Sloths are cute and slow, they hang upside down and live in trees. That’s about all most people know about them. But why are they so slow? And why are there so few other animals like them?
Sloths are folivores, meaning they just eat leaves. They live in the canopies of trees with their food, but forests cover a significant portion of the Earth, so you’d think folivores would be more common.
Not so. It turns out that living on leaves is difficult. Leaves aren’t a good source of nutrients, so in order to subsist of them, animals have to have some pretty specific evolutionary tools.
Sloths, for example, move really slowly so they don’t use much energy. They have the lowest energy expenditure of any mammal, and live off about a baked potato’s worth of calories.
A recent study shows that the sloth’s slowness is directly related to its evolutionary adaptation to its ecological niche. Sloths expend very little energy because they take in so little energy. Other creatures that live in the canopy such as birds and squirrels have a lot more energy, but that’s because they don’t just eat leaves.
It also means that sloths don’t have many close genetic cousins. This is because, in evolutionary parlance, they don’t radiate: They don’t spread out and evolve from a central ancestor. That, it stands to reason, would require more energy that sloths can be bothered with.
Evolution requires creatures to spread out and find new niches that allow them to make use of genetic mutations, slightly longer necks or higher metabolisms, which eventually become dominant traits. Usually that movement is because they need to find new food sources or get away from threats, but sloths don’t seem to do either of those things. They just hang out and eat leaves.