0215 GMT November 19, 2019
According to eurekalert.org, no other animals have a syrinx, and scientists aren't sure how or when it evolved.
In a new study in the Proceedings of the Natural Academy of Science, an interdisciplinary team of developmental biologists, evolutionary morphologists, and physiologists examined the windpipes of birds, crocodiles, salamanders, mice, and cats to learn more about how syrinxes evolved.
Their findings seem to confirm: The syrinx is an evolutionary odd duck. But it might have arisen from a reinforcement at the bottom of the windpipe that we still see in many other animals.
Chad Eliason, a postdoctoral researcher at Chicago's Field Museum and co-first author of the study, said, "Birds have a unique organ in their airway to produce sounds — it's like a flute.
"We don't know where that organ came from, how and why it evolved. But in this study, when we zoomed out and looked at the windpipes of other animals, we found deep similarities that we didn't expect."
Mammals, birds, and reptiles all have a trachea — a tube connecting the nose and mouth to the lungs. And we all have a larynx — a hollow, muscular organ at the top of that tube that helps air pass through to the lungs and keeps food from falling into the airway.
In mammals and reptiles, the larynx has little folds of tissue that vibrate when air passes over them, producing sound. In birds, the larynx can't produce sound — it's only there for breathing and eating. Instead, birds' sounds are produced at the bottom of the trachea, just above where the trachea branches into the lungs. This juncture, made of cartilage, is the syrinx.
"The syrinx is a tiny box of cartilage — in a sparrow, it's about eight millimeters in diameter, about four times the thickness of a strand of spaghetti," said Eliason. "It reinforces the airway, and when air passes over the folds in it, it produces a sound: birdsong."
Since birds have both a syrinx and a larynx, it's unclear exactly when the syrinx took over sound production duty for the larynx. "Birds' closest living relatives, crocodiles, produce sound in their throats with a larynx. So how did ancestors of birds transition from making sounds with their larynx to making sounds with their syrinx?" asked Eliason.
"If we found fossil evidence of a syrinx in dinosaurs, that would be a smoking gun, but we haven't yet. In the meantime, we have to look at other animals for clues."
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 crocs all have a set of fused cartilage rings at the base of the trachea, called the carina, that provides structural support where the trachea branches to the lungs — just like a syrinx does. "Perhaps the syrinx is just a byproduct of reinforcing the airway, like a carina does in other animals," said Eliason.