Meet the sloth

Photo by Daniella Maraschiello. Retrieved from Wikipedia on 04-13-2023.

It’s spring, which means it’s almost the end of the semester. I don’t know about you, but I always feel frantic this time of year. Everyone is rushing around trying to finish things in one final dash of effort before the summer break starts.

So, I thought it would be a good idea to take a second to breathe by taking a closer look at an animal known for being slow: the sloth.

There are six different species of sloths, all of which live in the tropical forests of South and Central America. There are four species of two-toed sloths (family Megalonychidae) and two species of three-toed sloths (family Bradypodidae). The difference between the two is – plot twist – not the number of toes they have! Both two-toed and three-toed sloths have three toes on each foot. A more accurate name would be two- or three-fingered sloths since the difference in the digits is on the front limbs.

What exactly are sloths? They’re mammals, like us, but unlike us, sloths aren’t primates. Instead, sloths are Xenarthans with an evolutionary tree that includes anteaters and armadillos.

Sloths are also not nearly as lazy as popular culture makes them out to be. According to the Sloth Conservation Foundation, they sleep around eight or nine hours a day. This isn’t that much – howler monkeys may sleep for 15 hours a day, and a koala can sleep up to 20 hours.

But even if they aren’t sleeping, it’s true that sloths don’t spend a lot of time moving around. They are physically very slow creatures, which is a result of the way their body functions. However, being slow is evolutionarily adaptive for sloths! Their main predators, big cats like jaguars and birds like harpy eagles, typically hunt by sight; sloths rely on stealth and camouflage to avoid getting caught. Moving slowly comes in handy!

Bad eyes

Brief vision lesson: our eyes have rods and cones in them. Rods work when there’s low light, and they’re what lets us see at night. In contrast, cones work in bright light and enable us to see color.

Sloths have no cone cells in their eyes and so are entirely colorblind! They also can’t see very well in dim light and are completely blind in full daylight.

Since they have such trouble seeing, sloths move through the world relying mainly on touch and smell. This poor vision may be one reason sloths are so slow – you don’t want to run through the trees if you can’t see where you are going!

Low calories and slow digestion

Sloths eat leaves. And if you’ve ever tried to survive exclusively off salads, you know that leaves don’t have a lot of calories. Sloths have stomachs with multiple chambers to help them digest their high-cellulose diet.

However, it takes a LONG time for sloths to digest their meals – a single meal may be processed over an entire week! This slow digestion time combined with a low-calorie food source means a sloth’s stomach is constantly filled. They need to stuff in as many leaves as possible to get enough nutrients! A sloth’s weight is about 30% stomach contents.

Photo by Roy Luck. Retrieved from Flickr on 04-13-2023.

Low metabolism

Since they have such a low-calorie diet, sloths have a very low metabolism. In fact, it’s so low that sloths are thought to have one of the lowest metabolic rates among mammals. They are surviving at the lower end of their energy needs; there’s not a lot of wiggle room in the energy bank, and sloths can’t go around wasting energy!

This results in sloths struggling to maintain body heat like other warm-blooded animals. While we humans keep our bodies pretty constant over the course of a day, sloths can have a body temperature that can range between 77 and 95 degrees Fahrenheit (25 and 35 degrees Celsius).

Unfortunately, this means that sloths can starve to death even with a full stomach of food. If their body temperature gets too low, the microbes that live in a sloth’s stomach that digest plant matter can die off. And with the microbes dead, the sloth can no longer break down the plant matter it eats.

Weird muscles

Sloths have very low amounts of skeletal muscle in their bodies – they have 30% less muscle mass than we would expect in animals of similar sizes.

This lack of muscle means that while suspending is easy, walking is not. In fact, sloths cannot support themselves to walk on the ground; instead, they must move by dragging themselves.

Dragging yourself across the ground is hard work. Photo by Syedida. Retrieved from Wikipedia on 04-13-2023.

But just because they can’t walk doesn’t mean that sloths aren’t strong! They are three times stronger than humans and can lift their entire body with just one arm. This strength is from specialized muscle arrangements – instead of having lots of muscles for pushing things, sloths basically only have muscles that allow them to pull and grip. Sloths also have tendons that lock their hands and feet in place, letting them easily cling to tree branches.

Sloths are also really good at swimming. Using their arms to doggy paddle, sloths can move in the water three times faster than they can on land. And, by suppressing their metabolism even further and slowing down their heart rate, they can hold their breath for 40 minutes underwater!

One last exciting body thing: sloth organs are anchored to their rib cage and not weighing down their lungs. This means that, unlike us, a sloth can hang upside down and breathe without issues.

An ecosystem of fur

Although sloths technically have brown or gray fur, they will often look greenish. This is due to the algae living in their fur. Sloths can eat these green algae to supplement their diet. In return, the algae get a place to live and water, as sloth fur is good at absorbing moisture.

Swimming in algae. Photo by Thowra_uk. Retrieved from Flickr on 04-13-2023.

Other creatures, like various invertebrates and even fungi, will also take advantage of this algae ecosystem and live on a sloth. Some animals, like the sloth moth, live exclusively in sloth fur!

You might be worried that an algae, fungi, and invertebrate-covered creature would smell bad. But never fear! Sloths actually smell great. Only two-fingered sloths can sweat, and even these species only sweat on the tip of their nose. This means that sloths don’t have body odor like humans do; instead, they smell like the jungle.

A weekly poop

Remember how it takes a long time for sloths to digest their food? This means that sloths only have to poop about once a week!

And pooping is a big deal for a sloth. Instead of pooping up in the trees, a sloth will first descend to the ground. Then, it will dig a little hole at the tree’s base with its butt and tail. Only then will a sloth poop. And it’s a big poop – a sloth can lose up to a third of its weight in one bathroom break.

So while I encourage you to take life slow like the sloth, I recommend pooping more than once a week at the base of a tree!



Smithsonian’s National Zoo and Conservation Biology Institute

World Wildlife Foundation

The Sloth Conservation Foundation

World Animal Protection

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