The Sloth’s Evolutionary Secret

On the surface of things, a two-toed sloth doesn’t look much like its closest fossil kin. The tubby, pug-nosed mammal is not quite as imposing or majestic as Megalonyx – the “great claw” Thomas Jefferson discovered and mistakenly identified as an enormous lion over two centuries ago. But the two are relatively close relatives. In fact, today’s shaggy two-toed sloths are more closely related to Megalonyx than that other variety of upside-down mammal, the three-toed sloths. Together, the two- and three-toed flavors of sloth represent a remarkable case of convergent evolution.

The distant relationship between the two modern sloth genera – species of two-toed sloth are grouped under Choloepus, and three-toed species under Bradypus – isn’t immediately obvious. Both suspend their bodies beneath tree limbs by their hooked claws, and they slowly clamber along upside-down. If we had no knowledge of the fossil record, we would expect that these animals are the descendants of some relatively recent sloth that moved in the same way.

What actually happened isn’t that simple. In addition to anatomical comparisons between modern and fossil sloths, genetic studies involving DNA extracted from the skin and dung of extinct species have found that two-toed sloths are most closely related to genera such as Megalonyxas Megalonyx, and three-toed sloths belong to a different sloth subgroup which includes behemoths akin to Megatheriumakin to Megatherium. Exactly when two- and three-toed sloths last shared a common ancestor isn’t clear, but, on the basis of their relationships, both lineages probably go back to a species which trundled along on the ground. This is consistent with what is known from the fossil record. So far, no one has found an extinct sloth which moved upside-down through the canopy like modern species do.

Both two- and three-toed sloths converged on a similar form despite their distant relationship. As anatomist John Nyakatura outlined in a recent Journal of Mammalian Evolution review, the deep history and evolutionary context of sloths may explain why.

Sloths are xenarthrans – their closest relatives include anteaters and armadillos. And, among other things, large, curved claws and powerful forelimbs for digging are common xenarthran traits. It’s likely that the last common ancestor of two- and three-toed sloths shared these features, too. If this is correct, then these traits may have been co-opted when smaller sloths moved into the trees and therefore allowed the peculiar mammals to create a new niche for themselves. Strong arms with hooked claws can be just as useful for climbing through the trees as they are for digging on the ground.

Nyakatura suggests that the silky anteater might serve as a rough intermediate between early, earthbound sloths and the modern, topsy-turvy species. Like other xenarthrans, this anteater has curved claws and powerful forelimbs, but, while climbing through the trees, is capable of moving both above and below branches. The ancestors of modern sloths probably went through a similar stage. “Similar to the silky anteater,” Nyakatura writes, “small to medium sized arboreal [sloths] with strong limb flexor groups and strong, curved claws already in place, may have been able to locomote above the branch, but also in a suspended posture.”

But why do sloths hang upside-down? Many other tree-dwelling mammals – from squirrels to monkeys – run along the tops of branches. What caused sloths to eventually adapt to a different perspective? The answer probably has something to do with saving energy. When a creature moves along the top of a tree branch, Nyakatura points out, the animal has to balance. Sloths sidestep this extra energy expenditure by simply hanging below branches. More than that, xenarthrans typically have low metabolic rates compared to other mammals, so the similar evolutionary patterns in two- and three-toed sloths may partially stem from a shared physiology.

Together, two- and three-toed sloths illustrate the relationship between constraint and convergence. To say that natural selection repeated itself would miss the broader picture. Starting from terrestrial origins, both sloth lineages were adapted in similar ways due to particular, shared constraints. Sloths may have taken to the trees thanks to the digging adaptations of their ancestors, but other inherited traits limited the ways they could climb through the canopy.

For more on sloth evolution and locomotion, see this WIRED Science article I wrote about Nyakatura’s previous work in 2010. And, since we’re on the topic, I think there’s no better time to share this insufferably cute video of baby sloths:

Top Image: A two-toed sloth. Image by Dave Pape, via Wikipedia.

References:

Nyakatura, J. (2011). The Convergent Evolution of Suspensory Posture and Locomotion in Tree Sloths Journal of Mammalian Evolution DOI: 10.1007/s10914-011-9174-x