Why three-toed sloths risk their lives to help moths

Yahoo

04-06-2025

For centuries, people encountering sloths for the first time have reacted by ridiculing them. In 1526, Spanish conquistador Gonzalo Fernández de Oviedo y Valdés wrote that the sloths he'd seen in the American tropics were 'ugly,' 'useless,' and 'the stupidest animal that can be found in the world.' In 1749, French naturalist Georges-Louis Leclerc, Comte de Buffon, called them 'the lowest form of existence' and judged that 'one more defect would have made their lives impossible.'
Harsh words about a creature that's survived for at least 50 million years.
Yes, sloths have poor hearing and eyesight, and they're slow—the slowest mammal on Earth. But their lethargy is an energy-saving strategy; despite the inability to outrun threats, sloths have figured out what works for them.
And while they may appear to be solitary, they don't succeed alone. Sloths operate as one-third of a partnership with moths and algae, which both live in the mammal's thick fur (along with fungi and ticks and mites, oh my). 'Sloths are these fascinating, fantastic, weird mammals that have enlisted really unexpected organisms to help them make their living,' says Jonathan Pauli, a wildlife ecologist at the University of Wisconsin­–Madison who has studied this symbiosis.
After a three-toed sloth has lounged for days in the forest canopy, napping and grazing on poisonous leaves—slowly, so that its liver and four-chambered stomach can break down the toxins—the time comes for its weekly poop. Rather than rain droppings from high up, the animal makes a drawn-out descent.
As captured on camera in National Geographic's Underdogs series, the move is exceptionally risky, burning a tenth of the sloth's daily calories and exposing it to predators on the jungle floor, where it's mostly—but not always—helpless to defend itself. More than half of the sloth deaths documented by Pauli's team of researchers in Costa Rica occurred on bathroom breaks. But if a jaguar isn't waiting at the tree's base, a sloth digs a small depression in the soil—a toilet bowl—and relieves itself at last.
Cue the moths. Females, including species found only on sloths, lay their eggs in the fresh dung. The eggs become coprophagous larvae, feeding on the feces, and larvae become adult moths that flutter off in search of their own hairy habitats. As many as 120 moths have been counted on a single sloth.
Moths fertilize sloth fur with nutrients, like nitrogen, whether by delivering fecal matter there or dying and decomposing. Along with rainwater, nitrogen encourages the growth of algae. And algae give sloths a green coloring—effective camouflage from birds of prey patrolling overhead. The more moths on a sloth, the more nitrogen and algae. Sloths also eat the green stuff as a sort of dietary supplement. Although researchers haven't observed the behavior, Pauli's team tested the contents of sloths' stomachs and found algae.
What seems clear to researchers is that sloths, moths, and algae all benefit from their shared arrangement. And now, a team of scientists in Costa Rica is exploring whether the microbiome living on sloths could boost human health, too. Many experts believe sloths are resistant to illness or infection; testing fur samples, the Costa Rican team isolated previously unknown bacteria that may lead to new antibiotics. As concerns mount about superbugs that are capable of defeating existing medications, what if remedies are discovered on the backs and bellies of sloths?
An animal maligned in the past as useless and an unlikely survivor would prove to be a hero, sophisticated enough to save human lives. Perhaps we were awfully slow to recognize the sloth's potential.
Underdogs will premiere on National Geographic June 15th and stream the next day on Disney+ and Hulu. Please check local listings.