All vertebrates that breathe the air have a larynx: a structure of muscles and folds that protects the trachea and, in many animals, vibrates and modulates to produce a surprising variety of sounds.
But birds, although they have larynges (plural of larynx), use a different organ to sing. It is low on the airway, down where the trachea ramifies to go to both lungs. Called syrinx, it is a unique avian characteristic. For decades, scientists, including those from the University of Utah, have been wondering why syringe exists and how it developed in the first place.
Now, a team that combines physics, biology, calculus and engineering finds that syringe confers an advantage: sitting so low in the airway, the syringe can produce a sound with very high efficiency. Your results have been published in PLOS biology.
"I am always excited when something is against intuitive," says Ingo Titze, director of the National Voice and Speech Center of the University of Utah and co-author of the study. "Most people would say" Put the sound source directly on the mouth or on the beak and you will receive the sound from the listener ". But that's not what we find."
Syringe evolution test
For more than 20 years, biologist U Franz Goller studied the mechanics of syringe. He found similarities between the control and the design of the syringe with the mammalian larynx, says Tobias Riede, a former member of the Goller laboratory and now an assistant professor at Midwestern University. "But the question of why birds have evolved into a syringe, although they also have a larynx, will remain open," says Riede.
A question in the understanding of evolution is to identify the compensation between two different structures with a similar function. Why low sirin is the best place for the vocal organs of a bird? "And how do you test this in a bird?" Laugh at. "You can not, because you can not move the vocal source up and down in the tract."
This led Riede and Goller to collaborate with Titze and Scott Thomson, a professor of mechanical engineering at Brigham Young University. Both bring different aspects of the acoustic simulation to the study: Titze brings computational modeling, while Thomson brings experience building physical models to simulate organs that produce sound. Now, says Titze, the team could combine their disciplines and methods. "With a methodology," he says, "if you are working on a live specimen or a physical model, it gives them fragments of the whole image. Now we often answer serious questions by doing combined measurements. We put all the fragments together."
The team followed a triple approach to measure the different effects of Syrinx's position on vocal efficiency: simplified physical models, computational simulation and real birds of different body sizes in the laboratory.
"We find that sound is produced more efficiently by a source of sound in the syrinx position," says Riede. The result supports a hypothesis that birds with a low sirine rate on the airway are more capable of communicating and gain an evolutionary advantage. Birds, which have the longest neck of all terrestrial vertebrates, can use the long neck as resonators to amplify the sound.
"If you are small, you do not have much energy available to produce a signal that is efficient and far away," adds Riede. "So simply by moving the sound source, you can make the sound stronger. It's the same problem an engineer faces. If you want to miniaturize your speaker, how does that speaker be as tall and intelligible as a great speaker? "
"In the old days we thought that the source of sound only produces the sound and the airway only modifies the sound," says Titze. "Our research has shown that there is a strong dependence on both the tube, the airways and the sound source. Where the sound source is in the tube, make the difference, be it in the middle, in front or behind."
There is much to learn from this discovery, especially in how birds place their bodies to take advantage of the position of syringe. Some birds have the need to communicate with a precise vocal "target" to get a message to a partner, for example, without warning predators. "So, they have to position themselves to selectively direct that sound," says Titze. "This requires a different way for the head, body and neck."
Next, Riede plans to examine the diversity of birds to see how their search is applied to birds of varying sizes (from hummingbirds to ostriches) and identifying other factors that can contribute to the evolution of the syringe in a given species. "What do you prefer to the woman?" Laugh at. "How often do you travel better in a certain habitat?"
Perhaps the responses to the tempting complexity of bird song show again the simple elegance of the answers that Riede and her colleagues have discovered so far. "The simplicity of the answer to an old and seemingly difficult question, he says," was the most surprising to me. "
This work was funded by the Gordon and Betty Moore Foundation.