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The spiral secrets of mammals' hearing abilities.


Whispering galleries are curious features of circular buildings. As whispers travel along the buildings' curved walls, they remain loud enough to be heard clear on the other end of even a large room. New research suggests we all carry a couple of them around in our heads.

Inside their inner ears, mammals (but not other animals) have cochleas--coiled shell-like tubes in which incoming sound energy tilts sensory hairs, generating electric signals to the brain. Scientists thought coiled cochleas evolved as a way to pack a longer tube length into the available skull space, giving mammals better low-frequency hearing than other vertebrates, which lack cochleas.

To test the idea, Darlene Ketten, a biologist at Woods Hole Oceanographic Institution (WHOI), and WHOI researcher Julie Arruda provided high-resolution CT scans, taken at the WHOI Computerized Scanning and Imaging Facility, of cochleas from 13 different land and mammal species. They included creatures whose hearing abilities are well-documented, ranging from an elephant, mouse, and human to a sea lion and a bottlenose dolphin.

From the CT images, Ketten's group made precise geometric measurements of all the cochleas and sent the images, drawings, and data to a research team led by Daphne Manoussaki, a mathematics professor at Vanderbilt University, and Richard S. Chadwick of the National Institute on Deafness and Other Communication Disorders, who calculated the cochleas' curvatures. They found that the more tightly wound the spiral in an animal's cochlea, the lower the frequency of sounds the animal could hear. The findings suggest that tighter curvature focuses more low-frequency sound waves against cochlea walls, channeling them more efficiently to the cochlea tips and making sounds more audible--just as in a whispering gallery.

The research, published April 22, 2008, in the Proceedings of the National Academy of Sciences as the featured news item, could be extrapolated to improve understanding of hearing (and the impact of human-made noises) in animals whose auditory abilities remain unknown, such as tigers and polar bears, and even extinct mammals, such as saber-toothed tigers and woolly mammoths, whose cochleas are often preserved as fossils.

This study was funded by the National Institutes of Health, Vanderbilt University, the Technical University of Crete, and the Office of Naval Research Marine Mammal Program.
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Title Annotation:RESEARCH NEWS
Author:Madin, Kate
Geographic Code:1USA
Date:Sep 1, 2008
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