Sloths have become so popular that using them in promotion isn’t so much a cliche as it is a requirement. Seriously, try to find a Costa Rican tour agency, hotel, or conservation org that doesn’t use sloths anywhere in their material. They’re on books. They’re on T-shirts. They’re lining the shelves of souvenir shops in plush form. They’re even on the money. The internet has gone sloth-wild recently. They’re the new Panda bear. They’re just too good of a symbol to pass up–what better representation of nature and easy-going lifestyle?
So I’m almost resistant to talking about sloths too much. After all, I tend to roll my eyes at anyone using sloths as a selling point or an icon. It’s just too easy. They’re too lovable. They eat leaves and sleep all day while constantly wearing the expression of a stoner roommate. What’s not to like about a sloth?
But they are fascinating creatures, even from the standpoint of a cynical biologist. And after the last few weeks of piloting my own sloth tracking study, I can add them to my list of focal animals.
So…what the hell? Let’s talk sloths.
Modern sloths are a remnant of a once-diverse and numerous clade that flourished in the Ice Age in many different niches. There were giant sloths the size of elephants, herds of sloths that ran on hooves, even large marine sloths that lived in oceans. But many factors (including, notably, the advent of early humans), selected for only the sloths that eventually evolved to live in trees in an extreme case of Convergence.
What I mean by that is that of the 6 surviving species of sloth today (4 3-toed, 2 2-toed for a mathematical and linguistic challenge), both have very similar body structure, physiology, habits, and habitat, despite having diverged over 30 million years ago. Both Families are strictly arboreal, mostly herbivorous, solitary, and very slow moving.
Why is this? It may have to do with the fact that sloths early on figured out a strategy that was very effective and very specialized. By living their entire lives in trees, they avoid most predators. But to do this they limit their access to food, leaving mostly leaves, some fruit, and the occasional lizard or insect when they’re feeling peckish or protein-starved. So in turn, they evolved an efficient digestive system, and a slow metabolism to maximize their energy-poor diet. They also developed other specific traits for living high in trees, including reduced bone density, symbiotic fur algae, and finger bones that extend out of their hands like claws for hanging.
All of this gives sloths an appearance that is just…odd. It has to do with how their limbs look out-of-proportion, scrawny yet strong. Their fermentation gut gives them a potbelly, and their long matted hair makes them look like a wet mop in bad weather.
But it’s all functional. Sloths may seem silly, but whatever they’re doing, they’re doing it right. After all, they’re quite abundant here in Costa Rica, despite being rarely seen. That’s that camouflage working. And it’s not like their food is going anywhere. Bottom line: sloths are not lazy, they are efficient.
But the truly weird candle on the sloth cake? Bathroom habits. With their slow metabolisms, all species of sloth have evolved the habit of only defecating once or twice a week but only do so directly on the ground, usually at the base of a tree. Why? No one knows. After all, the ground is where they are most vulnerable to predators. Why no just poop from the branches?
There are only hypotheses so far. I’ve heard many, but none have been tested as far as I know. A common idea is that they like to fertilize their favorite tree. This doesn’t really scan for me, since you don’t fertilize a rainforest tree at its base, and either way you could accomplish this just as well with an aerial drop from the branches. My current favorite is that the sound of several pounds of sloth dung landing on the forest floor from several dozen meters up would clue in predators, so selection favors sloths who climb down and take care of business discretely. A third explanation, while unlikely, still sticks with me–whiplash. A sloth dropping a turd (which, I should mention, can weigh up to 20% of the entire sloth) from a thin branch would, by physics, risk catapulting himself off the tree. It’s quite an image. Thoughts? Talk among yourselves.