Why does a sloth poop on the ground when it spends the bulk of its time high in the trees? The behavior—oddly fastidious for the languorous creatures—is a hallmark of tree sloths: sloths chill amid the tropical canopy, and they only descend from their treetop abodes once a week to, well, take a dump.

What seems lazy to us strikes scientists as downright weird. Sloths have extremely slow metabolisms and restricted leaf diets, so a simple excursion down to the forest floor demands a lot of their daily energy budget. On the forest floor, they also face more predators. “It’s like if you had to go to the bathroom, and you were programmed to go run a 5K on an interstate before you could go to the bathroom. It’s really risky, and it’s really energetically costly,” says Jonathan Pauli, a mammalian ecologist at the University of Wisconsin in Madison.

So, if the risk is so great and the trip burns so much energy, then why do sloths do it? Some have suggested that it’s actually a protective instinct to defecate more quietly compared to the noisy canopy, while others have linked their ritualistic pooping to socializing with other sloths, who also descend to poop, while serving the ecosystem function of fertilizing trees. Pauli and his colleagues propose a new explanation in Proceedings of the Royal Society B: sloths have mutually beneficial relationships with algae and moths living in their fur, and these relationships require the lumbering beasts to trek to the forest floor to defecate.

A surprising array of critters—beetles, moths, bacteria, and fungi—make their home in a sloth’s fur coat. Scientists have always thought that the relationship is mostly one-sided. “They just kind of haul [the critters] around and clearly the organisms that live in the fur benefit. They receive a safe haven,” says Pauli. But Pauli and colleagues noticed that green algae (Trichophilus spp.) and poo-eating sloth moths (Cryptoses spp.) represent the most prevalent inhabitants of sloth fur. Algae can be taxonomically and geographically specific, while moths use sloth fur as mating grounds. When sloths descend, moths lay their eggs in their host’s poop. When the eggs hatch, larvae eat the poo and hop on a sloth directly above them.

"Because this behavior in sloths dovetails so well with the lifecycle of [moths],” says Pauli. “We thought, well maybe there’s actually some kind of mutualism that exists at their end.”

To learn more, the UW team captured 14 two-toed sloths (Choloepus hoffmanni) and 19 three-toed sloths (Bradypus variegates) on a cacao farm in Costa Rica. The scientists collected hair samples, stomach contents, and moths amid the fur. From the hair samples, they could estimate the amount of nutrients such as nitrogen available in what's best described as the "fur ecosystem."

The researchers found that sloths with higher levels of nitrogen in their fur had more moths and algae on them. These sloths tended to be the three-toed variety, which typically stick to a rigid schedule of descending to the forest to poop and then climbing back up—by contrast, two-toed sloths have been known to sometimes do their business while remaining in the canopy. Members of both sloth species also had evidence of Trichophilus algae from their fur in their stomach—these algae were found to be rich in lipids.

The scientists' results point to linked mutualisms between the sloths, the algae, and the moths: the sloth climbs down the tree to poop and, because the ground around the tree is littered with poop from previous descents, moth larvae growing in the poop can hitch a ride on the sloth's back. The moths find shelter and thrive in the fur ecosystem. They also bring nutrients to their new home from the poop they were born in and when they die and decompose. Those nutrients fuel algae growth in the fur, and the algae supplement sloths’ foliage diets with lipids that the scientists speculate could serve as a high-energy snack. Then, when the sloths go down to do their business again, moths hop on their back and the cycle starts over again.

Such mutually beneficial life cycles are not uncommon in the animal kingdom. Leaf cutter ants similarly harvest leaves to feed fungi that live on their abdomen--the fungi is ultimately eaten by the ants. The ants also carry a Streptomyces bacteria on their cuticles, which produces an antiboitic fungi-killing mold. Another example comes from Yellowstone National Park where tropical panic grass (Dichanthelium lanuginosum) subsists at very high soil temperatures with help from a fungus living at its roots. The fungus in turn gets its heat tolerance from a virus infection.

Three toed-sloths in particular only frequent specific tree species, and individual sloths themselves have particular trees that serve as their home base and primary leaf source. When organisms face such habitat constraints, they have to get creative and work together, according to Pauli. “It’s fun to think about how organisms with highly constrained lifestyles like that are able to maybe get around it, kind of cheat on the margins by supplementing their diet with things like algae to be able to successfully exploit those resources,” says Pauli.

It's possible that the algae also helps camoflage sloths from one of their key predators in the canopy, the harpy eagle (Harpia harpyja). But the researchers think this is secondary to algae's nutritional benefit. And so life in the sloth's fur ecosystem and in it's own ecosystem goes on, fueled by poo.