Reputation: Sloths are lazy and stupid. They have to be because they look it. They are covered in algae. Yuck! They climb to the ground to perform a ritual defecation at the base of a tree, a risky business when there are eagle-eyed, fleet-footed predators around. They are bad at crossing roads. Silly sloths.

Reality: Slowness is the ultimate weapon in an evolutionary war against eagle-eyed, fleet-footed predators. What better way to blend in with the forest than to cosy up with algae and fungi. Ritual defecation is the sloth equivalent of speed dating, just without the speed.

Sloths get bad press. In just about every language on the planet, the common name for these creatures has roughly the same meaning. The English have the “sloth” (one of the seven deadly sins). The French go for the “la paresse” (“the lazy one”). In German, it’s “das Faultier” (“lazy animal”). Spanish gives us “el perezoso” (“the lazy bear”). And so on.

There is, of course, some truth to this common view. But stereotypes tend of obscure a greater underlying truth.

I suspect that sloths are not slothful at all, they are just bloody careful

Sloths are certainly slow (at absolute top whack they can travel at around 6 cm per second), but lazy they are not. Several years ago, I interviewed Rory Wilson, a biologist at Swansea University, in Wales, UK and the inventor of “the daily diary”, a nifty electronic gadget that records the movement of animals in incredible detail. At that time, the gizmo had mainly been used to study fast-moving creatures such as penguins and cormorants.

But Wilson was keen to use the device on slow creatures too and sloths were an obvious choice. The fact that they are slow doesn’t make them lazy, he tells me. “Nobody calls a bivalve lazy,” he says, talking about slow-moving shellfish such as mussels or clams. He has a point.

In a world populated by predators like big cats and raptors, you’d think that swift would be good. The monkeys that inhabit the same forest canopy as the sloths of Central and South America have gone for this option. But sloths just laugh in the face of such danger, slowly closing their eyes as the simians scatter through the treetops. Instead of running for cover, sloths have opted for an even more impressive strategy: invisibility.

For a sloth, one of the most feared predators is the harpy eagle. “They are great big eagles with the most appallingly powerful talons and a wicked beak,” says Wilson. “A sloth just doesn’t have the slightest chance.” Unless, that is, it can move so slowly that the eagle can’t see it. “I suspect that sloths are not slothful at all, they are just bloody careful.”

Pulling off this trick requires incredible strength. Imagine a male gymnast performing on the rings, muscles quivering as he holds himself in a crucifix position then raises his legs to the horizontal position. For a sloth, such acrobatics is small potatoes. “It’ll just move into the sitting crucifix position completely serenely as if there’s no muscular effort involved at all, as if gravity is just turned off.”

Making fur-dwelling friends

Under Wilson’s supervision, doctoral student Becky Cliffe has been using the daily diary on the captive inmates at the world’s only sloth sanctuary in Costa Rica. When I called at the appointed time, she was unable to make it to the phone due to a sloth going into unexpected labour. It’s nice that a pregnant sloth has the capacity to surprise, I think.

When I eventually get through to Cliffe, I ask about the microbes that thrive in the sloth’s lush coat. Each one of the sloth’s hairs is basically folded in half. “The only benefit we can think of to the hair having that structure is so that algae can grow,” she says.

So the sloths want fur-dwelling friends, but why?

A recent study of the sloth’s fur revealed it is home to a range of fungi that are bioactive against strains of the parasites that cause malaria and Chagas disease, and against some human breast cancer cells. For the authors, the sloth’s fur was a novel environment for discovering interesting fungi with medical uses.

You can’t imagine a sloth trying to lick the back of its head

But they also considered the possibility that these microbes confer some health benefit to sloths. “It wouldn’t surprise me,” says Cliffe. “Sloths are so bizarre, it’s perfectly realistic.”

Another paper suggested that sloths are in the farming business. As well as hosting fungi, they cultivate algal gardens in their fur, and these “highly digestible and lipid-rich” algae beef up their limited diet.

Cliffe gives this idea short shrift. “Sloths don’t groom with their mouths. They don’t lick their hair. They groom with their hands,” she says. What’s more, the algae tend to be concentrated around the head and the neck where the hairs are longest. “You can’t imagine a sloth trying to lick the back of its head.”

For Cliffe, the most likely explanation for the sloth’s interest in algae and other microbes is to use them to camouflage their fur, and hide from those nasty eagles. “Sloths in the wild are completely green and blend in to the canopy.”

Sloths have another talent that is not widely known: holding on (and we’re not talking about their ability to cling to trees).

“Their stomachs can weigh up to one-third of their body weight,” says Cliffe, more than twice that expected for an animal its size. This turns out to be part of an unusually continent digestive strategy. In comparison to other mammals, a sloth eats relatively little but what it does munch is subjected to an incredibly tortuous journey through the sloth’s intestine; one study from the 1970s recording the passage of food from the mouth to anus of one sloth at more than 50 days (cited here).

By the time the food reappears, the sloth has managed to extract every last nutrient. What’s left, says Cliffe, is “just a tough ball of very fibrous faeces.”

Ritual defecation

The sloth has an interesting toilet ritual that is the cause of much sniggering. Rather than going up in the canopy as one might assume, sloths climb down to the ground to perform a ritual dump at the base of a tree. This is an incredibly risky manoeuvre, one that is likely to lay these animals open to predation by big cats such as the jaguar. Mercifully, their impressive powers of retention mean that they don’t need to do this often, typically just once a week.

But why go to the trouble of going to ground at all? Why not release these fibrous nuggets like bombs?

An interesting truth strongly suggests that it must have something to do with sex. “When a female is in estrus, she will come down to the ground and go to the bathroom every single day and that happens between eight and ten days every single month,” says Cliffe. Given the amount of energy this requires and the increased risk of being spotted by a predator, she has to have a good reason to do this.

As is so often the case in the animal kingdom, sex is the obvious incentive. Leaving a humungous pile of fibrous dung, it seems, is the sloth equivalent of placing a personal ad. A female descends to inform any males in the area about her reproductive status and at the same time picks up on everything she needs to know about those who have been there before her. It’s like speed dating without the speed.

Sure, a gurning sloth looks like an idiot. But the truth about sloths is that humans have done a very bad job at figuring out why they do what they do. “There aren’t many mammals left on the planet that are so unusual yet we still know so little about,” says Cliffe.

Tweetable sloth facts:

A sloth at full tilt travels at 6 centimetres per second

A sloth’s stomach can weigh over one-third of its body weight

A sloth will typically only defecate once every 7 to 10 days

A sloth may take 50 days or more to digest its food