Why Are Sloths So Slow?

Why Are Sloths So Slow?

When you imagine a sloth, you probably think of a simple, lazy creature that does little other than sleep all day. In fact, you might wonder how such a slow-moving animal survives in the wild at all. Even the word “sloth” in most languages translates to a version of “lazy.”

In 1749 when sloths were first described in scientific literature they were labeled as the lowest form of existence. It is not surprising that sloths have been subject to such profound speculation and misinterpretation:

“Sloths are slow because they eat leaves that drug them”

“Sloths are so stupid that they mistake their own arm for a tree branch and, grabbing it, fall”

“If you cut the head off a sloth, the heart will continue to beat for 15 minutes……”

I have heard it all. But what does it really mean to be a sloth? What makes them so slow? And why does it work?

Illustration of a three-fingered sloth by French naturalist Georges Buffon who described them as the “lowest form of existence”/Image: https://www.idler.co.uk/article/book-of-the-week-the-unexpected-truth-about-animals/

The answer is surprisingly simple: Being slow is an incredibly successful strategy for survival. In fact, being slow has helped sloths to survive on this planet for almost 64 million years. It is obviously a winning tactic. But in order to understand exactly what it is, that makes them so slow and why it works so well, we have to look at the biology of these unusual animals in a little bit more detail.

Sloths have poor eyesight

The first piece in the puzzle to understanding the sloth’s slow pace is their eyesight. Research has shown that all sloths have a rare genetic condition called “rod monochromacy” which basically means that sloths lack the cone cells in their eyes that most other mammals have in order to see color.

This leaves them completely colorblind, only able to see poorly in dim light and completely blind in bright daylight. Sloths acquired this odd condition a long time ago – way before they broke off from anteaters on the evolutionary tree!

Sloths were originally ground-dwellers (check out the Giant Ground Sloths), and the sloths that we see today only took to the trees quite recently in their evolutionary history. As they were already mostly blind by this point, moving into the trees was a dangerous move. There are not many blind climbers, and those that do usually have amazing adaptations to cope with the lack of vision. You can’t run around in the trees if you can’t see where you are going – you will fall to your death! Slowness was the only option for sloths!

giant ground sloth
Modern-day sloths evolved from giant ground sloths who also had poor eyesight./Image: Nature

Sloths have a low-calorie diet

The second clue in the puzzle is the sloth’s low-energy diet. Both two and three-fingered sloths have a predominantly folivorous diet, meaning that they feed mostly on leaves with notably low caloric content.

Are sloths slow because of what they eat? Eating a low-calorie diet doesn’t explain everything – there are plenty of mammals that are folivores that move at a normal pace – howler monkeys for example. The difference lies in the sloth’s large, four-chambered stomach and extremely slow rate of digestion.

A diet of leaves doesn’t provide much energy. Have you ever tried to eat lettuce for a whole day?

For the majority of mammals, the digestion rate is proportional to body size, so larger animals take longer to digest their food. Sloths appear to break this rule quite spectacularly. Their exact rate of digestion remains unclear, but it could take anywhere from 157 hours to 50 days (1,200 hours) from the time a leaf is eaten to when it is excreted!

Sloths don’t eat much on a daily basis

In general, most folivores will compensate for a low-calorie, leaf-based diet by consuming relatively large quantities of food. For example, howler monkeys consume three times as many leaves per kilogram of body mass as sloths do.

So why don’t sloths just eat more? Due to their slow rate of digestion, the sloths’ four-chambered stomach is constantly full. More leaves could only be eaten when digested leaves exit the stomach and enter the small intestine.

This means that food intake and energy expenditure are likely limited by digestion rate and room in the stomach. In other words, sloths are not able to eat large amounts of leaves on a daily basis because their stomachs are already full of slowly digesting food! Indeed, the abdominal contents of a sloth can account for up to 37% of their ∼4.5 kg body weight! In short, this means that sloths barely have any energy at their disposal.

Brown-throated Three-toed Sloth Bradypus variegatusMaleAviarios Sloth Sanctuary, Costa Rica*Rescued and in rehabilitation program
As a result of their poor eyesight and energy-saving adaptations, sloths physically don’t have the ability to move very fast.

Sloths have a low metabolism

In order to survive on such a limited diet, sloths have one of the lowest metabolic rates amongst mammals – estimated to be just 40–74% of the predicted value according to their body mass! This means that sloths are probably surviving on the very edge of their energy budget – and therefore everything that they do has to be constantly geared towards saving energy. 

An obvious example of such energy-saving brilliance can be seen when we look at the sloth’s body temperature. Maintaining a stable core temperature is energetically very expensive, and sloths appear to have almost completely sacrificed this ability.

Similar to many poikilotherms, they rely on behavioral methods of thermoregulation (basking etc.) and their core temperature can change by 10◦C over the course of a day. This fluctuation is in stark contrast to most endothermic mammals, which are able to maintain a constant core temperature of approximately 36 ◦C regardless of the temperature outside.

Sloths have little muscle mass

In addition to a low and variable body temperature, sloths have also sacrificed muscle tissue. Although they might look quite large, most of a sloth’s visual mass comes from their unusually thick fur (probably another method of maintaining body heat).

Underneath all the hair, sloths are surprisingly skinny. Muscle tissue is metabolically expensive to maintain, and in order to save energy, sloths have just 30% of the muscle mass expected for a mammal of similar size. Despite this apparent deficiency, sloths have an unusual muscle arrangement which gives them surprising strength and very high resistance to fatigue.

Skkiny sloth facts suzi eszterhas
Sloths have remarkably little muscle mass underneath all of their fur/Photo: Suzi Eszterhas

Sloths can’t run but they can hide

As a result of their poor eyesight and energy-saving adaptations, sloths physically don’t have the ability to move very fast. They can’t run away from predators like a monkey would and instead, they have to rely on camouflage.

The sloth’s main predators (big cats – Jaguars, Ocelots; and birds – Harpy Eagles) all primarily detect their prey using sight. It is likely that sloths move at a pace that simply goes unnoticed in order to avoid being identified as prey.

They aren’t lazy, they are stealthy.

Brown-throated Three-toed Sloth Bradypus variegatus Male (covered in algae) Aviarios Sloth Sanctuary, Costa Rica *Digitally removed piece of wood in background
A sloth can’t run away from predators like a monkey would and instead, they have to rely on camouflage.


Dr. Rebecca Cliffe

Founder and Executive Director