Here’s a memorable encounter between David Attenborough and a three-toed sloth, as shown in Life of Mammals. Sloths normally spend their lives hanging from high branches, but this one ambles down to the ground at the 1:10 mark. “It wants to defecate,” says Attenborough, “and the only place it’s happy doing that, oddly enough, is down on the ground.”

This happens once a week. The sloth climbs down, digs a small bowl in the ground with its tail, and poos. It covers up its latrine with leaves before climbing back up.

This behaviour is bizarre. Sloths not only burn 8 percent of their daily calories on these laborious descents and climbs, but they are incredibly easy prey on the ground. Look at the video—Attenborough only has to slowly lean forward to scrag the animal. In fact, more than half of all sloth deaths are due to predators killing them while travelling to and fro their low latrines. If a sloth s**ts in the woods, predators seem to know.

So why do it? Surely, there must be some advantage. Otherwise, why waste energy and risk death, when they could just defecate from high branches and let gravity carry their poo away? Some people think that the sloths are fertilising their favourite trees, while others have suggested that they communicate with other sloths using the latrines.

But Jonathan Pauli from the University of Wisconsin-Madison has a different explanation. It involves thinking of a sloth as less of an individual, and more of a mobile ecosystem.

In their fur, sloths host a diverse community of fungi, algae, insects, mites and ticks. (In one case, scientists found 980 beetles in the fur of a single animal). Many of these residents are found nowhere else. To them, the sloth is the only world they know. The three-toed sloth even has adaptations that help it cultivate these partners. For example, its hair contains cracks that collect rainwater, and acts as miniature hydroponic gardens for growing algae.

The fur also contains moths. Cryptoses moths live exclusively on sloths, probably feeding on their skin secretions or algae. There can be up to 120 of them in one individual.

The moths are entirely dependent on the sloths, and specifically on their daring defecation descents. While the sloths do their thing, the female moths fly off and lay eggs in the fresh dung. The larvae eat nothing else. Surrounded by a banquet of delectable sloth faeces, they slowly transform into adults, before flying into the canopy and colonising more sloths. Beautiful.

So Pauli wondered: do the sloths also depend on the moths? Is that why they risk the dangers of terra firma, even though a squatting sloth is a sitting duck?

To find out, his team compared two types of sloth in Costa Rica—the brown-throated three-toed sloth, which always defecates on the ground, and Hoffmann’s two-toed sloth, which only sometimes does so. They cut locks of hair from the animals, sucked up all the moths using an “invertebrate vacuum”, and analysed the chemical composition of the remaining fur and algae.

They found that the number of moths, the amount of algae, and the nitrogen content of the fur were all connected. If an individual has more moths, it also has more algae and more nitrogen. And three-toed sloths have more of all the above than the two-toeds.

Pauli believes that the moths seed the sloth’s fur with nutrients that spur the growth of the algae. Maybe they’re transporting nitrogen-rich waste from the dung pile into the fur. Maybe they die in the fur and release nitrogen when they’re decomposed by fungi. Either way, they seem to fertilise the algae.

This matters because, according to Pauli, the algae are an important food source for the sloths. His team mixed the sloth fur with bacteria from a cow’s stomach and showed that the algae within can be easily digested. They also analysed the chemical make-up of the algae and found that it has as much carbohydrate and protein as the leaves that the sloths normally eat, but three to five times more fat. (It’s possible that the algae, by painting the sloths green, help to camouflage them from predators like harpy eagles.)

So, the moths help to fertilise the sloths’ algal gardens, which gives the sloths a valuable energy boost to supplement their otherwise poor diets. The sloths, in turn, defecate on the ground to help their partners-in-gardening to complete their life cycle. They may die, but that’s a risk they have to take. They are locked into this partnership.

View Images The sloth-moth cycle. Credit: Pauli et al, 2013. Royal Society.

But there are still a few loose threads. For example, the team haven’t calculated how much energy the algae could hypothetically provide, and whether that makes up for the cost of travelling to the ground once a week.

And Brazilian researcher Adriano Chiarello points out an even bigger problem with the hypothesis. He and his students have spent more than 1,000 hours watching maned sloths in the wild. “We never saw sloths behaving in a way that might suggest or indicate that they were somehow extracting algae or other nutrients from their fur,” he says. They’re not like cats; they clean their fur with their front paws rather than their mouths. “I don’t remember ever seeing a sloth licking or lapping its fur.”

So, how exactly are they eating the supposedly nutritious algae? “Perhaps sloths do this secretly, or solely at night when such behaviour would be even more difficult to witness,” says Chiarello. Or, perhaps they’re absorbing the nutrients directly through their skin. He’s not convinced by either possibility and, either way, “the smoking gun is missing”.

Still, Chiarello says the study is a strong piece of work. “Testing hypothesis with sloths is not easy as they are difficult to capture and observe in their natural environment,” he says. “The authors are making their best with available data.” He hopes that more will come.