Alligator Sound Perception Comes From Air-Filled Canals Between Their Ears
March 28, 2014

Hearing Canal Helps Gators Find Sound Direction

Brett Smith for - Your Universe Online

For an animal without external ears, alligators have a strong sense of directional hearing and a new study published on Wednesday in the Journal of Experimental Biology has revealed that the reptiles’ refined hearing is a result of large, air-filled canals connecting two middle ears.

"Mammals usually have large moveable ears, but alligators do not, so they have solved the problems of sound localization a little differently,” said study author Hilary Bierman, a biology lecturer at the University of Maryland in a recent statement. “This may also be the solution used by the alligator's dinosaur relatives.”

To reach their conclusion, the study team compiled anatomical, biophysical and electrophysiological specifications of alligators to examine the mechanisms they use to identify sounds.

"Different vertebrate lineages have evolved external and/or internal anatomical adaptations to enhance these auditory cues, such as pinnae and interaural canals," said Bierman.

The researchers analyzed how sound moved around the inside of an alligator's head to see if the animal in some way channels sound, to pick up for minor time and volume inconsistencies in the sound's arrival at the two ears – which help to identify the source. However, the researchers did not find any proof that the animal's body changes sound transmission adequately for the animal to be capable of recognizing the difference. When the team assessed alligators' brainstem reactions to sounds, they were too quick for the animals to detect these minor time inconsistencies.

The team then searched for internal features in the alligators' heads that may propagate audio between the two eardrums. By examining slices with the heads of young alligators, the team could clearly see two routes linking both the middle ears that could channel sound between the two eardrums.

The researchers said sound reaches each side of the eardrum, traveling externally to arrive at the outer side and through head features to the interior side, to enhance the sound at certain frequencies as soon as the head is aligned together with the sound. This enhances the pressure contrasts on the two sides of the eardrum, elongating the time difference between the sound reaching the ear drum by means of two different paths – thereby permitting the animal to identify the source of the sound.

When the team checked out the eardrum's vibration, they could see that it was boosted at certain frequencies, as they had suspected. The team added that this same mechanism was probably used by the ancient ancestors of modern crocodilians and birds to pinpoint the sources of sounds.

In January, a study published in the journal PLOS ONE revealed that alligators typically hunt at night – even though most prey is available in the morning.

To understand the reptile's nocturnal behavior, the study team used a camera ‘Crittercam’ strapped to the backs of 15 alligators from Merritt Island National Wildlife Refuge and Guana River in coastal Florida.

The video footage revealed the hunting and success rates of the alligators throughout the day.

While the gators were most actively hunting at night, the researchers calculated that the reptile should have the most success during the morning hours.