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University of Colorado School of Medicine

The Secret to Alligator Ears

School of Medicine

3/27/2014

By reptile standards, alligators are positively chatty. They are the most vocal of the non-avian reptiles and are known to be able to pinpoint the source of sounds with accuracy. But it wasn’t clear exactly how they did it because they lack the external ear structures, or pinnae, that most mammals have.

A new study by an international team of biologists shows that the alligator’s ear is strongly directional because of large, air-filled channels connecting the two middle ears. This configuration is similar in birds, which have an interaural canal that increases directionality in the absence of external ears.

The study was published online in the Journal of Experimental Biology on March 26, 2014. The research was funded by the National Institutes of Health, National Science Foundation, Danish National Science Foundation and Carlsberg Foundation.

Daniel Tollin, PhD, associate professor of physiology and biophysics at the University of Colorado School of Medicine—along with researchers from the University of Maryland, the University of Massachusetts Lowell, and University of Southern Denmark—collected anatomical, biophysical and electrophysiological measurements of alligators to investigate the mechanisms alligators use to locate sounds.

Acoustic measurements were performed in Tollin’s laboratory. Tollin and others from the School of Medicine – postdoctoral fellow Jennifer Thornton, PhD, neuroscience graduate student Heath Jones and postdoctoral fellow Kanthaiah Koka, PhD – were involved in the design, performance, analysis and figure production for acoustic measurements.

First, the team tested how sound travelled around an alligator’s head to investigate whether the animal somehow channels sound to help it locate the origin, listening for the minute time and volume differences between the sound arriving at the two ears. But the team found no evidence that the animal’s body alters sound transmission sufficiently for the animal to be able to detect the difference. And when the team measured alligators’ brainstem responses to sounds, they were too fast for the animals to be sensing the time difference between a sound arriving at the two ears.

Next, the team looked for internal structures in the alligators’ heads that might propagate sound between the two eardrums. Sound reaches both sides of the eardrum—travelling externally to reach the outer side and through head structures to the internal side—to amplify the vibration at some frequencies when the head is aligned with the sound. This maximizes the pressure differences on the two sides of the eardrum, magnifying the time difference between the sound arriving at the ear drum via two different paths to allow the animal to pinpoint the source.

Viewing slices through the heads of young alligators, the team could clearly see two channels linking the two middle ears that could transmit sound between the two eardrums. They also measured differences in the alligator’s brainstem responses to sounds depending on their location. And when the team looked at the eardrum’s vibration, they could see that it was amplified at certain frequencies, as they would expect if alligators use the pressure difference at the eardrum for orientation.

Assembling all of the evidence together, the researchers suggest that the reptiles rely on magnified time difference at the eardrum to locate noises. They also suspect that this is the mechanism that the archosaur ancestors of modern crocodilians and birds used to pinpoint sounds.

This research was supported by the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health under Award Nos. DC-00436, DC-04664 and DC-011555; the National Science Foundation under Award No. 0817208; the Danish National Science Foundation under Award No. DFF1323-00132; and the Carlsberg Foundation. The content of this article does not necessarily reflect the views of the funding organizations.

The research paper, “Biophysics of directional hearing in the American alligator

(Alligator mississippiensis),” Hilary S. Bierman, Jennifer L. Thornton, Heath G. Jones, Kanthaiah Koka, Bruce A. Young, Christian Brandt, Jakob Christensen-Dalsgaard, Catherine E. Carr and Daniel J. Tollin, will be published online on March 26, 2014 in the Journal of Experimental Biology.