these moths were laying their eggs in the sloths’ dung. When the slow-moving hosts returned to their bathroom spots, the eggs hatched and the new moths hopped aboard to keep the cycle going.

Apparently, when the moths die and decompose in the sloths’ fur, they provide nutrients that allow the algae to grow. As we have seen, the sloths benefit from the algae by way of camouflage and a handy snack. In short sloths, moths, and algae have worked out one of the strangest symbiotic ménage a trois in nature. This also explains why three-toed sloths living in cleaner captive conditions often aren’t as healthy as their wild counterparts.

The morale of the story? They may be cute and easy to catch, but you really don’t want to hug a sloth.