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Can Moths Explain Why Sloths Poo On the Ground?

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.

The sloth-moth cycle. Credit: Pauli et al, 2013. Royal Society.
The sloth-moth cycle. Credit: Pauli et al, 2013. Royal Society.
This would explain why the three-toed sloth always defecates on the ground while its two-toed cousin will often do it from the trees. The two-toed forages on a wider range of food so, as Pauli found, it isn’t so reliant on the moths or the algae.

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.

Reference: Pauli, Mendoza, Steffan, Carey, Weimer & Peery. 2014. A syndrome of mutualism reinforces the lifestyle of a sloth. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.3006

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13 thoughts on “Can Moths Explain Why Sloths Poo On the Ground?

  1. Glad you included the paragraph about Chiarello’s observations (or lack thereof). I’d still possibly put more emphasis on the alternative hypothesis of algal camouflage. Sure the algae might be nutritious, but growing your own ghillie suit might still provide sufficient advantage for selection of this bizarre scatological instinct.

  2. My first encounter with a sloth was in Panama, (Atlantic) side I was studying termites, living in a trailer, and one day this sloth came along and swam out into the ocean toward the Caribbean. It was doing a very slow crawl. The maintenance guy from the lab swam out 200 yards and brought the sloth back. The sloth then literally hung out in a palm tree for the next 6 hours getting warm … Didn’t return to the ocean. Very odd.

  3. This is a very interesting idea, but I think there may be a bigger problem for this hypothesis than either of the two shortcomings mentioned. If the system works as described, it’s hard to imagine how the sloths’ unique pooping behavior could have evolved in the first place. If the moths do indeed “[fly] into the canopy and [colonize] more sloths” after they pupate, and they then provide some benefit to the sloths, that benefit would be divided among all the sloths in the area. Unless there were some mechanism that somehow enforced host fidelity — i.e., that helped the offspring of a particular moth to find and re-colonize the same sloth, then from the sloth’s perspective it’s better to be a freeloader — i.e., to poop from the trees, avoid the risks associated with pooping on the ground, and reap the benefits of other individuals’ ground-pooping. On a population scale, ground-pooping might help maintain this moth-sloth symbiosis, but selection acts on individuals, not populations. So, if the natural history of this system is as described, it would pose a significant barrier to the evolution of the “ground-pooping” strategy in the first place.

    The simplest mechanism to enforce host fidelity would be highly constrained movements of both moths and sloths; if neither organism disperses much at all, then you might get the level of host-fidelity needed to make this work. I don’t know enough about either sloth or moth movements to know whether this is plausible, however.

    [Good points, Neil, although sloths do seem to defecate at the bottom of preferred trees. No idea if the moths have similar fidelity. – Ed]

  4. Interesting theory but doesn’t really seem plausible. I have been studying sloths extensively in the wild for four years and have never seen a sloth lick or chew it’s fur. Sloths maintained in captivity in Costa Rica with a natural diet (but without algae) survive perfectly well. And blood analysis shows no difference between these animals and those in the wild.

    Perhaps most significantly, female sloths descend to the forest floor daily to defecate during oestrus. I have discussed this further in my blog here: http://beckycliffe.com/sloths-poop-ground/

    But it’s nice to see sloths hitting the science headlines for once!

  5. My wild speculation about this would be: a virus, spread by moths, controlling the sloth’s behaviour to benefit itself (and the moths)?

    Yep, pretty wild. Forget it…};-)

  6. I´m a Chiarello´s student and I worked with the maned sloth (Bradypus torquatus) in the brazilian Atlantic forest; the hypothesis is interesting, but agreeing with my teacher, I never saw a sloth licking it´s fur.

  7. Very good questions (Losin, anonyatta). Here’s a little more background for those who are curious about the sloth-moth-algae relationship.

    First, it should be noted that there is much work yet to be done. The data presented, so far, articulate and support a very testable hypothesis.

    Regarding the question as to the lack of observations of cat-like preening:
    1) Sloths may not preen themselves as other animals do. However, if very few observations have been made during nighttime hours, it’s difficult to make the claim that sloths do not preen themselves at all. Perhaps they preen only a few times per week, and only at night. Many species reserve dangerous but necessary activities for the cover of darkness. That said, if maned sloths have been observed combing their fur with their claws, is it not possible that they are harvesting algae from their claws, either directly or indirectly? Each Bradypus species may do this somewhat differently. Bottom-line: the absence of proof is not proof of absence. What we know is that there is an obligate algal species living on/in the sloth’s fur, and that it makes its way into sloth guts in measurable quantities. The exact pathway that this algae takes will require more work to illuminate. (Future work might involve motion-sensing night-vision cameras).

    How might the ground-pooping behavior have evolved in the first place, given that these moths can fly and might disperse their benefits equally among all sloths in the population? First, consider things from the moth’s viewpoint. They are competing with many other coprophagous and detritivorous species on the forest floor. When the sloth poo hits the ground, it’s a mad dash for this precious resource. If you, as a moth, can ride the sloth and hop from the sloth to its dung while the dung is still steamin’ fresh, you’d have a huge advantage colonizing that resource before the beetles, isopods, and flies get there. Not only that, the dung pellets are all piled together, neatly consolidated for your young caterpillars’ efficient feeding. When the caterpillars are done feeding and have pupated into adults, they likely do not have to search much of the forest canopy because “their” sloth is right overhead. As the paper states, our three-toed sloth species tends to remain within a modal tree, so for the moths, searching out your sloth (which likely smells quite strongly of decay, ammonia) is made much easier and safer. Thus, there may be less sharing of moths among sloths. Still, it’s a very cool question and worthy of testing by an eager graduate student.

    Finally, this mutualism between sloths, moths, and algae may have started before sloths spent much time up in the tree canopy. Is it possible that as sloths spent more and more time up in trees, there was strong selection against pooping in the canopy, which would effectively scatter your pellets across the forest floor, making itreally hard for your moths to find your dung? Those three-toed sloths that climbed down their tree to poop and then stayed nearby would have had more moths in their fur, more dead moths as fertilizer for algae, and so more lipid-rich algae to eat in support of their main goal: making sloth babies. The two-toed sloth is much more active, less committed to a modal tree, and has a more varied, protein-rich and lipid-rich diet. It appears to need the moths less, and so doesn’t cater as much to the needs of its moth population. The paper bears this out: the two-toed sloth individuals had fewer moths, less ammonia, and less chlorophyll! Speaking of chlorophyll, this is a nice segue for the algae-as-camouflage idea. If algae is important as camouflage, why would the two-toed sloths, which are much more active than three-toed, have less? Presumably they would need more camouflage since they are spending more time moving from tree to tree, even dispersing on the ground at times. Maybe algae does help as camouflage, but we don’t have solid evidence of this yet.

  8. Thanks for the clarifications, Shawn. I hadn’t seen the original paper (paywalled) so I didn’t realize how circumscribed an average sloth’s movements are. Those constrained movements could certainly provide the kind of host-specificity I suggested would be required for the evolution and maintenance of this behavior. Cool!

  9. I wonder if there is some essential fatty acid or vitamin that the three toed sloth which has lost the ability to synthesize himself but still gets from the algae. Perhaps the two toed sloth still has the abiity to synthesize this nutrient and does not need so much algae on his fur. We as humans do this sort of thing but we keep our stinky symbiotes on the inside.

  10. Might be interesting to look at vit d synthesis between the two and three toed species. These canopy dwellers in chronicly low sunlight and thick fur might need the algae to provide enough vitamin D for the skeltal growth that seems critical for keeping them in the trees. Just a thought.

  11. Are all the three toed sloths always prefer to poo on the ground or they sometimes do it anyway when the height seems safe enough from predators? A couple of meters above ground perhaps then we can measure up the average distance and compare it with the algae-moth parameters.

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