Why Are Sloths So Slow?

When you imagine a sloth, you probably think of a simple, lazy creature that does very little other than sleep all day. In fact, you might wonder how such an animal survives in the wild at all. Even the very name “sloth” in most languages translates as a version of lazy. In 1749 when sloths were first described in the scientific literature they were labelled as “the lowest form of existence” – it is little 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? Why are they so slow? And why does it work?

Brown-throated Three-toed Sloth Bradypus variegatus Male (covered in algae) Aviarios Sloth Sanctuary, Costa Rica *Digitally removed piece of wood in background
Sloths have a rare genetic condition that leaves them almost completely blind.

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. The first piece in the puzzle to understanding the sloths slow pace is their eyesight. Recent 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.  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!

The second clue in the puzzle is the sloths low calorie diet. Both two and three-fingered sloths have a predominantly folivorous diet, meaning that they feed mostly on leaves with a notably low caloric content. But 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 sloths 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, digestion rate scales with body size, so larger animals should take longer to digest their food. Sloths appear to break this rule quite spectacularly. The exact rate of digestion remains unclear, but current estimations range from 157 hours to 50 days (1,200 hours) for the passage of food from ingestion to excretion! 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. So why don’t sloths just eat more? In accord with their slow rate of digestion, the sloths’ four-chambered stomach is constantly full, and so more leaves can only be ingested when digesta leaves 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. Indeed, the abdominal contents of a sloth can account for up to 37% of their ∼4.5 kg body mass! In short, this means that sloths can’t eat very much on a daily basis and therefore have barely 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.

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 given for 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 sloths 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 can exhibit daily fluctuations in core temperature of up to 10 ◦C. 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 ambient temperature.

In addition to a low and variable body temperature, sloths have also sacrificed muscle tissue. Although they might look quite large, most of a sloths visual mass comes from their coat of 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 a highly unusual muscle arrangement which gives them surprising strength with a very high resistance to fatigue.

A sloth can’t run away from predators like a monkey would and instead, they have to rely on camouflage

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 sloths main predators (big cats – Jaguars, Ocelots; and birds – Harpy Eagles) all primarily detect their prey visually, and it is likely that sloths move at a pace that simply goes unnoticed – sloths move slowly to avoid being identified as prey. They aren’t lazy, they are stealthy.


Dr. Rebecca Cliffe

Founder and Executive Director