Birdsong and the human voice built from the same genetic blueprint

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

A trio of recent studies led by researchers from the University of Texas at Austin are changing that.

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

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

“To me, this is as big as the transition from flippers to limbs,” said Clarke, who co-directed or co-authored the studies. “In some ways, it’s even bigger because the syrinx is not a modified organ with a new function, but an entirely new organ with an ancient, shared function.”

All three studies build 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 in what is now Antarctica during the Late Cretaceous. The specimen is the oldest syrinx that has been discovered. But when she tried to compare the fossil syrinx with the syrinxes of modern birds, she found a dearth of scientific literature. Many of the studies date back to the 19th century, before the advent of modern scientific imaging, or cited claims from those older studies without verifying them.

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

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

Over the years, Clarke and members of her lab have developed new methods for dissection, preservation and CT scanning of syringes that have helped reveal the syrinx in greater detail. These extended views of the ostrich and hummingbird vocal organ have 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 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.) However, even though both sexes have the same vocal apparatus, male ostriches tend to make a greater variety. wider range of sounds than women. ostriches, with sounds often associated with aggressive behavior among noisy males.

On a visit to an ostrich farm in Texas, researchers recorded 11 types of calls, ranging from high-frequency chirps and gurgles in young ostriches to low-frequency squeaks and grunts in adult males. These included several types of calls that had never been recorded before. The only sounds eventually recorded by adult female ostriches were whistles. What the females lacked in range, they made up for in attitude, said Michael Chiappone, who became involved in the ostrich research as an undergraduate at the Jackson School and is the study’s lead author.

“They were pretty prolific whistlers,” said Chiappone, who is now a doctoral student at the University of Minnesota.

For the study of hummingbirds in Zoological Journal of the Linnean Society, researchers compared the hummingbird’s syrinx with that of swifts and nightingales, 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 nightingales work with a limited repertoire of instinctive calls, while hummingbirds are able to elaborate calls by learning complex songs from each other, a trait called vocal learning.

According to Lucas Legendre, a Jackson School research associate who led the hummingbird research, the findings suggest that the common ancestor of all three birds also had a similar structure of vocal folds — and that it may have helped lay the groundwork for on evolution in vocal learning. in the hummingbird.

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

Before the study, it was uncertain whether swifts even had vocal folds. As part of the research, Lezhandre created a 3D digital model of the fast vocal track that takes viewers from the wash to the syrinx and to the vocal folds that sit near the tip of each branch of the syrinx. The model — called a “magical mystery journey” by Clarke — shows the advances in anatomical knowledge of the syrinx that her lab is leading.

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

At the same time Clarke and her team were developing methods to preserve and capture syrinx anatomy across avian species, they were collaborating with Clifford Tabin, a developmental biologist at Harvard University, to investigate the evolutionary origins of the syrinx by tracing the expression of genes that accompanied the development of vocal organs in the embryos of birds, mammals and reptiles.

Research published in Current Biology is a culmination of that collaboration. The study details how scientists discovered the deep connection between laryngeal and syrinx tissues by observing that the same genes were controlling vocal organ development in mouse and chicken embryos, respectively, even though the organs arose from different embryological layers.

“They form under the influence of the same genetic pathways, ultimately giving vocal tissue similar cell structure and vibrational properties in birds and mammals,” said Tabin, a co-leader on the study.

The study also analyzed the development of the syrinx in bird species – which involved looking at gene expression in embryos from 14 different species, from penguins to puffins – and found that the common ancestor of modern birds probably had a syrinx with two sound sources , or two independently functioning vowels fold. This feature is found in songbirds today, allowing many to create two different sounds at the same time. The research suggests that the birds’ common ancestor may have made similar, different calls.

These results may shed light on the origin of the syrinx, but it is still unknown when the syrinx first developed 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 continue to study vocalization as it exists today in birds, dinosaurs that are still with us, and other reptilian cousins.

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

Chad Eliason, a senior research scientist at the Field Museum of Natural History and former postdoctoral researcher at the Jackson School, was also a major contributor to these syrinx projects and others.