The three-toed sloth may struggle to get by on a diet of leaves, but the insects and algae living in its wet, fertile fur may provide important supplementary nutrition. Photo: M.Z. Peery/UW-Madison

For the three-toed sloth, a trip to the restroom is no rest at all. It’s a long, slow descent into mortal danger from the safety of home among the upper branches of the forest.

But the harrowing, and excruciatingly slow trip may be key to staving off starvation.

“What is striking about this behavior is the vulnerability,” says Jonathan Pauli, a professor of forest and wildlife ecology at the University of Wisconsin–Madison who studies the deliberate, molasses-slow animals in northeast Costa Rica. “It’s very dangerous. And the energy required is non-negligible for an animal that has such a restricted diet.”

Unlike the two-toed sloth — which is, shall we say, less restrictive in its choice of latrine — the three-toed sloths creep down trees every eight days or so to the base of their tree. Once on the ground, they dig a hole with their tails, defecate in it, and cover the pile with leaf litter. Then, it’s back up the tree in an achingly sluggish climb.

Jonathan Pauli

The fastidious ritual — nearly the only reason a sloth leaves the limbs of just a few trees — may be the leading cause of death among the sloths. More than half the deaths Pauli and collaborators documented during field research came at the claws and teeth of predators pouncing on sloths on or near the ground.

“There were historically more native large cats and canids, like foxes, jaguars and ocelots, and now more and more feral dogs hunting in these forests,” Pauli says. “A sloth on the ground is such an easy meal for them. So this risky behavior must confer some sort of advantage.”

Previous explanations for the sloth’s dangerous choice included communication with other sloths and a gracious gift of fertilizer to the just one or two trees a three-toed sloth calls home.

Neither of those notions seemed worth it to Pauli, who was struck by another possibility while watching a David Attenborough video describing the mold, insects and other crud that resides in the plodding animal’s thick fur.

Among the fur fauna are small pyralid moths with a particular attachment to the sloth’s near-weekly trip to poop on the ground.

When the sloth squats to do its business, some female pyralid moths will emerge from the sloth fur to lay their eggs in the sloth’s dung. The moth larvae then eat their way out of the sloth waste, emerging as moths that flutter back up into the tree overhead. There, they find a sloth and render themselves nearly flightless, damaging their wings to burrow into the wet, matted fur to mate and renew their life cycle.

“A sloth on the ground is such an easy meal for [predators]. So this risky behavior must confer some sort of advantage.” Jonathan Pauli

“That is a lot of reliance on the sloth,” Pauli says. “The moth is strictly dependent on the sloth in each step of its life. That made us wonder if the sloth was making this dangerous trip for the moth because the moth provides something relatively important to the sloth.”

In fact, Pauli’s research shows that the moths may give their all to the sloth in return for nursery for larvae and shelter and mating grounds for adults.

“Sloths live on the nutritional red line,” Pauli says. “Judging from their diet — which is all leaves from the tree they live in — they shouldn’t be able to maintain even the slow lifestyle that makes them so fascinating to a lot of people.”

Pauli and graduate student Jorge Mendoza turned to the sloth’s fur in search of another dietary contributor. Three-toed sloths tend to appear a mottled green color, thanks to algae growing in a combination of water trapped by unique cracks in the sloth’s hair and nitrogen released by fungi breaking down dead pyralid moths.

More moths, more nitrogen, more algae (which may also provide camouflage to the treed sloths, protecting them from flying predators). And the broad team of researchers — Pauli tapped entomologists, limnologists and bacteriologists — found the algae in samples taken from the stomachs of three-toed sloths.

Why does the sloth poop in the woods? Maybe because it’s hungry. Maybe to better hide among the leaves. Probably to lend the moths a three-toed leg up.

“It could be that even just small amounts of the algae makes ends meet, if only because it’s so rich in lipids,” Pauli says. “Having this highly-digestible, high-fat algae could be an important input that makes the difference when malnutrition is at stake.”

At least one question remains. It’s not clear how the algae get into the sloth’s stomach or how much of it they’re actually consuming.

“We think they’re getting it from themselves,” Pauli says. “They spent a non-negligible amount of their time raking their fur with their claws, and we know they’re so slow at it that the moths can stay ahead of the claws. So it’s not grooming. It may be part of ingesting the algae.”

Why does the sloth poop in the woods? Maybe because it’s hungry. Maybe to better hide among the leaves. Probably to lend the moths a three-toed leg up.

And that, according to Pauli, is another lesson in the complex and unusual way organisms as different as a tiny moth and a seemingly over-sized, tree-confined mammal need each other to get along.

“There’s some grandeur in these systems of mutualism,” he says. “It makes us think about organisms that exploit such narrow niches.”