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The birds sing of the heart. While other four-membered animals, such as mammals and reptiles, emit sounds with vocal cans in their throats, chirping birds come from a single vocal organ called syrinx, located in their bosom. . No other animal has syrinx, and scientists do not know how or when it has evolved. In a new study in the Proceedings of the Natural Academy of Sciences, an interdisciplinary team of developmental biologists, evolutionary morphologists and physiologists examined the tracheas of birds, crocodiles, salamanders, mice and cats to learn more about the evolution of syrinxes. Their results seem to confirm: syrinx is a strange evolutionary duck. But it is possible that it comes from a reinforcement at the bottom of the trachea that we still see in many other animals.
"Birds have a unique organ in their airways to produce sounds – it's like a flute," says Chad Eliason, a postdoctoral researcher at the Chicago Field Museum and co-lead author of the study. "We do not know where this organ was born, how and why it evolved, but in this study, when we zoomed out and looked at the trachea of other animals, we found similarities deeper than we expected. "
Mammals, birds and reptiles all have a trachea – a tube that connects the nose and mouth to the lungs. And we all have a larynx – a hollow muscular organ at the top of this tube that helps air to pass through the lungs and prevents food from falling into the airways. In mammals and reptiles, the larynx has small folds of tissue that vibrate as the air passes over them, producing a sound. In birds, the larynx does not produce sound – it is only there to breathe and eat. Instead, the sounds of the birds are produced at the bottom of the trachea just above the branch of the trachea in the lungs. This joint, consisting of cartilage, is syrinx.
"The syrinx is a small box of cartilage – in a sparrow it is about 8 millimeters in diameter, about four times the thickness of a spaghetti strand," explains Eliason. "It strengthens the airways and when the air passes over the folds, it produces a sound: the song of birds."
Since birds have both syrinx and larynx, it is unclear exactly when syrinx has assumed a healthy production duty for the larynx. "The closest relatives of the birds, the crocodiles, produce sound in the throat with a larynx, so how did the ancestors of the birds make sounds with their larynx to make sounds with their syrinx?" Eliason asks. "If we found fossil evidence of a syrinx in dinosaurs, it would be a smoking gun, but we have not done it yet." In the meantime, we need to look for other clues in animals . "
Eliason and his fellow scientists, including co-first author Evan Kingsley of Harvard Medical School, examined the tracheas of a suite of animals. Cats, mice and fangs all have a set of cartilaginous rings fused to the base of the trachea, called carina, which provides structural support where the trachea is moving towards the lungs, as does a syrinx. "Maybe the syrinx is only a byproduct of airway strengthening, as does a hull in other animals," Eliason explains.
The existence of structures like syrinx and carina are not new to science – people have known them for decades. But this study provides a new way to look at them. "We are deepening our understanding of how these structures relate to each other, and this could help us understand how syrinx evolved in the first place," says Julia Clarke, professor at the University of Texas at Austin and author study.
The study also highlights the ability of birds to produce sounds throughout their evolutionary history. According to Eliason and his colleagues, there are two ways to go. In Scenario A, the ancestors of the missing birds made sounds with their larynx, then they lost the ability to make sounds with their larynx and they had a time when they could not sing. . In scenario B, the ancestors of the birds made sounds with a larynx, then they developed a syrinx for the structural support of the airways, the syrinx became able to produce sound and the birds did not make sounds with their larynx . This study suggests that scenario B is more likely. "It seems to me more intuitive that the development of syrinx by birds would have been progressive, that they would have retained their ability to produce sounds with both the larynx and the syrinx, and then progressively lose the ability to make sound with the larynx, "says Eliason." The other option would be that the birds and their dinosaur ancestors live some kind of quiet zone when they could not make noise. "Looking at the skulls of the dinosaurs, we know they could hear, and it would make no sense, able to hear but not to be able to make sounds to communicate. "The researchers also note that the presence of a perfectly functional larynx in the next-to-no birds makes scenario B less likely than the first scenario.
Kingsley says he hopes this study will be useful for all scientists studying the evolution of new features. "One of the great challenges of biology is to understand how new structures appear, why we see strange traits in some organisms," he says. "This study gives us an idea of how one of these new structures, the syrinx, could have been created."
Eliason also notes that the evolution of syrinx is inextricably linked to the evolution of bird song, and therefore of the birds themselves. "Birdsong is what they use to communicate, find partners and prevent each other from predators.It is thanks to syrinx that birds can produce all these different sounds.There are over ten thousand species of Birds, each with a different appeal the ability to produce all these sounds could have played a role in the diversity of birds we see today. "
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The research was funded by the Gordon and Betty Moore Foundation (Grant 4498).
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