Birdsong and human voice built from the same genetic blueprint

People have long been fascinated by birdsong and the cacophony of other bird sounds – from coos and honks to quacks and peeps. But little is known about how birds’ unique vocal organ – the syrinx – varies from species to species, or about its deeper evolutionary origins.

Three recent studies led by researchers at the University of Texas at Austin are changing that.

The studies include high-resolution anatomical scans of syrinxes of hummingbirds and ostriches – the smallest and largest bird species in the world – and the discovery that the syrinx and larynx, the vocal organ of reptiles and mammals, including humans, share the same developmental programming.

According to Julia Clarke, a professor at UT’s Jackson School of Geosciences, this genetic connection between the vocal organs is an exciting new example of “deep homology,” a term that describes how different tissues or organs can share a common genetic link.

“To me, this is as big as the transition from flippers to limbs,” says Clarke, who co-led or co-authored the studies. “In some ways it is even greater because the syrinx is not a modified organ with a new function, but a completely new organ with an ancient and common function.”

The three studies are built on a foundation of collaborative and interdisciplinary syrinx research with physiologists and developmental biologists that Clarke has led for more than a decade.

The research began in 2013 when Clarke, a paleontologist, discovered a syrinx in a fossil of a duck-like bird that lived during the Late Cretaceous in what is now Antarctica. The specimen is the oldest syrinx discovered. But when she tried to compare the fossil syrinx with the syrinxes of modern birds, she found the scientific literature was lacking. Many of the studies dated from the 19th century, before the advent of modern scientific imaging, or quoted claims from those older studies that were done without double-checking them.

This set Clarke on a mission to modernize and maximize syrinx data collection.

“We had this new three-dimensional structure, but we had nothing to compare it to,” said Clarke, describing CT image data of the fossil syrinx. “So we started generating data that hadn’t existed before about the structure of the syrinx in many different groups of birds.”

Over the years, Clarke and members of her laboratory have developed new methods for dissecting, preserving and CT scanning syrinxes, allowing the syrinx to be visualized in greater detail. This improved look at the vocal organ of the ostrich and hummingbird has shown that bird behavior may be as important as the syrinx when it comes to the repertoire of sounds these birds produce.

For example, in the study of the ostrich syrinx, in the Journal of Anatomythe researchers found no significant differences in the anatomy of the syrinx between adult male and female birds (previous studies focused only on male ostriches). Although both sexes have the same vocal equipment, male ostriches tended to make a wider variety of sounds than female ostriches. ostriches, where the sounds are often associated with aggressive behavior between noisy males.

During a visit to an ostrich farm in Texas, the researchers recorded 11 types of calls, ranging from high-frequency chirps and gurgles in baby ostriches to low-frequency boos and booms in adult males. These include a few call types that had never been recorded before. The only sounds definitively recorded from adult female ostriches were hissing. What the females lacked in range, they made up for in posture, says Michael Chiappone, who became involved in the ostrich study as a student at the Jackson School and is the study’s lead author.

“They were pretty productive hissers,” says Chiappone, who is now a doctoral candidate at the University of Minnesota.

For the hummingbird research in the Zoological Journal of the Linnean Societythe researchers compared the hummingbird’s syrinx with the syrinx of swifts and nightjars, two close relatives, and found that all three birds have similar vocal folds in their syrinx, despite having different ways of learning their calls. Swifts and nightjars operate with a limited repertoire of instinctive calls, while hummingbirds can elaborate on the calls by learning complex songs from each other, a trait called vocal learning.

According to Lucas Legendre, a Jackson School research fellow who led the hummingbird study, the findings suggest that the common ancestor of all three birds also had a similar vocal fold structure – and that this may have helped lay the foundation for evolution in vocal learning . in hummingbirds.

“Having it all [vocal fold] Structures that were already present before hummingbirds acquired vocal learning likely made it easier for them to acquire vocal production learning,” he said.

Before the study, it was uncertain whether swifts had vocal folds at all. As part of the research, Legendre created a digital 3D model of the rapid vocal track that takes viewers through the trachea to the syrinx and the vocal folds that rest at the top of each branch of the syrinx. The model – which Clarke calls the ‘magical mystery journey’ – shows the advances in anatomical knowledge of the syrinx that guides her laboratory.

“This is a structure that was not known to exist outside of hummingbirds, but our CT scans showed that swifts have these vocal folds in the same position,” Clarke said. “This is the kind of journey we had to take to get these answers.”

At the same time, Clarke and her team developed methods to preserve and record the anatomy of the syrinx in all bird species. They worked with Clifford Tabin, a developmental biologist at Harvard University, to investigate the evolutionary origins of the syrinx by monitoring the gene expression that guided the development of vocal organs in the embryos of birds, mammals and reptiles.

The research published in Current biology is the culmination of that collaboration. The study describes how scientists discovered the deep connection between the larynx and syrinx tissues by observing that the same genes controlled the development of the vocal organs in mouse and chicken embryos, respectively, even though the organs emerged from different embryological layers.

“They form under the influence of the same genetic pathways, ultimately giving the vocal tissue similar cellular structure and vibration properties in birds and mammals,” says Tabin, co-leader of the study.

The study also analyzed syrinx development in several bird species – observing gene expression in embryos of fourteen different species, from penguins to parakeets – and found that the common ancestor of modern birds likely had a syrinx with two sound sources, or two independently functioning vocal sources. folds. This trait is found in songbirds today, allowing many to create two different sounds at the same time. The research suggests that the common ancestor of birds may have made similarly divergent calls.

These results may shed light on the origins of the syrinx, but it is still unknown when the syrinx first evolved and whether non-avian dinosaurs — the ancestors of today’s birds — had the vocal organ, Clarke said. No one has yet found a fossil syrinx from a non-avian dinosaur.

According to Clarke, the best way to understand the possibilities of ancient dinosaur sounds is to study vocalization as it exists today in birds, the dinosaurs that are still with us, and other reptiles.

“We can’t start talking about sound production in dinosaurs until we really understand the system in living species,” she said.

Chad Eliason, senior researcher at the Field Museum of Natural History and former postdoctoral researcher at the Jackson School, also made significant contributions to this and other Syrinx projects.