Three-toed sloths only poop about once a week, and to do it they climb all the way down from the safety of the treetops to the forest floor — one of the most dangerous things a sloth ever does — before hauling themselves back up

You live in the treetops, safe from many things that want to eat you. You move slowly, rely on being hard to spot, and staying put is basically your whole survival plan. So what do you do about going to the bathroom?

If you are a three-toed sloth, you climb down to the forest floor, one of the most dangerous places in your world, make a small hollow, do your business, cover it up, and climb all the way back. But you only do it once a week or so.

That is a real behavior, not a joke. And the more you look at it, the stranger it gets. Jonathan Pauli, an animal ecologist at the University of Wisconsin-Madison, put it well: “It’s like if you had to go to the bathroom, and you were programmed to go run a 5K on an interstate before you could go to the bathroom.”

The slowest digestion in the mammal world

Start with why a sloth doesn’t need to go very often. Three-toed sloths eat almost nothing but leaves, which provide little usable energy. To extract enough from them, the sloth runs one of the slowest guts known. In feeding trials, markers took between 11 and 30 days to pass through, averaging around 16.

Run everything that slowly and waste accumulates gradually. Three-toed sloths poop and urinate only about once every four to eight days. When they finally descend, they may lose roughly 30 percent of their body mass after urinating and defecating. That alone would make the event worth talking about. What makes it a real puzzle is where the sloth chooses to do it.

The descent: what actually happens on the ground

A three-toed sloth climbs to the base of a tree, where it makes a small dip in the ground with its stubby tail, deposits its dung, covers it with leaf litter, and starts the long haul back up. The time spent on the forest floor is among the most exposed stretches of its week.

Sloths survive largely by not being noticed. They are built for stillness and camouflage, not speed, so the forest floor weakens their main defense. Ground predators and large owls are among the threats. One field team recorded a radio-collared sloth apparently killed by a spectacled owl during this descent, with feces still in its rectum and old droppings at the tree base. The researchers wrote that the event “suggests there are high risks for sloths climbing to the ground to defecate, a puzzling behavior with no clear evolutionary advantage discovered yet.”

You’ll often read that more than half of adult sloth deaths happen on the ground. That figure needs context. It comes from one research group, which reported that more than half of the adult sloth deaths it recorded were caused by predators on the ground; it is not a universal estimate for all sloths. The journey is also energetically costly: the descent has been estimated at about 8 percent of a sloth’s daily energy budget.

Why the risk may be worth it: the moth theory and other ideas

So why do it? This is where the science gets interesting and unsettled.

In 2014, Pauli and colleagues published a study in Proceedings of the Royal Society B that laid out the puzzle bluntly. A three-toed sloth, they wrote, “will, however, descend its tree weekly to defecate, which is risky, energetically costly and, until now, inexplicable.”

Their proposed answer involves the tiny moths that live in sloth fur. By coming down, the sloth carries those moths to the dung where they lay eggs. As the team put it, “by descending a tree to defecate, sloths transport moths to their oviposition sites in sloth dung, which facilitates moth colonization of sloth fur.” More moths were associated with more nitrogen and algae in the fur. The researchers proposed that the sloth then consumes the lipid-rich algae, supplementing its thin leafy diet.

Keep in mind that this is a tidy story built largely on one study showing associations, not proof that one step causes the next. A 2021 review in Cuadernos de Investigación UNED examined five explanations, including fertilizing trees, hiding scent, chemical communication and acquiring trace nutrients, and argued against the moth model. It proposed instead that ground defecation may simply be an ancestral behavior that has not faced enough selective pressure to disappear. More research is needed.

Even with the best explanations laid side by side, the weekly trek still looks like a bad bargain. An animal whose survival plan is to be slow, quiet and unseen gives up all three to visit the forest floor, paying in both risk and calories. Maybe the moths explain it. Maybe it is scent, communication, ancestral habit or something no one has tested cleanly yet.