Air pushed past vocal folds force trapped air in the vocal tract to vibrate, resulting in resonances that facilitate animal communication. These resonances also have frequencies, which can be used to reflect an animal's size, but it is unclear if reptiles also use them to communicate.
Stephen Reber from the University of Vienna, Austria was curious about whether or not reptilian dinosaur relatives like alligators and crocodiles can utilize resonance for communication. However, he was stumped as to how he will distinguish vibrations from vocal folds and resonance vibrations from air from the rest of the vocal tract. He asked Tecumseh Fitch, his thesis advisor, for advice and was told to use heliox, a breathing gas combining helium and oxygen.
Reber realized that sound does travel faster through heliox than normal air. Remembering how inhaling helium can produce voice changes, he figured that reptiles using resonances as vocal communication will shift frequencies if they inhale the gas.
Having solved one part of the problem, Reber was then faced with the task of steeping a large reptile in heliox. He went to the St. Augustine Alligator Farm Zoological Park and was fortunate enough that his visit coincided with a small Chinese alligator being kept in isolation in a tank. According to the staff at the alligator farm, the Chinese alligator would bellow when other alligators, particularly the big ones in the other enclosures would bellow.
As the alligator was already in isolation, this gave Reber the idea to convert its tank into a sound box filled with heliox. Once the enclosure was created, he played recordings of the alligator's voice and found that that can be used to get it to bellow on command. Reber recorded the alligator's grunts in both heliox and normal air and heard that they sound different.
Reber worked with Takeshi Nishimura to analyze the frequencies in the resonances he recorded from the Chinese alligator and discovered that that while the bellows in the breathing has sounded deeper, they were of higher frequencies. With this, reptiles like alligators and crocodiles are officially labeled as using resonances to communicate.
Reber is also excited as this discovery is another evidence supporting the possibility that dinosaurs may have also used resonance to relay their size to others of their kind.
"If you see it in the last two groups that share a common ancestor with all extinct dinosaurs, we can infer that dinosaurs probably used [resonances] too for communication," he said.
Photo: City of Albuquerque | Flickr