The Story of Sloth Poop

What if you had to risk your life to go to the bathroom? Imagine what it would be like to know that you might not return home. For three-toed sloths (genus Bradypus), a once-a-week bowel movement can be life-threatening, yet they apparently are not aware of the risk they are taking or of the system they play a role in. The story of sloth poop involves a complex system with three species.

As everyone knows, and their name indicates, sloths are slow. Because of that, they are threatened by fast-moving predators. They have relative safety in the tree tops, but on the ground, they are sitting ducks, or sitting sloths. Being on or near the ground is the main cause of sloth deaths. Their slow climb down and back up is the most energy-intensive thing they do all week. Why don’t these sloths just do their business in the trees and let it fall to the ground? The answer involves another species.

Enter the sloth moths (Cryptoses choloepi) into our story. These moths live nowhere else but in the fur of the sloths. They spend their entire adult lives in sloth fur. The newly emerged moths find a slow-moving sloth, make a home in its fur, and permanently lose their ability to fly. After spending a short adult life in the sloth’s fur, one of two things will happen. The moth will probably die there. A second option for pregnant female moths is to hop off the sloth, land in the pile of poop, lay their eggs there, and die. The larvae that hatch from those eggs will feed on the sloth dung. As they eat and burrow their way into the pile, they create a chamber where they enter the pupal stage.

When the sloth moths complete their pupal stage, they emerge as adult moths. They fly for their first and only time upward into the tree canopy in search of a three-toed sloth. When they find their target, the moths are permanently grounded in the sloth’s fur. There they will live until they die, unless they are lucky enough to hop onto a pile of sloth poo during the animal’s weekly bathroom trip.

But that is not the end of the story of sloth poop. Another species enters the picture. The third participants in this system are the Trichophilus algae, which live only on sloth fur. As we said, many of the moths die in the sloth’s fur. As they decompose, they release nitrogen and phosphorus into the fur, and the algae use that in their photosynthesis. The chlorophyll in the algae turns the sloth’s fur green, giving it natural camouflage in the tree canopy. In addition to giving the sloth a way to hide among the leaves that it eats, researchers have found small amounts of the algae in the stomachs of sloths. Perhaps the lipid-rich algae also provide additional nutrition that the sloth does not get from the leaves.

Completing the circle of life, the following week, when the sloth makes that dangerous trip to the ground, more pregnant moths plop into the precious pile of poo, and the cycle repeats.

It seems challenging to describe a completely natural way for this system to get started. The sloth could just drop its dung from the tree canopy and be safe. The Cryptoses choloepi moths, which live only on sloths, would not be needed. The sloths could probably survive without the Trichophilus algae that live in sloth fur and nowhere else on Earth. Does the story of sloth poop suggest a designed system, or did it just accidentally evolve over time?

— Roland Earnst © 2026

Reference: popsci.com