Sluggish Sloths Have Little Use for Balance
by AMNH on
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.
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.
OUR SENSES: WHAT SLUGGISH SLOTHS TELL US ABOUT BALANCE
Published December 1, 2017
[The Museum's logo appears.]
[An illustrated ear, with arms and legs, wobbles on a tightrope.]
ROB DESALLE (Curator, Division of Invertebrate Zoology):
Balance and hearing are usually associated with each other because the organs that do the sensing of sound waves...
[Ear falls off the tightrope. Tightrope folds up into the shape of sound waves.]
DESALLE: ...and that detect position, they're very close to each other.
[Diagram of the human ear. A sound wave enters the outer ear. In the inner ear, two organs are labeled, "Position (Semicircular canals)," and "Sound (Cochlea)".
DESALLE: They're in the inner ear.
[Curator Rob DeSalle speaks in his office. On-screen text gives his title as "Curator, Division of Invertebrate Zoology."]
DESALLE: They work in somewhat the same fashion, too. The cochlea that detects sound waves...
[An animated soundwave moves behind an illustration of the human cochlea.]
DESALLE: ...has fluid in it, and interacts by...
[Camera zooms into the cochlea, and tiny hairs – represented by pipe cleaners – move back and forth]
DESALLE: ...the hairs being bent by the fluid.
[Illustrated semicircular canals float in a room with graph paper walls. Text identifies them as "SEMICIRCULAR CANALS." An X, Y, and Z appear on the walls, representing the coordinate planes. Arrows pointing in each of the three directions.]
DESALLE: Balance, on the other hand, has three little structures called semicircular canals arranged in an X-Y-Z coordinate fashion.
[Close up of the semicircular canals, as they are filled by animated fluid.]
DESALLE: These three semicircular canals are filled with fluid, too...
[Hairs, represented by pipe cleaners, pop into the frame.]
DESALLE: ...and are lined with hairs.
[A human silhouette, made from graph paper. The semicircular canals are placed in their approximate position in the head. Arrows indicating the X, Y, and Z-coordinate planes are to the left of the silhouette. The head bends forward.]
DESALLE: And so, if you bend a little bit forward...
DESALLE: ...what you're going to do is cause the fluid to go in the Z direction...
[The graph silhouette now has an illustrated brain, and bends upside down.]
DESALLE: ...and that's going to tell your brain, "Oh, I'm bending forward."
[A thought bubble comes out of the head with the text, "Oh, I'm bending forward."]
[An illustrated sloth crawls along a tree branch.]
DESALLE: Sometimes, balance is not that important for animals. Three-toed sloths are awesome because they live in trees...
[Close up of an illustrated sloth's face. The sloth blinks. Slowly.]
[The sloth crawls along a branch. A turtle and a snail race past it.]
DESALLE: ...and they've evolved to be very, very slow.
[A sloth climbs down a tree, holding a roll of toilet paper.]
DESALLE: They rarely come out of the tree except to poop.
[Footage of a sloth crawling awkwardly across the ground.]
DESALLE: And when they do their movements and balance are very, very herky-jerky.
[DeSalle speaks in his office.]
DESALLE: The reason for this is that their semicircular canals have started to degrade.
[Sloth moves slowly along a branch.]
DESALLE: They're a really great example of how a sense is no longer needed and becomes vestigial.
[Wider shot reveals that the sloth is crawling along an illustration of semicircular canals and a cochlea.]
[Card appears saying: "American Museum of Natural History, Our Senses: An Immersive Experience, Opens November 2017. Our Senses is generously supported by Dana and Virginia Randt."]
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.