Getty Images

Correction appended Jan. 23, 2014

Among the greatest mysteries of the tropical rainforest are the pooping habits of sloths. Really. Those furry, slow-moving tree dwellers almost never descend from the safety of the tree tops—except for once a week, when nature calls. It’s a dangerous and often lethal potty break. On the forest floor, they are spectacularly vulnerable to predators, and the question biologists have been asking for years is, why descend at all? What possible benefit could make this life-or-death journey better for the sloth than simply cutting loose, as it were, from the safety of a tree?

Theories abound: Maybe the sloths are somehow picking up minerals from the soil that their leafy diets don’t provide them. Maybe they are fertilizing their favorite trees with their poop. Or maybe it has to do with the other species that call the sloths themselves home. Sloth fur is populated by colonies of moths and flourishing coats of green algae. The moths are known to leap onto sloth poop to lay their eggs before returning to their host when the bathroom break is through. But symbiosis being what it is, there ought to be some benefit to the sloth from this arrangement too.

Like many sloth scientists, Jonathan Pauli, an assistant professor at the University of Wisconsin, Madison, has pondered the poop question in his free moments. In this week’s Proceedings of the Royal Society B, he and his colleagues present data suggesting that the moths may indeed be helping the sloths, by somehow feeding the algae in their fur, which the sloths in turn might be eating to supplement their diet.

Pauli’s has long studied the population dynamics of radio-collared sloths in Costa Rica, and the riddle of the weekly poop treks had always puzzled him. Sloths don’t eat the moths, eliminating one obvious explanation for their willingness to tolerate their guests and brave the forest floor for them. And the moths themselves, which Pauli says are little more than “flying genitals” by the time they reach maturity, don’t produce their own poop that could somehow benefit the sloth’s fur.

But that doesn’t mean that the moths, once they die, don’t in some other way contribute to the sloth-moth-algae ecosystem, perhaps by decomposing into nutrients that the algae can use. To probe idea, Pauli and his team needed to see whether more moths meant more nutrients in the fur and in turn, more algae. They also needed to establish that the sloths might be eating the algae in the first place—which had always been theorized but never proven.

Their first step was to vacuum all of the moths off a group of sloths, snip a lock of algae-coated hair from each animal, and, using a long thin tube and a syringe, sucked up samples from the sloths’ forestomachs to check for the presence of algae. They found that more moths correlated with more ammonium (NH4+) in the fur, a potential nutrient source for the algae, as well as with more algae. Eight out of twenty-eight sloths had the algae in their stomachs.

To Pauli, those data suggest that the moths are making a significant contribution to the sloth’s upkeep. “It seems like the sloths are potentially obtaining something in terms of a nutritional input from cultivating or at least helping algae to grow on their fur,” he says. Supporting that idea is the fact that the algae is easily digestible and rich in fat, which could make it a quicker source of calories than the tough leaves that make up most of sloths’ diet.

The story has at least one big hole, however. There’s no evidence that sloths lick themselves, says Bryson Voirin, a post-doctoral research at the Max Planck Institute of Ornithology, who studies sloth behavior; their grooming consists mainly of scratching with their claws. The algae is deeply embedded in individual hairs which means it would take some effort to extract. It’s hard to see how the sloths could consume enough of it to make a difference in their diet—and to make the journey to the ground worth it.

“It seems like a really convoluted way to get whatever they’re absorbing from the algae,” Voirin says. Pauli acknowledges that difficulty, but maintains that this hypothesis is promising. “It’s been the best thing, in my opinion, that’s been proposed before,” he says.

To explore the mystery more fully, a next step might be to feed the moths a bit of radioactively tagged nitrogen that could be traced through the algae and the sloth. Another way to come at it might be to calculate exactly how many calories sloths are getting from the algae, to judge the true value of it as a food source. Regardless, the scientific world is richer for the fact that there are people out there who wonder about sloth poop at all—and richer still for the fact that they’re actually coming up with answers.

Correction: The original version of this story left unclear whether moths lay eggs in sloth droppings. They do. The uncertainty concerns the symbiotic payoff of that act.