Size matters.
Tiny semicircular canals, shown in this computer-reconstructed adult female baboon, are larger for more agile creatures, according to a new study.

Alan Walker lab/Pennsylvania State University

Pirouette or Plod?

You can tell how nimble an animal is without even looking at its legs: Simply check the size of its inner ear. A new study shows that agile animals, such as tree-swinging gibbons or brown bats, have relatively larger ear canals than their lumbering counterparts the sloths or dugongs, a relative of the manatee. The finding may provide an innovative way to check how quick-footed extinct species were. However, critics point out that in this study, agility is in the eye of the beholder.

Organs in the inner ear help steady an animal's motion by synching the body's movement to visual stimuli. The inner ear has three fluid-filled semicircular canals, one circling each spatial dimension, that act like little gyroscopes to detect changes in speed in the direction of motion. Fluids in the semicircular canals flow when an animal jerks its head, in the same way water in a bucket will slosh if you're running with it and suddenly stop. Scientists had noticed that some agile animals, such as graceful gibbons, had larger semicircular canals relative to their body sizes than less maneuverable creatures such as sloths. That would make sense because the bigger hoops should be more sensitive to acceleration, and animals that change direction and speed rapidly have jerkier head motions and experience bigger accelerations. But in spite of anecdotal evidence, it wasn't clear that larger canals always belonged to quicker creatures.

To test the pattern, paleontologist Alan Walker of Pennsylvania State University in State College and his team surveyed more than 200 mammals. Using personal knowledge, data from previous studies, and video footage, Walker and his trained field experts gave each animal an agility rating ranging from 1 for "extra slow" to 6 for "fast." Sloths, who crawl at 1.5 meters per minute, anchored the lowest end of the scale, and gibbons, tarsiers, and several types of skittering rodents hit the high end. Giraffes, elephants, hippopotamuses, and humans were scattered in the middle.

With a microcomputed tomography (CT) scanner, Walker measured each animals' semicircular canals, correcting for body mass, as he reports online this week in the Proceedings of the National Academy of Sciences. Walker's team found exactly what he'd hypothesized: The jerkier the movements of the animal, the larger the semicircular canals. That correlation gives a new way to check how extinct species may have moved, an issue that is sometimes controversial. Walker is currently studying the fossilized remains of semicircular canals from extinct animals.

The method "can give a good idea of how animals might have moved," says Timothy Hullar, an otolaryngologist at Washington University School of Medicine in St. Louis, Missouri. But Hullar, who is doing similar studies, also notes that the agility ratings are essentially subjective. "It's hard to just look at an animal and tell how fast it's moving," he says. There are better, more objective ways to measure agility, but he won't say exactly what they are "because we're still collecting data."

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