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Computer models of the larynx of an alligator (left) and syrinx of a duck. The 3D models were created using specimens scanned at the University of Texas high resolution tomography facility. Julia Clarke et al
The birds surround two vocal organs inside their body, but only one works.
New interdisciplinary research suggests that this distinctly avian anatomy arose from the fact that birds, somewhere in their evolutionary history, have opted for the construction of a new vocal organ – the syrinx – instead of modifying an existing larynx .
The researchers, a team of scientists including developmental biologists, paleontologists and evolutionary biologists, said the evolution of syrinx – unique in birds – raises questions about changes in the vocalization of birds. new structures in animals. The syrinx is a particularly interesting case because it is one of the rare cases where a new structure has evolved without serving as a new function.
"Syrinx is a kind of novelty that you do not often see in the tree of life," said lead author and researcher Julia Clarke, a professor at the University of Texas at the Austin Jackson School of Geosciences. . "What's strange is that in many biological novelties, structures change in response to a new function, but in this case, you seem to have the same function."
The diverse team, including Evan Kingsley and Cliff Tabin of Harvard Medical School, as well as UT affiliates Chad Eliason and Zhiheng Li, have published their study in the journal. Proceedings of the National Academy of Sciences September 24th.
The syrinx is at the base of the trachea near the heart in birds. The larynx is located above the trachea. While most terrestrial animals with a spine produce sounds with vocal cords in the larynx, the birds do not use it. Instead, the birds create calls in the syrinx by passing air on soft tissues supported by cartilaginous rings.
The study examines how and why birds have developed this brand new way of producing sound.
Some animals modify the sounds produced by the larynx with the help of other structures. For example, male koalas have vocal cords above their larynx that create deep bellows, and toothed whales can alter vocal sounds by using folds in their nasal cavity.
However, the syrinx of the birds stands out because it is a distinct vocal organ that exists next to another vocal organ. Although the larynx is now extinct from birds, the fact that it is functional across the animal kingdom presents a possible evolutionary scenario in which the ancestors of living birds could have two functional vocal organs – a larynx and a syrinx – that sounded together. .
Researchers are also exploring an alternative scenario in which the larynx has stopped working and the syrinx has evolved to give back to the ancestors of modern birds.
A table illustrating the proposed vocal organs and vocal organs of the ancestors of birds and their functioning (blue box). Archosaurs, ancestors of birds that include avian dinosaurs, may have a larynx and syrinx that have resonated together during a period of overlap. Alternatively, the evolution of syrinx could have ended an "area of silence" where the ancestors of the birds had no functional vocal organs. Understanding whether the switch to syrinx occurred sooner or later in the evolution of the archosaur will require further comparative genomic and paleontological work. Julia Clarke et al
"Instead of being born as an accompaniment, the syrinx evolved as a creator of sound after the larynx lost its main function of sound production, ending a" quiet zone "of evolutionary history where the ancestors bird, postdoctoral fellow at the Field Museum of Natural History and former postdoctoral fellow at the Jackson School.
However, the researchers note that the presence of a perfectly functional larynx in the closest living relatives and improved hearing in multiple dinosaur lineages make the operation of syrinx and larynx more likely that the "silent zone".
In addition to examining the changing environment, the team studied the history of syrinx development relative to the larynx.
"Despite their associated function, syrinx and larynx have a distinct history of development – forming from different tissues," said Kingsley.
The origin of a unique development path and a new bird organ could be linked to other amazing avian innovations – from the dawn of the flight to the ########################################################################################### 39, to the evolution of complex bird songs and the development of long extinct ancestors dinosaur necks of modern birds.
"What we show about the development of syrinx suggests fundamental differences from the larynx and has important implications for how the huge variation we observe in birds in song and structure has emerged," he said. said Kingsley.
Although syrinx may only be for birds, Dr. Clarke said that better understanding of what led to its origins and development could help scientists learn more about how it works. biological innovation works in the big picture.
"When we put all this in the context of biological novelty, we get a glimpse of how new structures and functions appear in the story of life," Clarke said.
The research was funded by the Gordon and Betty Moore Foundation.
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