Why sloths risk their lives to poop

Popular Science...

Every week, without fail, the three-toed sloth takes a breathtaking, almost suicidal risk—all for the sake of a bowel movement. Or, to put it in terms familiar to anyone who has sat through a long Zoom meeting, a “bio-break.”

With fast-moving predators lying in wait, being on or near the ground is the number one cause of mortality in sloths. And because sloths have among the slowest metabolisms ever recorded in animals, the climb down the tree and back up represents one of the biggest energy expenditures of their entire week.

“It’s like if I had to go on a 5K run down the middle of an interstate, just to use the bathroom,” University of Wisconsin-Madison wildlife ecologist Dr. Jonathan Pauli tells Popular Science. “It’s really costly, and it’s really risky.”

Which begs the question: Why do three-toed sloths take such risks for a poo? Why not just do the sensible thing and poop from the trees?

The answer involves mutualism—a relationship where all parties benefit—between sloths, moths, and that precious pile of dung sloths risk their lives to leave behind.

Sloths are home to flightless moths

The key to this whole system turns out to be a much smaller and less glamorous creature: sloth moths (Cryptoses choloepi). These moths spend their entire adult lives in sloth fur—yes, entire. The moment a moth finds and colonizes its slow-moving host, it loses the ability to fly. Permanently.

That’s OK, because sloth moths don’t need to fly once they find their sloth homes. Instead, the moths hitch a ride with the sloth to the base of the tree for its weekly poop session.

Some sloths do a little wiggle or dance when they’re trying to poop. Video: Have you ever seen a Sloth POOP Dance?, The Sloth Conservation Foundation

After the sloth has deposited its dung on the forest floor, pregnant female moths jump off the sloth into the dung pile (because they can’t fly, they literally hop), lay their eggs, and that’s pretty much the end of the moth.

Meanwhile, a new generation of sloth moths is dreaming big dreams. After hatching within the dung, the newborn larvae quickly commence chowing down on the dung that spawned them.

“Larvae will feed off the sloth dung. They actually chew a chamber into the sloth dung,” Pauli says. “The larvae then pupate and emerge as moths.”

And then, for one fleeting moment, sloth moths can fly. The newly emerged moths drift up into the canopy of the tree, find and inhabit a sloth host, and the cycle begins again. The moths are permanently grounded. Until, one day, their offspring will make that brief, one-way flight all over again.

Algae creates a sloth’s green camo coat

Enter the third player in this strange triumvirate: algae.

Because the moths are flightless, many of them live out their entire lives in the sloth’s fur and die there. As they decompose, they release nitrogen and phosphorus directly into the sloth’s coat.

Pauli describes the sloth’s peculiar water-absorbing hair as “almost like a hydroponic growth area” for algae.

More moths means more fertilizer, and more fertilizer means more algae, specifically Trichophilus, or “hair-loving algae,” a species found nowhere on earth except sloth fur. Pauli likens the effect to a ghillie suit (the head-to-toe camouflage gear snipers wear to vanish into foliage). The algae turns the sloth’s fur green enough to disappear into the forest canopy.

A sloth up high on a tree branch in a forest in Costa Rica with green fur. — The algae living on sloths gives their fur a green hue, helping the slow-moving animals blend into the forest canopy. *Image: Getty Images /* zen rial

But that algae also serves another purpose beyond being cool living camo. It’s also a potential food source for these slow-moving mammals.

Do sloths farm algae on their bodies? Maybe.

To find out whether sloths were actually eating this nutrient-rich algae, Pauli and his colleagues did something that sounds alarming but is apparently just a normal Tuesday in wildlife ecology: They pumped the stomachs of roughly a dozen three-toed sloths.

What they found wasn’t all that surprising: lots of Cecropia leaves, a staple of sloths’ diet. But they also found Trichophilus algae. And since Trichophilus exists nowhere on earth except sloth fur, there was only one way it could have gotten there: The sloth ate its fur. Testing the algae, Pauli and his team found it to be digestible and lipid-rich—a potentially valuable supplement to a diet of nutritionally poor leaves.

What the team of researchers don’t know is whether it matters. Is the sloth cultivating, munching on, and extracting nutrition from its own self-grown snack?

“It could be totally trivial and unimportant,” Pauli says. “It could be that they incidentally get a little bit in their stomach, it’s all by accident. It would be like the equivalent of me eating a Snickers bar too quickly and accidentally eating part of the wrapper.”

Or it could be that sloths are deliberately consuming it, extracting real nutrition from the algae growing on their own bodies. Whatever is driving it, Pauli is fairly certain of one thing: The sloth isn’t doing it on purpose.

“It’s not conscious—I don’t think the sloth is ever thinking ‘Time to re-up my algae.’ I think it’s more that individuals that have these behaviors, that fortify these relationships with these other species, see fitness benefits. That’s why we see these behaviors persist.”

In other words, this whole system—from flightless sloth moths to algae to sloth diets—may be helping sloths survive.

Which brings us back to that suicidal weekly commute. It turns out the sloth’s trip to the forest floor may be doing a lot more than answering nature’s call. In fact, it may be the key to maintaining the entire system. No ground trip, no moth-to-dung delivery. No moth delivery, no fertilizer. No fertilizer, no algae. And no algae means no camouflage, and possibly no nutritional supplement for an animal that can barely afford to lose either. Not bad for a bathroom break.

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