Why Are Sloths So Slow? How Slow Are Sloths?

A lot of us love sloths, and one of the main reasons is we can relate to them. Just like our buddies down in the rainforest, we’d love to spend all day just hanging out (pun intended) letting the warm tropical breeze blow through our hair, at least those of us who still have hair!

Being slow is such a part of a sloth's identity, that this what their name pretty much means. (Read about that here: Is it a sin to be a sloth?)

The slowness of sloths has also inspired some great products:

Light Autumn's super sloth pun plush

But why are sloths so slow? And how do they get away with it? There are two main reasons:  the sloth’s diet and its defense against predators.

Although some sloths have a varied diet, three-toed sloths are entirely vegetarians, or as scientists call them ‘herbivores’. And though human vegetarians usually eat plenty of high-energy food such as fruits and nuts, sloths subsist almost entirely on leaves, which are a plentiful food source in the rainforests where they live. I guess if you only ate lettuce you might be a bit lethargic too!

To keep their bodies functioning on such a diet, and the reason that they are so low, sloths have an incredibly low metabolic rate. In fact it is the slowest metabolic rate of all mammals.  As you can see in the chart, while a sloth only uses 450 kilojoules a day, a cat uses twice as much, and humans and elephants use much, much more energy. This means that sloths can’t spend their energy darting around quickly, and instead are one of the slowest creatures around, moving on average at about 100 feet per day from branch to branch, or taking 1 entire minute to move just one foot when crawling on the forest floor. (However, because of that upper body strength and a natural buoyancy, sloths are actually the fastest when they are swimming!) 

Likewise, since their diets have such little protein, and as a way to use nutrients efficiently, sloth’s bodies do not produce much muscle. Their legs are not strong enough for them to stand on them, the way humans and some primates do.  So, even if sloths are on the ground they primarily depend on their arms and upper body strength to move around. So, make sure you get enough protein in your diet so you don’t wind up crawling slowly around your apartment!

Does this leave you slightly less impressed by these cute tropical tree-dwellers? Well, they aren’t just lazy salad-munchers. According to a study carried out in Costa Rica by researchers from the University of Wisconsin-Madison, sloths have something of a superpower in that they are heterotherms, meaning they can adjust their body temperature when they need to preserve energy. That’s like you dropping your body temperature down below 98.6 if you don’t get that afternoon snack. 

Going back to the sloth’s diet, what they eat and how they process their food is actually quite remarkable. Although some sloths do eat some fruit and even some meat (insects and other small animals), the three-toed sloth almost only eats plants, more specifically leaves, and in particular the leaves from particular trees, namely the Cecropia tree.

You might love leafy foods like lettuce, spinach and kale, but don’t start working up an appetite for Cecropia leaves—they are extremely tough and full of toxins. Sloth may have evolved to eat the Cercropia’s leaves because most other animals avoid this plant, but in order to do so the sloth needs a special digestive system, including a multi-chambered stomach (yours just has one chamber) and special bacteria living inside the sloth to help breakdown the leaves. Even with that unique digestive system, it takes a sloth around an entire month to process its food (humans do this in about 7 hours). Another fun fact comes in at the end of that digestive process:  they leave their trees and go down to the forest floor to ‘do a poo’ only about once a week!

So, the sloth’s diet makes it slow. Is that the only reason why?  And isn’t it dangerous to be a slow moving creature in a jungle full of predators?  Well, actually, the fact that sloths are slow actually serves as a defense. The jungles of Central and South America where sloths live are also home to some of the most fearful predators such as jaguars, panthers, ocelots, boa constrictors, anacondas and hawks. Almost all of these animals depend on their vision to catch prey. When an animal like a monkey or a parrot scurries along a branch or takes off in flight, it catches the eye of one of the jungle predators and can wind up being dinner. Since a sloth basically never makes any sudden movement it can avoid getting spotted (and eaten) by the predators of the rainforest.  The next time your parents, partner, or roommates say you should get up off the couch and do something, maybe you can use the same excuse.

This is going to be our excuse for lying on the couch,

When sloths were first encountered by European explorers, they were given the name sloth as an accusation of being idle and lazy, but we now know why sloth seems to be lazy and it’s all about survival!

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