Once a week, three-toed sloths slowly descend from the leafy forest canopy to poop on the ground. Why do these sluggish mammals go on such a long and potentially dangerous journey instead of just letting it fly from the treetops? Scientists now believe the answer has to do with the odd, symbiotic relationship between sloths and moths.

A dangerous toilet

Only about 10 mammalian species — less than 0.2 percent of all mammals — have evolved to eat leaves and fruit that can only be found by climbing trees. Mostly that's because living high in the trees imposes certain lifestyle constraints. First, the animals must be small and light enough to live in that habitat; and second, must be able to digest a huge amount of plant matter, which is rich in fiber but low in nutrients.


Sloths are one of a few animals that make this lifestyle work, and they come in two flavors. Two-toed sloths have a relatively large home range of up to 140 hectares, and a comparatively diverse diet of fruits, leaves, insects, small lizards and carrion. By contrast, three-toed sloths have small home ranges (0.3 to 15 hectares) and dine exclusively on leaves from only a few select trees. As a result of this nutritionally poor, toxic diet, three-toed sloths have the slowest rate of digestion for any mammal. And, as you probably already know from watching cute sloth videos, they have an incredibly slow metabolic rate that amounts to less than half of what's expected for their mass.

The two sloth types also differ in their pooping habits. About once a week, a three-toed sloth will descend from the canopy, create a depression in the ground near the base of a tree with its tail, and defecate in the hole. It then covers its latrine with leaves and climbs back up. Two-toed sloths aren't so ritualistic — they often poop from the canopy, more than once a week.


This difference has long left scientists scratching their heads, wondering why three-toed sloths don't just let loose in the canopy like their two-toed relatives. After all, ground pooping puts the animals in the crosshairs of predators; in fact, more than half of all documented sloth mortalities occur when the mammals are near the ground. What's more, this weekly trek is estimated to use up approximately 8 percent of a sloth's daily calories.

"Given the heightened risk and energetic cost for a sloth to defecate on the forest floor, one would expect it to be an important fitness-enhancing behavior," wrote University of Wisconsin-Madison wildlife ecologist Jonathan Pauli and his colleagues in a recent study on the matter.


The Sloth's Moths

Scientists wondered whether the sloths do it to fertilize their tree homes. Or maybe they're using these latrines to communicate with other sloths.

But Pauli and his team believe the sloth's risky toilet quest is all to help the crawly denizens in the sloths' fur. You see, sloth fur isn't just a bunch of hair. It's packed with green from algae and is loaded with sloth moths, among other things. When a sloth poops on the ground, the adult female moths leave the mammal to lay their eggs in the feces. The eggs eventually hatch and the moth larvae feed on the poop; then, as adults, they fly up into the canopy to nestle in a sloth's fur.


But if the sloths are pooping on the ground to help the moths flourish, what do they get out of it? Do sloths and moths really have a symbiotic relationship or are the moths just mooching off the sloths? Pauli and his team reasoned that the sloths must get some sort of nutritional benefit by becoming these roaming, micro-ecosystems. "We hypothesized that this behavior sustains an ecosystem in the fur of sloths, which confers [unknown] nutritional benefits to sloths," they wrote. And maybe this added nutrition has something to do with the algal mat in their fur.

To test this, the researchers captured two-toed and three-toed sloths near San José, Costa Rica. They vacuumed all the moths off of the sloths, and counted how many critters each sloth had. They then cut off locks of fur, analyzed the nitrogen and phosphorous content in the fur and measured the amount of algae present. Finally, they used a long tube and syringe to suck up samples from the sloths' forestomachs, an organ where their food collects after being swallowed.


The link between sloths, moths and algae. Credit: Pauli et al., Proceedings of the Royal Society B.

The team found that the three-toed sloths harbored far more moths than the two-toed sloths. And as the number of moths increased, so did the amount of algae in their fur. Some of the sloths also had algae in their forestomachs.


The researchers believe that when the sloths help the moths thrive, the moths in turn help the algae grow, either by directly transporting nutrients or when they die and decay (fungi in the sloth fur break down the dead moths, producing more nutrients for the algae). And here's where the sloths actually get something out of this Swamp Thing scenario. The animals eat the moth-fed algae in their fur to supplement their nutrient-poor diets. The algae is easy to digest, and contains as many carbs and proteins as leaves, but up to five times more fat. "An unaccounted food source would help to explain why three-toed sloths are difficult to keep well-nourished in sanitized captive facilities," the researchers wrote. The algae would also have the added benefit of camouflaging the sloths from their natural predators in the canopy, such as the harpy eagle.

Still, some mysteries remain. The researchers have yet to show that the nutrition gained from eating the algae actually offset the energy the sloths use to poop on the ground once a week. There's also the business of showing that the sloths do, in fact, routinely eat the algae — the mammals aren't known to clean or lick their fur like some other animals.

If Pauli and his colleagues' theory does stand the test of further evidence-gathering, it suggests that sloths are actually a lot more industrious than their slow movements would suggest. They are farming their next meal by creating an entire ecosystem out of the fur on their backs.


Check out the study in the journal Proceedings of the Royal Society B.

Top image via Jerry Kirkhart/Flickr. Inset image via Christian MehlfĂĽhrer/ Wikimedia Commons.