Sloths' dirty bum fur grows nutritious algae

Sloths have a reputation for laziness, but these easy-going mammals could be the world's best recyclers, suggests a new study that finds the poop of three-toed sloths attracts moths that colonize sloth fur and lead to nutritious "algae gardens," which the sloths eat.

The trees that the sloths live in, the moths, and the sloths themselves all benefit from the unusual chain of events, according to the paper published in the latest issue of the Proceedings of the Royal Society B.

Lead author Jonathan Pauli of the University of Wisconsin-Madison's Department of Forest and Wildlife Ecology first suspected something unusual was up after watching a David Attenborough video on YouTube called "Mouldy Sloth."

While footage shows a sloth covered in gloppy green algae and moths, Attenborough says, "Imagine an infestation in a shag carpet on top of a compost heap."

Pauli knew this seeming mess had to be for a reason. He and his colleagues went to San José, Costa Rica, to study three-toed sloths in their native habitat.

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Sloths spend most of their time resting or eating in the forest canopy but, once a week, they laboriously -- and at risk from wild dog and cat predators -- descend their trees to defecate, thereby fertilizing the trees, Pauli told Discovery News.

The researchers determined that the dung harbors dung-eating moth larvae that, as Pauli said, "eats its way out of the waste." Sloths obviously don't have toilet paper, so some waste sticks to their bums. When adult moths emerge from the poop -- both from the forest floor and from the sloth bums -- the moths fly into sloth fur.

But the unusual set of events doesn't end there.

The moths themselves then poop, die and decay. The rather gross mixture, when combined with other organic materials on sloth fur, results in the growth of what the researchers call green "algae gardens."

"Sloths eat the nutritional algae, which contains nitrogen and phosphorous," Pauli said. "It's like pepper on top of sloth waste. The green coloration also provides sloths with camouflage."

The relationship between sloths and moths is known as mutualism, where two species mutually and closely benefit from each others' lifestyles. Other examples include cleaner fish that eat parasites off of reef fish, and ants that defend acacia trees.

In this case, the moth presence could be a lifesaver for sloths.

Adriano Chiarello, a conservation biologist at the University of São Paulo, found that sloths are strict arboreal herbivores -- meaning plant-eating tree dwellers -- that primarily eat leaves, "survive on an energy-poor diet," and have "a very long passage time of digesta." In other words, their metabolism is so slow they can wait a week before pooping.

Pauli said the sloths groom each other while in the trees. While doing so, they ingest the algae, which provides the much-needed nutritional boost.

It's a tightly structured system without much flexibility.

Pauli said less than .2 percent of all animals are arboreal herbivores. Another well-known example is the koala.

"Herbivores (like cows) tend to be very big because they often carry within themselves a ton of machinery to hold and emulsify their food," he said. "On the other hand, life as a prey animal in trees necessitates a smaller body size, so arboreal herbivores are caught between these two extremes and have a constrained lifestyle as a result."

Pauli and his team are conducting a larger study of sloths and their genetics in hopes of learning more about them and improving their conservation. Some species of three-toed sloths, such as the pygmy and maned three-toed sloths, are among the world's most critically endangered species.