Balance Organ Indicates How Extinct Species Moved

Researchers have discovered a relationship between an animal's style of locomotion and the dimensions of the semicircular canals of its inner ear, an organ that controls balance. The results, published in the Proceedings of the National Academy of Sciences, could allow scientists to predict more accurately how extinct species moved and develop a more complete history of the evolution of different forms of movement.

"We have shown that there is a fundamental adaptive mechanism linking a species' locomotion with the sensory systems that process information about its environment," explained Alan Walker of Penn State University, one of the lead researchers. The team had hypothesized that, since the semicircular canals both coordinate an organism's movements and help stabilize its vision, the structure of this organ should be closely related to how it moves, with more complex and rapid movements necessitating more highly evolved balance regulation.

The semicircular canals are filled with a fluid that moves as an organism moves, triggering sensory hair cells that signal the body's orientation to the brain. To examine them, the scientists scanned skulls of 91 primate and 119 other, mostly mammalian, species using a high-resolution x-ray CT scanner. They measured the size of each skull's semicircular canal and the radius of curvature, then factored in the organism's body size. Each species' style of movement was classified in one of six categories of increasing complexity, and then the canal size was compared with this classification.

The results indicated that, the more rapidly-moving and agile a species is, the larger the radius of curvature of its semicircular canals, both for primates and across the sample range of mammals from mouse to elephant.

"Now we have a way to reconstruct how extinct species moved that is completely independent of analysis of the limb structure," Walker said. "For the first time, we can test our previous conclusions using a new source of information."

Written by Emma Wear

Former JYI staff members have gone on to win Rhodes, Marshall, and Fulbright Scholarships, as well as NSF Graduate Research Fellowships and other graduate research funding.
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