There’s a reason why humans can gracefully walk a tight rope, master the Tree Pose in yoga class, or spin in a circle while dancing without getting dizzy, and it’s all thanks to the vestibular systems of our inner ear.
The mammalian inner ear features a system of three semicircular canals, each filled with fluid and cells topped with microscopic hairs, called cilia. When our head moves, fluid flows, causing hairs to bend—and the cells fire off signals to our brain and help us keep our balance.
Slow-Moving Sloths
But what happens when you don’t move—or, don’t move much? Tree-dwelling sloths, best known for their limited, languid locomotion and affinity for hanging upside down, don’t have much need for balance.
They spend most of their lives in trees, attaching themselves to branches with hooklike claws so securely that they can sleep while suspended. Sleep fills up to 20 hours of a sloth’s day. And when they do wake up, they typically move less than 300 feet—usually so they can find a spot on the ground to defecate—before taking another nap. Sloths rarely hustle faster than 0.31–0.36 miles per hour, and at this pace there’s little need for them to have a fine-tuned motion sensor.
A Vestigial Organ in the Making
Such extreme sluggishness makes sloths an interesting test case for one of Charles Darwin’s hypotheses, which informed his ideas about vestigial organs. Vestigial features are structures that have lost their function in the course of evolution, like appendixes in humans or tiny hind leg bones in pythons and boa constrictors. In his 1859 work On the Origin of Species, Darwin proposed that when traits weren’t useful to a species, and so not under natural selection, there would be greater variation of that characteristic, and even degeneration.
In a 2012 study published by the Proceedings of the Royal Society B, Guillaume Billet and his colleagues set out to see if lethargic sloths showed more variation of their semicircular canals than faster-moving species that rely on their inner ear motion sensors, as Darwin would have predicted. They examined the inner ears of 14 species of Xenarthra, the group of placental mammals that includes two- and three-toed sloths and nine-banded armadillos, as well as other mammalian species such as the red squirrel and European mole.
Their findings seemed to confirm Darwin’s hunch: the inner ears of two-toed sloths (Choloepus), and three-toed sloths (Bradypus) in particular, did show higher degrees of variability in shape than other species. In the case of three-toed sloths, variations were more extreme—likely because they move even slower than two-toed sloths. “What’s significant about the work is actually catching the semicircular canals on their way to becoming vestigial,” writes Rob DeSalle, curator of the new exhibition Our Senses: An Immersive Experience, in his forthcoming book.
Learn more about the senses of different species in our new exhibition Our Senses: An Immersive Experience.