Jean Baptiste Van Damme

GEORGE-LOUIS LECLERC, the Comte De Buffon, was the most famous naturalist on the planet in the middle of the 18th century, and he didn’t think much of the New World. He proclaimed the Americas “degenerate”, a sodden, miserable land filled with weak and inferior species. But Buffon reserved his most biting contempt for one creature in particular.

He wrote of their “too short” and “badly terminated” legs, of their “slowness, stupidity… and even habitual sadness”. “These sloths,” he continued, “are the lowest term of existence in the order of animals with flesh and blood. One more defect would have made their existence impossible.”

Buffon couldn’t have been more wrong. What he saw as shortcomings we now realise are exquisite adaptations that have allowed sloths to thrive in an exceedingly austere niche for at least 30 million years. In fact, the closer we look at sloth biology, the more we see just how hard evolution has had to work so that these notorious dawdlers can take it easy.

One reason we know so little about sloths is that they are surprisingly difficult to study. They live high in the canopies of South and Central America and are extremely hard to spot: they are small, they rarely move and their fur often gets matted with green algae, making them blend in with the leaves.

To figure out exactly how slow they are, in 2014 Jonathan Pauli at the University of Wisconsin-Madison and colleagues went to Costa Rica to measure the metabolic rates of three-toed brown-throated sloths and Hoffmann’s two-toed sloths. He found that while both species have extremely slow metabolisms, the three-toed sloth is a record-breaker. The rate at which it expends energy in the wild, known as the field metabolic rate, came in at 162 kilojoules per day per kilogram, meaning it has lower energy needs than any other mammal that isn’t hibernating, including renowned slouches like koalas (410 kJ/day/kg) and giant pandas (185 kJ/day/kg).

The long way down

Part of the reason sloths are such extreme energy savers is their diet. They are arboreal folivores, meaning they live in trees and eat leaves. It is a deeply unpopular lifestyle choice, occurring in just 0.2 per cent of mammal species, and for good reason: leaves tend to be rather difficult to digest and contain few nutrients. Some tree-living leaf-eaters, such as howler monkeys, get around this by gorging on massive quantities of the stuff.

Sloths have adopted a different strategy: they nibble a bit here and there, making sure to keep their stomachs full. And they don’t rush digestion. It can take anywhere from two days to nearly two months before swallowed food emerges again as dung, which makes this the longest digestive process on record for a plant-eating mammal. That’s particularly weird when you consider that among mammals, the digestion rate typically depends on body size, with big animals taking longer to digest their food.

A long and winding alimentary canal isn’t the only way sloths conserve energy. They also allow their body temperature to vary wildly compared with other mammals. Whereas humans hover within a degree of 38°C, the three-toed sloths Pauli studied allowed swings of nearly 5°C as the forest cooled or warmed around them. “That’s a huge cost saving,” says Pauli, because maintaining a core body temperature is energetically expensive.

But sloths still need a way to warm up. Shivering, favoured by most warm-blooded animals, is for creatures with energy to burn. Instead, Pauli says, three-toed sloths climb higher into the canopy each morning to make the most of the sun’s generosity. “They’re on the reptile end of being a mammal,” says Rebecca Cliffe, a sloth researcher at Swansea University in the UK.

Life as an extreme energy saver does have its drawbacks. “Sloths can’t jump,” says Cliffe. “They never do anything unless they’re holding on with at least two hands.” But even beyond saving energy, the sloth’s characteristic slow-motion upside-down walking might have another benefit: camouflage. One of the sloth’s main predators, the harpy eagle, relies on seeing its prey move. “Everything in the forest can eat them,” says Sam Trull, co-founder of the Sloth Institute in Costa Rica. “So they have to be careful to go undetected, and one of the best ways to do that is to be very slow and very quiet.”

Hanging upside down, completely still, for hours on end seems to do the trick. Sloths can do this in part thanks to their long, curved claws, which their giant ancestors used to excavate tunnels (see “My ancestor the anteater“), but now operate more like coat hangers. The constant grip is made possible by a lattice of tendons in the hands and feet that draw the digits closed while at rest.


Hummingbirds burn energy faster than any other vertebrate

Matthias Breiter/Plainpicture

But there seems to be more to their muscular abilities than that. We usually think about muscles as doing one thing well, says Michael Butcher, a zoologist at Youngstown State University in Ohio. An Olympic weightlifter, for instance, has muscles capable of small, powerful movements, whereas a marathon runner’s muscles are geared towards sustaining long periods of exertion. “But sloths break that rule,” he says. They have an uncanny ability to resist fatigue, as well as a surprising amount of strength.

“Why sloths climb all the way down to the forest floor to defecate, then bury the mess, is still a mystery”

To better understand how they do it, Butcher dissected a dozen sloth cadavers. He was surprised to see they had very little muscle tissue – roughly 10 per cent less than you find in other arboreal mammals. But what muscle there is appears to be extraordinary.

Most strikingly, sloth muscles seem to contain a unique set of enzymes that confers tolerance to heavy accumulations of lactic acid, which may help them resist fatigue as they hang out or move in super-slow motion. Butcher’s latest work, which hasn’t yet been published, suggests that sloths have enzyme profiles similar to fast-running cats such as cheetahs. “Sprinting is all about anaerobic power for short durations,” says Butcher. “So it is odd and fascinating that a sloth that hangs for extended periods of time matches that metabolic profile.”

The plot thickens when you consider that anaerobic power is less efficient because, while it can create energy quickly, it can only create a fraction of the energy that aerobic mechanisms produce. And yet for an animal that has spent millions of years trending toward energetic thrift, a little bit of immediate energy from time to time seems to be the least wasteful way to power their occasional perambulations.

For all these fresh insights, there is still a lot to learn about sloths. We don’t know why they climb all the way down to the forest floor to defecate, for instance, never mind why they bury the mess. It doesn’t seem very frugal. Perhaps it is a form of communication, says Cliffe. It could even be linked to a putative nutrient cycle involving the algae that colonise the sloths’ fur and certain moths that share this just-about mobile home, Pauli suggests.

One thing is clear, though: the more we learn about these extraordinary creatures and their unhurried lifestyle, the easier it is to appreciate how diet and metabolism can drive evolutionary adaptation. And that applies to us humans too.

In 2016, Herman Pontzer at the City University of New York and his colleagues compared human energy expenditure to that of chimpanzees, bonobos, gorillas and orangutans, and found that we burn calories 27 per cent faster than other primates. The researchers reason that this dramatic uptick afforded us not only enhanced brainpower, but also the opportunity for faster reproduction and longer lives. These changes probably occurred as we diversified our diets and ate more high-calorie foods, like meat. We also started to get fatter, probably as insurance against food shortages.

If that last fact hits a little too close to home at this time of year, don’t look to the sloths for lifestyle tips. It has taken them millions of years to acquire the behaviours and anatomy to live the way they do. You are unlikely to reproduce it in the six weeks you might invest in a New Year’s resolution.

My Ancestor the Anteater

Look at a three-toed sloth and a two-toed sloth side by side and you might think they descended from a common, tree-living ancestor. The truth is much stranger.

Genetic studies suggest that the two branches of living sloths hark back to entirely different genera of giant ground sloths. Two-toed sloths, it seems, come from a family of beasts called Megalonyx, roughly the size of grizzly bears, while three-toed sloths are most closely related to the elephant-sized Megatherium.

It is a great example of convergent evolution. But how did they both move from the ground to the trees? John Nyakatura at Humboldt University in Berlin has an idea.

Sloths are xenarthrans: they belong to the same group as anteaters and armadillos, both of which boast large, curved claws and powerful forelimbs. Nyakatura suggests that the last common ancestor of today’s sloths probably inherited these features from their giant ancestors, which were powerful diggers, before co-opting them for an arboreal lifestyle. If so, the common ancestor might have looked like the silky anteater, which can move upside down beneath branches.

Living, Fast and Slow

Sloths aren’t the only creatures with extreme energy lifestyles

FAST: Humming birds Flapping your wings 70 times a second is hard work, so it is little wonder these tiny birds have super-fast metabolisms. If humans had the same metabolic rate, we would have to eat 155,000 calories per day – that’s 77 times as much as we actually consume.

SLOW: Giant tortoises Living on islands prone to long droughts and food shortages, these behemoths can go months without sustenance. Alas, such superpowers backfired when European seafarers realised they could keep them below deck as a living larder, and their numbers plummeted.

FAST: Camel spiders Most arachnids are sit-and-wait predators, relying on webs or ambush tactics to take down their unsuspecting prey. Not camel spiders, properly known as solifugids: they run and run and run until they find something, anything, to satisfy their extremely high energy needs.

SLOW: Geckos Bradfield’s Namib day geckos use about one-quarter as much energy as other desert geckos. Not only do they move and eat very little, they also boast special scales that grab heat from the sun. They even have the ability to absorb 70 per cent of their daily water intake from fog.

FAST: Swordfish To help them zip around at lightning speed, swordfish have huge hearts relative to their body size and blood containing an unusually high concentration of oxygen-carrying haemoglobin. They even boast a gland in their heads to produce a lubricating oil that seems to reduce drag.

SLOW: Greenland sharks With lifespans exceeding 400 years, Greenland sharks are in it for the long haul. They swim slowly, grow just 1 centimetre a year and the females may not become sexually mature until they are 156 years old.

This article appeared in print under the headline “Life in the sloth lane”