TWO-TOED AND THREE-TOED SLOTHS AREN'T ALL THAT SIMILAR. The cute little babe pictured. It's a good thing sloths don't have to go to school. They'd never make it on time. These drowsy tree-dwellers sleep up to 20 hours a day! And even when they are awake, they barely move at all. In fact, they're so incredibly sluggish, algae actually grows on their fur.

1. How Fast To Sloths Move
2. How Sloths Move

Sloths’ reputation as lazy, slow and stupid creatures owes much to French naturalist Georges Buffon, who described the tree-dwelling mammal as the “lowest form of existence” back in 1749. Buffon’s assessment has endured for centuries, but much of the criticism directed at sloths is unwarranted. As zoologist Lucy Cooke explains for The Day, the sloth’s sluggish lifestyle is a deliberate survival strategy that has enabled it to maintain a place on Earth for nearly 64 million years.

But sloths don’t just live life in slow motion: They might even be able to put their metabolism on pause, a new study published in PeerJ suggests. The slowpoke is now the only mammal known to be capable of temporarily shutting down its metabolism without entering into a state of lethargy or hibernation, a behavior that’s more typical of reptiles and birds, Jason Bittel reports for National Geographic.

According to a blog post published on the Costa Rica-based Sloth Conservation Foundation’s website, this scenario plays out when sloths experience very hot or cold temperatures. Most mammals facing similarly extreme conditions, or those that fall outside of a comfortable temperature range known as the thermoneutral zone, respond in a manner completely anathema to the sloth’s slow-down.

Within the thermoneutral zone, most mammals can control their body temperature without using up too much energy, but outside of it, they must expend vast amounts of energy. As zoologist Rebecca Cliffe, the study’s lead author and co-founder of the Sloth Conservation Foundation, tells Bittel, animals’ physical responses to hot and cold, such as shivering, sweating and panting, help them regulate their internal temperature but take a heavy toll on energy levels.

Reptiles and birds, on the other hand, don’t have a thermoneutral zone. When it’s cold, they expend minimal amounts of energy, and when it’s hot, they use lots of energy. As the foundation notes, this occurs because the animals are unable to control their body temperatures, and metabolic processes tend to work faster at hotter temperatures regardless of whether they’re operating in mammals, cold-blooded critters or avian fliers.

Scientists have long known that sloths defy easy categorization. Sloths are often likened to reptiles, Cliffe writes for The Conversation, because they both adopt a slow pace in order to conserve energy. It would make sense, therefore, for sloths experiencing high temperatures to exhibit a higher metabolic rate and sloths experiencing low temperatures to use up very little energy.

To test this hypothesis, Cliffe and her team placed eight three-fingered sloths into individual, temperature-controlled chambers and monitored their oxygen levels as the temperature rose and fell. As the chambers got hotter, the sloths used up more oxygen (and energy), but once the thermostat hit 86 degrees Fahrenheit, energy levels started sliding back down.

The results were surprising, to say the least, as the foundation’s blog post explains:

This reduction in metabolic rate at high temperatures is the exact opposite of what typically happens in all other animals.

Instead of using up vast amounts of energy when trying to cool down, the sloths in the study actively depressed, or slowed down, their metabolism—and they did it without entering a state of torpor, aestivation or hibernation (essentially all synonyms for periods of intentional inactivity).

When other large mammals face extremely hot or cold temperatures, they can enter a similar hypometabolic, or lowered metabolism, state, but in doing so, they tend to become lethargic. As Roberto Nespolo, an evolutionary biologist at the Austral University of Chile, tells National Geographic’s Bittel, such states find the animals’ body temperatures dropping dramatically and rendering them unresponsive. The sloths, however, maintained their body temperatures while remaining fully awake.

Nespolo says the team’s new findings remind him of birds’ energy patterns. King penguins, for example, appear to conduct deep sea hunts without warming their stomachs, potentially saving energy and enabling them to stay underwater for longer periods of time.

The explanation behind this unexpected strategy is likely related to the “metabolic knife edge” Cliffe says sloths navigate on a daily basis. All animals must balance the energy they consume with energy taken in to ensure their survival; for sloths, this is a highly tenuous task. Sloths can only eat a specific group of leaves, and unlike most animals’ nutrient-rich food sources, these leaves are both lacking in nutrition and difficult to digest. As a result, sloths have to keep a careful eye on the amount of energy they use each day.

Ultimately, sloths facing scorching temperatures can probably do little beyond moving into the shade and lying still rather than wasting energy on panting, sweating or similar cool-down efforts.

“You depress your metabolism and you just sit still and wait for the heat to pass,” Cliffe tells Bittel. “So it does make sense, but it was totally unexpected.”

Oct. 20 is International Sloth Day. Impress your friends (and ace your next Buzzfeed quiz) with these fun facts about sloths.

What is a sloth?

Sloths are the stars of memes and viral videos. They make us laugh, and they make celebrities cry. But these social media darlings have been around far longer than the internet. Millions of years ago, giant ground sloths the size of elephants roamed the planet. Some were nearly 20 feet long from snout to tail, with massive claws for pulling tree branches down to eat.

How Fast To Sloths Move

Today’s sloths are a little less intimidating. They max out at about 2.5 feet and weigh between 9 and 17 pounds (about the size of a small dog). They have coarse, brown fur, long limbs and curved claws for hanging tightly onto tree branches — the adaptation to upside-down tree living. And they’re one of the slowest-moving animals on earth!

There are two living families of sloths: two-toed sloths and three-toed sloths. Truth be told, all sloths have three toes on their back feet. But two-toed sloths only have two toes on their front feet, whereas three-toed sloths (you guessed it) have three. Two-toed sloths are also slightly bigger than their three-toed pals.

So, are sloths marsupials or primates?

Trick question. Sloths are mammals, but they aren’t primates or marsupials – though the groups do share some similarities. Koalas, for example, are marsupials that live in trees, eat leaves and have slow metabolisms. But sloths and koalas developed these traits independently of each other. The two aren’t closely related. Sloths actually belong to the superorder Xenarthra and the order Pilosa, with a family tree that includes anteaters and armadillos.

Where do sloths live?

Sloths are found throughout Central America and northern South America, including parts of Brazil and Peru. They live high in the trees of tropical rainforests, where they spend most of their time curled up or hanging upside down from branches. Sloths will sleep, eat, mate and even give birth upside down!

Why are sloths so slow?

Sloths have leafy, low-calorie diets and very slow metabolisms to match. Their metabolic rate is only about 40-45% of what would be typical for their body weight.

Because of this specialized metabolism, sloths need to be frugal with their energy use. So, they move slowly and tend not to wander far from their small home ranges.

Can sloths swim?

Believe it or not, sloths are strong swimmers. Despite their clunky movement on the ground, sloths are nimble in the water where they use a sort of doggy paddle to move around.

Are sloths endangered?

The smallest sloth, called the pygmy three-toed sloth, is found only on a small island off the coast of Panama where it is critically endangered. Some species, like the maned three-toed sloth are considered vulnerable. Others, like the southern two-toed sloth (also called the Linne’s or Linnaeus’ two-toed sloth) are designated as least concern by the International Union for Conservation of Nature.

Are they dangerous?

You tell us:

In all seriousness, you’re unlikely to cross paths with a sloth in the wild (they rarely leave their treetop hideouts). But if you do, like any wild animal, it’s best to keep your distance.

Sloths typically rely on their camouflage to protect themselves from predators. However, when threatened, they can use their 3- to 4-inch-long claws and teeth to defend themselves. And despite their slow movements, sloths are surprisingly strong.

And there are other reasons not to snuggle up to a sloth. Each strand of a sloth’s coarse fur has grooves that run from top to bottom where two types of blue-green algae grow. The green tint of the algae helps sloths blend in to their leafy surroundings, but it also invites ticks, mites, beetles, moths and other creepy crawlies to the party. This little ecosystem created by the algae is so unique that some species, like the sloth moth, live exclusively on sloth fur!

So, do sloths have predators?

Jaguars and eagles are common predators of sloths.

How Sloths Move

Do sloths have tails?

They’re not much to look at, but three-toed sloths do have short, stubby tails.

What do sloths eat?

Sloths eat leaves, fruit and some select fresh green shoots (they’re not keen on crunchy twigs). Though mostly herbivorous, they may occasionally snack on a larval insect or other passive, protein-packed snack (like a bird egg). They lap dew drops off of leaves and fruits, and get additional water from their food.

Did you know that a sloth’s teeth grow continuously throughout its life? As sloths chomp on plants, their teeth get worn down, which prevents them from getting too long.

It takes a long time for a sloth to digest its leafy diet. So long, in fact, that a sloth usually only climbs down from its tree about once a week to poop!

How much do sloths sleep?

True to their name, sloths sleep for about 15 hours a day. They’re mostly nocturnal, sleeping during the daytime and waking up at night to eat.

How can I help sloths?

Sloths live in rainforests, where habitat destruction is a serious threat. Supporting the responsible use of resources (like choosing Smithsonian Bird Friendly Coffee) can help protect them from habitat loss.

They’re also impacted by wildlife trafficking. Sloths may be amazing and adorable but, like other exotic animals, they don’t make good pets. Their unique diet, lifestyle and health needs require specialized care. Instead, get your fix by visiting sloths at the Smithsonian’s National Zoo or taking home a sloth plush through the Adopt a Species program.