Birds reinvent voice box in novel evolutionary twist

Birds tote around two vocal organs inside their bodies, but only one works. New interdisciplinary research suggests that this distinctly avian anatomy arose because birds, somewhere in their evolutionary history, opted for building a brand new vocal organ–the syrinx– instead of modifying an existing one that is present in an array of animals but silent in birds–the larynx.

A singing bird (Dickcissel, Spiza americana). Birds are able to sing thanks to a unique organ
called the syrinx [Credit: John Bates, Field Museum]

The researchers, a team of scientists including developmental biologists, paleontologists and evolutionary biologists, said that the evolution of the syrinx– which is unique to birds–raises questions about changes in bird vocalization over time and can help shed light on the mechanisms driving the development of new structures in animals. The syrinx is an especially interesting case because it is one of the rare instances where a new structure evolves without serving a new function.

“The syrinx is a kind of novelty that you don’t see commonly in the tree of life,” said lead author and principal investigator Julia Clarke, a professor at The University of Texas at Austin Jackson School of Geosciences. “What’s kind of peculiar about it is that in a lot of biological novelties the structures change in response to a new function, but in this case you have apparently the same function.”

The diverse team, including Evan Kingsley and Cliff Tabin from Harvard Medical School as well as UT affiliates Chad Eliason and Zhiheng Li, published their study in Proceedings of the Natural Academy of Science. The study presents a look at how and why birds developed a completely new way of producing sound.

Most land-dwelling animals with a backbone produce sounds using the larynx. Some animals modify the sounds with the help of other structures. For example, male koalas have vocal folds above their larynx that create deep bellows, and toothed-whales can modify vocal sounds using folds in their nasal cavity. However, the syrinx–located deep inside the chest near the heart–stands out because it is a distinct vocal organ in its own right. Part of the research involved investigating how the syrinx and larynx may have once worked together.

These are 3D models of the larynx of an alligator (Alligator mississippiensis) and the syrinx of a muscovy duck
(Cairina moschata). Specimens were dissected out, stained and scanned at The University of Texas
High-Resolution CT Facility [Credit: Julia Clarke et al. PNAS, 2018]

The researchers argue that the presence of a functional larynx in closely related bird relatives presents a possible evolutionary scenario where the ancestors of living birds may have had two functional vocal organs–a larynx and a syrinx–that sounded together, until eventually the larynx went silent and only the syrinx remained functional.

“We also present an alternate scenario where the syrinx, instead of arising as an accompaniment, evolved as a replacement noisemaker after the larynx lost its primary sound producing function, ending a potential ‘quiet zone’ in evolutionary history where bird ancestors didn’t make a peep,” said Eliason, a postdoctoral associate at the Field Museum of Natural History and a former Jackson School postdoctoral researcher.

The researchers note that the presence of a perfectly functional larynx in the closest living relatives of birds and improvements in hearing in multiple lineages of dinosaurs make this scenario not as likely as the first.

In addition to looking into the evolutionary environment, the team also investigated the developmental history of the syrinx in comparison to the larynx.

Two primary hypotheses for the evolutionary transition from a laryngeal sound source to a syringeal sound source
(blue shaded box). Auditory innovations are shown as black dashes and suggest a sustained role for acoustic
communication in archosaurs. Understanding whether the shift to a syringeal sound source occurred early
or late in bird-lineage archosaurs will require further comparative genomic and paleontological work
[Credit: Julia Clarke et al. PNAS, 2018]

“Despite their related function, the syrinx and larynx have distinct developmental histories–forming from different tissues,” said Kingsley.

The origin of a unique developmental path and novel bird organ could be linked to other amazing avian innovations–from the dawn of flight to the evolution of complex bird song and the development of long necks in extinct dinosaur ancestors of modern birds.

“What we show about syrinx development suggests fundamental differences from the larynx and has important implications for how the enormous variation we see within birds in song and structure arose,” Kingsley said.

Even though the syrinx may be just for the birds, Clarke said that understanding more about what led to its origin and development could help scientists learn more about how biological innovation works in the big picture.

“When we put this all into the context of biological novelty, we gain insight into how new structures and functions arise in the history of life,” Clarke said.

Source: The University of Texas at Austin [September 24, 2018]

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