Getting the hang of it. (Photo by Masteraah)
Ah, the sloth. Has any animal’s name ever been so efficiently descriptive, so elegantly appropriate? It’s all right there, one word, five letters — sloth.
Sloths are curious creatures found in the jungles of central and South America. The word “sloth” existed before the scientific community knew about them, a word that meant slow, lazy, lagabout, mud-for-bones, or any number of terms both real and perhaps imagined by the author that come round to the same concept — just plain doesn’t do much. Like a teenager on summer vacation, the sloth fits this term perfectly, so perfectly that we slapped the word right on it as a name.
These creatures are highly distinctive. They are shaped somewhat like large monkeys with their long arms and flat faces, but they are not in any measurable way related to primates. As you know, they are most closely related to anteaters. It’s only that long arms and a generally monkey-like body shape are best suited for climbing in trees, and that’s what sloths do, so this is what they evolved to look like.
Sloths in fact spend almost their entire lives in trees, because that’s where they are safest from predators. But there are many predators in South America that can also climb trees, from the jaguar to large snakes, so what’s this all about?
The key to life as a sloth is in the name. Sloths are known for moving very, very, extremely, quite rather incredibly very slowly. But it’s not laziness. The slowness of a sloth is in fact its greatest defense against predators.
“Don’t mess with me. I’m warning you, if you do, I will sit here. I will sit here so hard, you won’t even know what didn’t hit you.” (Photo by Stefan Laube)
If that makes no sense to you, you’re just not thinking like a sloth. When you don’t move, you don’t attract attention, and when you don’t attract attention, predators don’t try to eat you.
With this in mind, sloths have taken the fine art of not moving to new levels. They spend about ten hours a day sleeping, and the rest is spent hanging upside-down from branches and either not moving or very, very slowly reaching out for leaves to eat. A sloth will only carefully move around in a tree to find new food, or for a bathroom break. When a sloth needs to do its business, it doesn’t simply squat and let it go off the side of a branch. No, the sloth is a more refined creature. About once a week a sloth will very slowly climb down to the ground and relieve itself, then very slowly climb back up.
In fact, scientists have wondered why they do this, because it’s easier to just go from the tree and let it drop, not to mention safer. We don’t really know for certain. Some think it is how sloths find each other for breeding purposes; others think the sloth is worried that going from the tree will make too much noise and attract a predator; still others think the sloth is trying to nourish the tree by burying its excrement near the trunk, thereby helping its own food supply. Or maybe they’re just really fussy. Who knows?
The best part of all this is that it means not only that the sloth digests its food very slowly, but that it stores its excrement inside its own body until it’s ready to do its business. If you kill a sloth close to its bathroom day, there’s a good chance that up to one-third of its body weight will be feces and urine that it was waiting to get rid of. So have fun with that.
This lifestyle serves the sloth very well. For one thing, when you don’t move much you don’t use energy, which means you need to eat less, which means you need to move less. It’s a very happy cycle. For another, the sloth is perfectly adapted for hanging upside down in trees. It is the only mammal with hair that grows toward the body instead of away from it. Look at your arm — the hairs all grow out toward the hand. Look at a dog’s leg — the hairs grow down toward the feet. Not so for the sloth. Its hairs grow the opposite direction, back toward the body’s core, because that way it will still protect against rain when the sloth is hanging upside-down, which it nearly always is.
In addition, sloths have long, curved claws that are not for digging or fighting — though they will take a swipe at you if you threaten it — but for hanging. The claws allow the sloth to hang without any effort, no use of muscle at all. They just dig the claws in, let themselves relax, and hang. This is so effective that even human poachers don’t bother sloths as much as they otherwise might. If you shoot a sloth while it’s hanging, it will simply keep hanging even when it’s dead, held up by its claws, and the poacher would have to climb up and get it. It’s the ultimate post-death screw-you.
Even a sloth that dies a natural death, say from old age, can sometimes remain hanging. They give birth while hanging upside-down, they eat upside-down, they sleep upside-down. To a sloth, the world looks crazy and backward when they stand up straight.
You might not expect it, but sloths can swim. However with all this talk of how slow they are, you might be wondering, really, how slow are they? A sloth in extreme danger of its life will turn up the volume and dash about at an incredible 13 feet per minute. Yes, per minute. Try moving that slowly across your living room without going insane. But remember, that’s only the speed of a really terrified sloth. Normally they only move half that fast at best, and usually even slower.
Two hours later, it finished this bite. (Photo by Fruitwerks)
There’s even a bit more to a sloth’s defense system than sheer slowness, though. They also have camouflage, and it’s about the grossest camouflage you can think of.
Each sloth is essentially its own slow-moving ecosystem. Their hair is host to a number of bacterial colonies that just love sloths, love them so much that a sloth is the only place they will grow, and each baby sloth gets its own bacterial colony from its mother. The bacteria go nuts and cover the sloth’s hair and skin with patches of colour that help the animal blend in with the branches. In exchange for making it harder to see, the sloth lets the bacteria live on its body forever. The bacteria also tends to attract insects, and so you might also find plenty of happy bugs snuggled up with the sloth. They all get along famously, but for the love of God don’t try to pet one.
There are two basic types of sloths alive today, the two-toed sloths and the three-toed sloths. Despite their similar appearance in all things not toe-related, these two types are not very closely related at all.
All sloths have three toes on the back feet, but the two-toed sloths have only two toes on the front feet. These two types of sloths have been evolutionarily separate for more than 35 million years, which is a very long time for two things to be apart and yet still look so similar. The two-toed sloths are much more closely related to the extinct giant ground sloths than to their three-toed distant cousins.
We don’t even actually know what three-toed sloths are related to, or how they came to be. Their line on the tree of evolution is drawn in the shape of a question mark. But it is believed that the two types of sloth evolved independently from ground-dwelling sloths, and that it is sheer coincidence and convergent evolution that they look and behave so much alike. They even both do the inexplicable bathroom thing.
There is, however, one other major difference between the two types of sloth, and it’s a difference that sets each of them apart from all the other mammals as well. For whatever reason, almost every single mammal has seven neck bones, known as cervical vertebrae. You have seven, I have seven, your cat has seven, your hamster has seven, the elephant that hopefully didn’t just step on your hamster has seven, even the long-necked giraffes and the no-neck whales have seven.
But sloths? Sloths said no, no way, that won’t do, and they said it in opposite directions. The two-toed sloths have six neck bones; the three-toed sloths have nine. Why? No one knows. It’s preposterous and mysterious. They don’t need more or fewer neck bones. They’re both practically the same animal, why did they go different directions in terms of more or fewer? What purpose does it serve? Are they just trolling scientists?
We do actually have some idea how this happened, even if the result serves very little purpose. With nine neck bones the three-toed sloth has a more flexible neck than other creatures, but it doesn’t need it; nor does the two-toed sloth needs its more rigid neck. But it comes down to genes.
It turns out that the reason every other type of mammal has seven neck bones, no matter how big or small its neck gets, is that the gene that controls how many neck bones we have is the same gene that helps determine how our nervous systems and our cells grow. If that gene were to change our number of neck bones, there’s a good chance that it would also give us brain problems and cancer, which would keep us (or any other mammal) from passing its different-number-of-neck-bones gene along to any offspring.
So how did the sloth get away with it? It is useful to consider that one other type of mammal also pulled off this trick, and that’s the manatee. We haven’t looked at them yet, but they are slow animals as well, and have six neck bones. The very slow metabolisms of the manatee and the sloth may actually make them resistant to cancer and DNA damage, which would mean that when evolution gave their neck bones a slip, they survived the change and even though it didn’t necessarily confer any specific advantage or disadvantage, they passed it on to their kids with a shrug. A very slow shrug.
Sloths. So weird.