A three-toed sloth hangs out in Madidi National Park, Bolivia. PHOTOGRAPH BY JOEL SARTORE, NATIONAL GEOGRAPHIC.

Three-toed sloths are so, well, slothful, that they come down out of their tree canopy homes only once every three weeks, to go to the bathroom.

Part of the reason for the limited time in the loo is that dumping could actually be deadly for these animals. Half of all documented adult three-toed sloth deaths are thanks to predators that pick them off near the ground. Climbing down from their treetop homes could also lead to accidental tumbles. (See “Were People Killing Giant Sloths in South America 30,000 Years Ago?”)

But new research finds that this bathroom time is necessary—and not just for the obvious reasons.

A symbiotic web of algae and moths enables the three-toed sloth to maintain its life in the slow lane, and it all hinges on the plodding animal’s monthly deposits.

Low Nutrients, Low Energy

The sloths of Central and South America, including the three-toed sloth, spend their days placidly munching leaves in the jungle canopy. They move so infrequently that fungi and algae grow on their fur.

Don’t be fooled, though. Their laziness serves a purpose.

Leaves don’t contain a lot of calories or nutrition, which means that sloths have to conserve their energy to survive on a relatively low-nutrient diet. They aren’t so much lazy as they are energy efficient.

Jonathan Pauli, a biologist at the University of Wisconsin-Madison, was studying these sloths when a question began to nag at him. Besides the risks the sloths incurred on their tri-weekly potty trips, the climbs are costly in terms of energy demands.

Each trip to the loo accounts for 8 percent of the animal’s daily energy budget—the equivalent of a human talking a brisk, 30-minute walk. Why, Pauli wondered, did these sloths expend so much energy just to poop?

Eats, Poops, and Leaves

Pauli started by looking at the dung itself. Pyralid moths (Cryptoses spp.) deposit their eggs in steaming piles of fresh poo, where they live and feed as larvae. (Related: “Moths Vibrate Genitals to Avoid Bats.”)

The larvae turn into moths, which go on to mate in the fur of the canopy-loving sloths. This relationship at first seemed one-sided, with the sloth neither helped nor harmed by the moths.

But since the energy demands of such climbs were so high, Pauli and his colleagues believed that the sloths were getting some sort of hidden nutritional benefit from defecating on the forest floor, likely through their association with the pyralid moths.

To test this idea, Pauli and colleagues compared the number of moths infesting the fur of the brown-throated three-toed sloths (Bradypus variegatus) with those in another species of sloth, called Hoffman’s two-toed sloth (Choloepus hoffmanni). The researchers also compared the concentration of nitrogen and phosphorous on the fur and the amount of algae colonizing each type of sloth.

It turns out that the three-toed sloths carried more moths, algae, and nutrients on their bodies than their two-toed cousins did, researchers report online January 21 in the journal Proceedings of the Royal Society B.

The moths are somehow adding nitrogen to the three-toed sloths’ fur, which allows for the growth of more algae.

Researchers aren’t exactly sure how the moths are doing this: Insect droppings could be seeding sloth fur with extra nitrogen, or the moths could be transporting the sloth’s own dung back to the animal. The sloths, then, are going to the ground not so much to find a comfortable pooping platform but to get some quality time with some moths. The moths, then, seem to spur the growth of another food source for the sloths: algae.

Algae is quite nutritious and contains a lot of lipids. As the three-toed sloths groom themselves, they also ingest the algae. Thus, going to the bother of providing room and board for pyralid moth eggs on the forest floor really did benefit the sloths. The moths bring nitrogen and algae, which means more food for the sloths.

Two-toed sloths eat a wider range of plant products, descend from the trees more regularly, and readily poop from anywhere. They don’t seem to need the extra nutrition.

Pooping may be a dangerous activity for three-toed sloths, but the benefits appear to outweigh the costs.

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