What Lives in a Sloth's Fur? Algae, Fungus, and a Moth Found Nowhere Else

Q: What is the green algae in sloth fur, and does anything else on earth have it?

A three-toed sloth grows a green algae called Trichophilus, and one species of it has never been found anywhere on earth except in sloth fur — not in a pond, not on a rock, only on the sloth. When scientists ran the genetics on the stuff growing up there, the algae came back as its own host-specific species, hiding in plain sight on the w

Q: What is the sloth moth, and how does it end up living in the fur?

The sloth moth — Cryptoses choloepi and a handful of close cousins — is a small moth that spends its entire adult life crawling around in a sloth's coat, and it lives nowhere else. One study counted more than 120 moths on a single three-toed sloth; another put the average closer to 150. Sources genuinely disagree on the headcount, which m

Q: Why does a sloth climb all the way down to the ground just to poop?

A three-toed sloth descends to the base of its tree about once a week to defecate on the ground — and it's the most dangerous thing it does. Over half of all sloth deaths happen during this weekly toilet trip, when the slowest mammal alive is stuck on the forest floor with no escape plan. A single trip can drop up to a third of its body w

July 18, 2026

Slow Gains Club sloth t-shirt reading I Don't Sweat, I Marinate with a chubby muscular three-toed sloth

By Slow Gains Club · Field Notes

Here's a fact that reframes every sloth you've ever seen: that greenish, slightly damp-looking coat isn't neglect. It's a garden. A three-toed sloth walks around wearing a full-blown ecosystem — algae, fungus, and bugs that live nowhere else on the planet — and it does absolutely nothing to earn it except move slowly and refuse to shower. Respect.

What actually lives in a sloth's fur?

A three-toed sloth's fur is a self-contained ecosystem — green algae, several species of fungus, and moths, all sharing the same shaggy real estate. And it's crowded up there. Researchers have counted up to 950 individual invertebrates living on a single sloth at once: moths, beetles, cockroaches, mites, even the occasional worm.

The whole thing is possible because a sloth basically never moves and almost never grooms itself off. Most animals shed their hitchhikers by running, scratching, and bathing. A sloth just… doesn't. So the fur becomes the one thing a rainforest floor is short on — a stable, undisturbed place to live. Nobody's kicking you off. The tenant never gets up.

What is the green algae in sloth fur, and does anything else on earth have it?

A three-toed sloth grows a green algae called Trichophilus, and one species of it has never been found anywhere on earth except in sloth fur — not in a pond, not on a rock, only on the sloth. When scientists ran the genetics on the stuff growing up there, the algae came back as its own host-specific species, hiding in plain sight on the world's least busy mammal.

It grows because sloth hair is built for it. Each strand has tiny cracks and grooves running along its length that catch rainwater and hold it, so the coat stays damp enough to farm. The payoff is camouflage — a green sloth against green leaves is a lot harder for an eagle to spot than a brown one.

What is the sloth moth, and how does it end up living in the fur?

The sloth moth — Cryptoses choloepi and a handful of close cousins — is a small moth that spends its entire adult life crawling around in a sloth's coat, and it lives nowhere else. One study counted more than 120 moths on a single three-toed sloth; another put the average closer to 150. Sources genuinely disagree on the headcount, which mostly tells you nobody enjoys combing a wild sloth for moths.

The life cycle is the gross-but-brilliant part. When a sloth climbs down to poop, the female moths hop off, lay their eggs directly in the fresh dung, and the larvae grow up eating it. Once they hatch into adults, they fly up into the canopy, find a sloth, and move in. The sloth is both the apartment and the delivery service.

Why does a sloth climb all the way down to the ground just to poop?

A three-toed sloth descends to the base of its tree about once a week to defecate on the ground — and it's the most dangerous thing it does. Over half of all sloth deaths happen during this weekly toilet trip, when the slowest mammal alive is stuck on the forest floor with no escape plan. A single trip can drop up to a third of its body weight, too.

So why risk it, when pooping from the safety of a branch would be so much easier? Nobody's fully sure — and that's the interesting part. One theory says the sloth comes down specifically to keep its moths' life cycle going, since the moths need dung on the ground to breed. Other biologists think that's giving the sloth too much credit and the poop-on-the-ground habit came first. The argument's still open.

Does the sloth actually get anything out of hosting all this?

A three-toed sloth appears to eat its own algae — grooming it out of its fur and swallowing it as a nutrient boost. The algae is rich in fats and nitrogen, which is a big deal for an animal living on a diet of leaves so low-calorie it has to move slowly just to afford them. The green coat may double as a food shelf.

Whether the moths belong in that thank-you note is where scientists still spar. The popular version says moths fertilize the algae, algae feeds the sloth, everybody wins. Skeptics point out the moths might just be freeloaders in a very comfortable coat. Either way, the sloth isn't fighting any of it. It leans in.

Which is the whole philosophy, isn't it. The sloth doesn't burn its garden off with a workout or scrub it away in the shower. It hangs there and lets the whole thing steep — algae, moths, fungus and all — until the coat is basically seasoned. It doesn't sweat the process. It marinates. Wear the shirt, own the strategy.

See the “I Don't Sweat, I Marinate” shirt →