The Mystery of Sloth Poop: One More Reason to Love Science

A long-puzzling relationship among sloths, moths and algae at last has an answer

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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.


Might the algae exist as symbiotic camouflage? Mammals arent green, but theres no better color for a defenseless slow moving creature in the jungle.


Not a big thing, but the sloths who climb down from the trees to bury their poop are three-toed, and the picture is of a two-toed.


NOT MADE BY NATURE! Just because something exists in nature doesn't mean it was invented or made by Nature. If all the chemicals necessary to make a cell were left to themselves, "Mother Nature" would have no ability to organize them into a cell. It requires an already existing cell to bring about another cell. The cell exists and reproduces in nature but Nature didn't invent or design it! Nature didn't originate the cell or any form of life. An intelligent power outside of nature had to be responsible.

Natural laws can explain how an airplane or living cell works, but it's irrational to believe that mere undirected natural laws can bring about the origin of an airplane or a cell. Once you have a complete and living cell then the genetic program and biological machinery exist to direct the formation of more cells, but how could the cell have originated naturally when no directing code and mechanisms existed in nature? All of the founders of modern science believed in God. Read my Internet article: HOW FORENSIC SCIENCE REFUTES ATHEISM

Only evolution within "kinds" is genetically possible (i.e. varieties of dogs, cats, etc.), but not evolution across "kinds" (i.e. from sea sponge to human). How did species survive if their vital tissues, organs, reproductive systems were still evolving? Survival of the fittest would actually have prevented evolution across kinds! Read my Internet article: WAR AMONG EVOLUTIONISTS! (2nd Edition). I discuss: Punctuated Equilibria, "Junk DNA," genetics, mutations, natural selection, fossils, genetic and biological similarities between species.

Natural selection doesn't produce biological traits or variations. It can only "select" from biological variations that are possible and which have survival value. The real issue is what biological variations are possible, not natural selection. Only limited evolution, variations of already existing genes and traits are possible. Nature is mindless and has no ability to design and program entirely new genes for entirely new traits.

What about genetic and biological similarities between species? Genetic information, like other forms of information, cannot happen by chance, so it is more logical to believe that genetic and biological similarities between all forms of life are due to a common Designer who designed similar functions for similar purposes. It doesn't mean all forms of life are biologically related! Also, "Junk DNA" isn't junk. These "non-coding" segments of DNA have recently been found to be vital in regulating gene expression (i.e. when, where, and how genes are expressed). Read my popular Internet article: HOW FORENSIC SCIENCE REFUTES ATHEISM

Read my popular Internet article, HOW DID MY DNA MAKE ME?

Visit my newest Internet site: THE SCIENCE SUPPORTING CREATION

Babu G. Ranganathan*
(B.A. theology/biology)


* I have had the privilege of being recognized in the 24th edition of Marquis "Who's Who In The East" for my writings on religion and science, and I have given successful lectures (with question and answer time afterwards) defending creation from science before evolutionist science faculty and students at various colleges and universities


@BabuG.Ranganathan "Author of the popular internet article"

  Gee well it cant be that popular if you have to advertise for it in the sloth poop article. 

Typically i would enjoy a hearty internet debate but i doubt i would stand a chance against someone with a BA in theology. I mean geez this guy has so much experience and knowledge he made it all the way into the comment section! They dont let just anyone in there.

  And moreover this guy has given lectures. But not just any lectures. Successful lectures. Wow. Keep blowing my mind man. So glad i found you under the mystery of sloth poop. Would you mind telling us why your omnipotent deity makes those poor sloths climb all the way down the tree to drop a load?



Is it possible that they are just creatures of habit and not terribly bright? ;-)