Study links low-frequency hearing to shape of the cochlea

Ketten provided Manoussaki and her collaborators with high-resolution CT scans of the cochleae of a number of different species of land and marine mammals. Together with her biophysicist colleagues, Manoussaki analyzed these shapes and found that low- frequency hearing limits of species ranging from mice to cats to cows to whales varied in step with the ratio of the radii of curvatures at their cochlea’s base to that of its apex. This ratio varies from about two to nine: The larger it is the lower the frequencies that the animal can hear.

“This makes sense because the bigger the ratio, the tighter the spiral is wound and more of the sound wave energy in the low-frequency waves is forced against the cochlea’s walls,” Manoussaki says.

Animals like mice, which have a radii ratio of about two, can’t hear much below 1000 hertz (Hz). Species like cows and elephants, which have a ratio of about nine, hear sounds as low as 20 Hz. The power of this approach is illustrated by the cat, guinea pig and sea lion. The cochlea of the cat is longer than that of the guinea pig, but the guinea pig has a ratio of 7.2 and can hear down to 47 Hz, while the cat, with a smaller ratio of 6.2, has a higher threshold of 55 Hz. Similarly, the sea lion has a basilar membrane three times as long as that of the guinea pig. But its radii ratio is 5.2, lower than either the cat or the guinea pig, and it cannot make out sounds below 180 Hz. (This limit is for the sea lion’s hearing in air; under water it can hear down to 200 Hz.)

“What I like about this is that a macroscopic feature of the ear has such a major effect on our hearing,” says Manoussaki. “As colleagues have pointed out, so much research today is done at the genetic and cellular level that you don’t often see cases like this where simple geometry proves to be so important.”

Other contributors to the research are Emilios K. Dimitriadis, National Institute of Biomedical Imaging and Bioengineering; Julie Arruda of Woods Hole Oceanographic Institution and Jennifer T. O’Malley of the Massachusetts Ear and Infirmary.

The research was supported by the National Institutes of Health, Office of Naval Research, Technical University of Crete and Vanderbilt University.

[Note: A multimedia version of this story is available on Exploration, Vanderbilt’s online research magazine, at http://www.vanderbilt.edu/exploration/stories/shapematters.html]