Whether Sloths Mistake Their Own Arms for Branches, and Other Adorable Sloth Myths

Sam Westreich, PhD

Are these animals really dumb enough to fall out of trees due to their own incompetence?

Sloooooowly…. caaaaaarefully….Photo by Meg Jerrard/Unsplash

Sloths are one of the internet’s favorite animals, along with capybaras and baby elephants. There’s something undeniably cute about them; maybe it’s their always-squinting faces, or maybe it’s just how slowly they move in all their actions?

But there are also some myths about sloths, some of which are downright insulting. Commonly shared:

  • Sloths often fall out of trees and die because they mistake their own arm for a tree branch
  • Sloths have to climb down from trees in order to poop
  • Sloths can starve to death even with a full belly, because their digestive system is also super-slow

Are these myths true? Or are these nothing but falsehoods and fictions to bring down our beloved slow forest friends?

Let’s get some answers, from science — and hopefully you won’t need hours and hours to digest this article!

Do sloths mistake their own arms for tree branches?

Most myths about animals seem to come from everywhere and nowhere; “I heard this/I read it on the internet” are common origins.

But not this one! This myth has been widely publicized thanks to science fiction author Douglas Adams, who wrote a quote in his posthumous collection, “The Salmon of Doubt”:

“My absolute favourite piece of information is the fact that young sloths are so inept that they frequently grab their own arms and legs instead of tree limbs, and fall out of trees.”

Who knows where Douglas Adams picked it up, but it’s not true.

There’s a number of reasons why this is false:

  1. Sloths are widely observed and studied, as well as kept in captivity, and there are not documented examples of sloths grabbing onto their own arms.
  2. Sloths use long claws to climb along a branch, hanging from it, placing each arm on the branch in front of the other. (It’s the same way a child might shimmy along a rope, hanging upside down from it.) Because they place each arm in front of the other, the position doesn’t really let them grab their own arm.
  3. Sloths do have poor eyesight, but they mainly locate branches by touch. They try to verify that the branch can support their weight before climbing onto it; a sloth’s own arm would fail this test.

Check out this short video of a sloth climbing on branches:

Sloths do occasionally fall out of trees, but it is usually due to a branch breaking, rather than because the sloth was dumb and tried to grab its own arm.

Some articles also suggest that sloths can fall up to 100 feet without injury, surviving falls from trees, although I wasn’t able to find a peer-reviewed citation.

Do sloths mistake their own arms for tree branches? No.

Do sloths have to climb down from trees in order to poop?

Ah, a myth that has actual documentation! But like most questions that have a scientific answer, the answer is “things are more complex than can be answered in a single sentence.”

Specifically, there are two families of sloths: two-toed sloths, and three-toed sloths. Two-toed sloths sometimes descend to the forest floor to poop, but also sometimes defecate from up in the tree branches. Three-toed sloths, on the other hand, stick to a stricter schedule of descending to defecate.

Descending is definitely dangerous for sloths. Sloth predators include ocelots, jaguars, and eagles — and for most predators, the best time to get some tasty sloth meat is when it’s down on the forest floor. Most sloths killed by predators are caught on the ground.

So why descend? The most popular, scientifically accepted theory is because of a symbiotic relationship with certain species of moths (Cryptoses spp.) that live in sloth fur!

Here’s how that relationship works:

  1. When a sloth descends to the forest floor to defecate, sloth moths will lay eggs in the feces.
  2. The moth larvae hatch and develop, becoming adult moths that fly up to the canopy and seek out sloths.
  3. The adult moths will burrow into the sloth fur, where they mate with each other and die.
  4. Fungi will break down the dead moths, producing nitrogen, which fuels the growth of algae on the sloth’s fur (giving them a mottled, green appearance).
  5. Some moths survive, ready to deposit more eggs on the next sloth pooping session.

Sloths regularly groom themselves, and they swallow a significant amount of the algae growing on their fur. This algae may serve as a vital component of their diet, helping them get enough nutrition.

So the sloths may put themselves at risk by descending to the forest floor to poop, in exchange for maintaining a mutualistic relationship with the moths that live in their fur and fuel the growth of nutritious and calorie-rich algae.

Do sloths have to climb down from trees in order to poop? They often do — although some species do it regularly and others only do it some of the time.

Can sloths can starve to death even with a full belly, because their digestive system is so slow?

Another interesting myth! The answer here is also going to be tricky, because this claim is partially correct and partially incorrect.

Let’s start with sloth digestion. Sloths eat leaves (and, as we saw above, some algae). These are tough materials, so they need to ferment in the sloth’s belly in order to break down enough to be absorbed. It’s similar to how cows obtain nutrients from grass.

However, sloths — and the microbes living inside their gut — need to be at the perfect temperature in order to facilitate digestion. Too hot, or too cold? Those microbes won’t work.

To make matters worse, sloths are not very adept at regulating their body heat. They mostly move about in the tree canopy, exposing themselves to more or less sun to control their own temperature, but this won’t work at night or when temperatures are too extreme.

In extreme temperatures, mostly when it gets too cold, the microbes in the sloth’s gut die off, and the sloth loses its ability to digest the plant matter that it eats. If the sloth isn’t able to restore those microbes, such as by human intervention, it will starve to death — even though its stomach is full of food. The food is present, but cannot be broken down enough to be absorbed.

Can sloths can starve to death even with a full belly? Yes — but not because of slow digestion. It happens because extreme temperatures kill their gut microbes.

In summary: sloths are smarter than expected, have symbiotic relationships, and starve from climate change

Myths evaluated! Our lovable sloth friends have to deal with a decent amount of misconceptions on the internet — and even when a factoid is true, it may not be fully accurate.

Sloths are smart enough to not grab their own arms, mistaking their limbs for branches. Usually, when one falls out of a tree, it’s either due to a branch breaking, or from a territorial dispute with another sloth.

Sloths do descend to the ground to poop; it’s likely part of a symbiotic relationship with moths that live in their fur, providing nutrients to the algae that gives a sloth camouflage and extra nutrition.

Sloths can starve on a full stomach — but it’s due to their gut microbes dying from cold temperatures, since sloths struggle to regulate their internal body temperature. Climate change will likely lead to more starving sloths, due to increased temperature variation.

It’s great that the internet loves these slow, adorable creatures — and if we want to keep them alive, we should work to reduce our climate footprint and preserve their habitat. Sloths may be slow, but we can act fast!

Any other weird sloth myths or stories that you’ve heard?


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A microbiome scientist working at a tech startup in Silicon Valley, Sam Westreich provides insights into science and technology, exploring the strangest areas of biology, science, and biotechnology.

Mountain View, CA

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