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Why do birds have their own unique organ for producing sound?

While most animals use the larynx to produce sounds, birds use an organ that is deep in the airway called a syrinx. For decades, scientists have wondered how and why the organ developed in the first place, considering that birds have multiple larynges.

Experts at the University of Utah have discovered that the syrinx offers a clear advantage. By sitting so low in the airway, the organ can produce sound with very high efficiency.

“I’m always excited when something is counter-intuitive,” said study co-author Ingo Titze, who is the director of the National Center for Voice and Speech. “Most people would say ‘Put the sound source right by the mouth or the beak, and you’ll get the sound to the listener.’ But that’s not what we’re finding.”

Biologist Franz Goller has studied the mechanics of the syrinx for more than 20 years. Tobias Riede, a former member of Goller’s lab, said that he found similarities between the control and design of the syrinx with the mammalian larynx.

“But the question of why birds have evolved a syrinx, although they also have a larynx, had remained open,” said Riede. A question in understanding evolution is to identify the trade-offs between two different structures with similar function. In this case, the question is centered around what makes the low syrinx the best place for a bird’s vocal organs.

“And how would you test that in a bird?” said Riede. “You can’t, because you can’t move the vocal source up and down the tract.” This is what led the team to use a multidisciplinary approach which combined physics, biology, computation, and engineering.

“With one methodology, whether you’re working on a live specimen or a physical model, it gives you fragments of the whole picture,” said Riede. “Now we often answer serious questions by making combination measurements. We put all the fragments together.”

To measure the various effects of syrinx position on vocal efficiency, the team studied simplified physical models, computational simulation, and real birds of varying body sizes in the laboratory.

“We find that sound is produced with greater efficiency by a sound source in syrinx position,” said Riede. This finding supports the theory that birds with a syrinx low in the airway are better able to communicate and gain the evolutionary advantage of amplified sound.

“If you’re small, you don’t have too much energy available to produce a signal that is efficient and carries far,” said Riede. “So, by simply moving the sound source, you can make the sound much louder. It’s the same problem an engineer faces. If you want to miniaturize your speaker, how do you make that speaker equally loud and intelligible as a big speaker?”

“In the old days we used to think that the sound source just produces the sound and the airway just modifies the sound,” explained Titze. “Our research has shown that there is strong dependence on both the tube, or airway, and the sound source. Where the sound source is in the tube makes a difference, whether it’s in the middle, front or back.”

The study is published in the journal PLOS Biology.

By Chrissy Sexton, Staff Writer

Image Credit: Natasha Verzhbitskiy

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