The secret ecosystem living inside a sloth's fur

HostMost of us think of sloths as these slow, cute lumps that just hang from branches and sleep all day. They stay way up in the high trees where hunters like jaguars can't reach them, which makes a lot of sense. But once a week, they do something that looks like a total suicide mission. They climb all the way down to the forest floor just to go to the bathroom.

HostIt seems like a huge risk for such a small payoff. Why would an animal that moves like molasses leave its safe spot to do something it could just do from the branches?

GuestWell, it's a massive gamble. For a sloth, moving at all is a luxury they can barely afford. Their bodies are built to save every bit of fuel. Their body clock is so slow that a single leaf they eat might take up to thirty days to fully break down in their gut. So when they decide to climb down a hundred feet of tree trunk, they're spending about eight percent of all the energy they have for the day. That's a huge chunk of their budget.

GuestAnd the real cost isn't just the workout. It's the danger. Sloths are basically helpless on the ground. They're clumsy and slow, which makes them easy targets. Because of that, roughly half of all adult sloth deaths happen while they're on the forest floor. They don't just drop their waste and run, either. They use their short, stubby tails to dig a little hole, do their business, and then climb back up. It's easily the most dangerous thing they do in their entire lives.

HostI just don't get why they wouldn't just let it go from the canopy. It's not like they have neighbors to worry about. There has to be a better reason than just being clean.

GuestThere's, and it's because they're actually keeping a deal with a roommate. Deep inside a sloth’s thick, coarse fur, there are often over a hundred tiny moths living their whole lives. These are called pyralid moths, and they're what we call roommates that can't live anywhere else. They stay in the fur, but they need the sloth to go to the ground so they can have babies.

GuestWhen the sloth hits the floor to poop, the female moths fly off and lay their eggs right into that fresh waste. The moth babies have to have that dung to eat so they can grow. That weekly trip to the ground is the only window of opportunity these moths ever get to finish their life cycle. The sloth is basically running a shuttle service so these insects can keep their family line going.

HostBut what does the sloth get out of that? Having a hundred moths in your hair sounds like a nightmare, not a reason to risk getting caught by a jaguar.

GuestIt's because the moths are actually gardeners. Once those moth babies grow up, they fly back into the canopy to find a sloth to live on. As these moths live and die in the fur, they break down and leave behind special plant food called nitrogen. This nitrogen acts like a really strong fertilizer. That fertilizer helps a very specific type of green algae grow in the tiny grooves of the sloth’s hair.

GuestIf you see a sloth that looks a bit green or dusty, that's not moss or dirt. It's a thriving crop. The sloth’s fur works like a carefully managed water-based garden, and the moths are the gardeners providing the nutrients for the algae to grow. Without the moths, the garden would fail.

HostSo they're carrying around a farm on their back. I guess that helps them blend in with the leaves, but is that really worth the trip to the ground?

GuestIt's much more than just a disguise. The hidden deal is finally finished when the sloth eats its own garden. We have found that they groom themselves and snack on that green algae right off their own skin. This is the big payoff because their main diet of leaves is very poor in food value and very hard for them to break down.

GuestThis algae is different. It's easy to digest and full of fats and sugars that the sloth can soak up quickly. It basically gives them a high-energy snack that bypasses their slow-motion stomach. By risking that trip to the ground, they make sure the moths can breed, which makes the algae grow, and that algae gives the sloth the energy to make it through another week.

HostThe dangerous walk to the base of the tree is the only thing that keeps that whole tiny world alive.