The Incredible Reason Sloths Grow Algae on Their Fur

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Written by Jennifer Geer

Published: January 17, 2025

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A side view of a Brown-throated three-toed sloth who is hanging on one branch with his face towards you.
Rob Jansen/Shutterstock.com

While humans wouldn’t be very happy to find that organisms were growing on their skin, particularly fungi, algae, and insects, it works out pretty well for sloths. Sloths may be hosting entire ecosystems in their thick, dense fur, and algae growth on sloths can grow so great that it tinges their fur green.

The video above from the Natural History Museum in London details what’s going on in a sloth’s fur. Its skin and fur create a habitat filled with fungi, beetles, moths, and sandflies. But it’s the algae that may be the most important organism growing on a sloth. While hosting ecosystems on your skin sounds like a nightmare for humans, as the video explains, it may be a mutually beneficial relationship for the sloth.

sloth mom with baby

Sloths are solitary animals and live alone, except for mothers and their babies.

Where Do Sloths Live?

Sloths spend most of their time high up in the tree canopies of the tropical rainforests of Central and South America, including Brazil and Peru. They curl up in the branches or hang upside down for a snooze most of their day. They’re only awake a few hours per day and when they are awake, they are sluggish and slow. Though they look a little bit like monkeys with their long arms and shaggy fur, sloths are not primates. They’re actually more closely related to armadillos and anteaters.

How Algae Grows On Sloths

Sloths move incredibly slowly, traveling around 125 feet through the treetops in one day. When on the ground, they’re even slower and crawl about one foot per minute. If a sloth were to sprint, it could only go about 1.5 miles per hour. Because of the sloth’s slow nature and the many hours it spends sleeping in the tree canopy, organisms have a chance to not only land on the sloth’s fur, but also to make it their home.

There is a species of green algae that only lives on sloth fur. Sloth hair is unique among mammals in that it contains microcracks, making it perfectly suited for algae growth. In fact, when viewed under a microscope, you can see algae living inside the cracks.

As algae grows on the sloth, its fur turns a green color, camouflaging the sloth with its surrounding lush rainforest environment. The slow-moving sloth, with its long greenish coat, blends perfectly with the trees. Predators that hunt by sight will likely pass right by a sloth without even knowing. But the algae and the fungi take it one step further. They disguise the smell of the sloth so that predators that hunt by scent will only smell the jungle and not know they are near a sloth.

It’s in this way that sloths and algae have a mutually beneficial relationship, called mutualism. The sloth benefits from the algae’s disguise, and the algae has a place to live.

What Do Moths Have to Do With It?

The mutually beneficial relationship between algae and sloths involves another species living on the sloth as well. The algae needs nutrients to survive on the sloth’s hair. That’s where moths come in. There is a species of pyralid moth (Cryptoses choloepi) that only lives in sloth fur.

The moths lay their eggs in the dung of the sloth. The larvae hatch and eat the sloth poop. When they are adults, they fly up to the sloths living in the trees and colonize their hair. The moths are weak fliers and once they land on a sloth, they stay put. The moths spend their entire adult lives in the sloth fur. When they die, they stay in the fur and get broken down by the organisms growing there, such as the algae. A sloth can have up to 950 insects (including moths) living in its fur.

Why Moths (And Algae) Benefit From the Strange Way Sloths Poop

Sloths only poop once a week and they always leave their safe trees and head to the ground to do it. Fascinatingly, the moths don’t venture far from the sloths and can only reach the sloth dung to lay their eggs when the sloths are near it on the ground.

Scientists don’t know why sloths risk their lives to defecate on the ground when it would be much safer to stay hidden in the trees. More research needs to be done to find out the answer. What we do know is, the moths benefit when the sloths leave their trees to poop. The more moths that make the sloth fur their home, the more the algae can grow, and the greener the sloth fur becomes. The sloth has a perfect disguise, and the algae and the moths have a perfect home.


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About the Author

Jennifer Geer

Jennifer Geer is a writer at A-Z Animals where her primary focus is on animals, news topics, travel, and weather. Jennifer holds a Master's Degree from the University of Tulsa, and she has been researching and writing about news topics and animals for over four years. A resident of Illinois, Jennifer enjoys hiking, gardening, and caring for her three pugs.

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