Only Birds Have a Syrinx and That's Why They Sing

The ovenbird (Seiurus aurocapilla) is a small songbird of the New World warbler family. He uses his syrinx to sing. Larry Keller, Lititz Pa./Getty Images

Convergent evolution is the idea that two organisms that aren't very closely related can independently evolve a very similar trait. For instance, the last ancestor we shared in common with an octopus probably looked a lot like a worm. Even though that animal might have been able to detect light and dark, it took something like 750 million years of both human and octopus ancestors working on being able to see properly to evolve the incredibly similar eye structures humans and octopuses now share. Nifty!

But this isn't how it always happens. Take, for instance, the voice box of a bird. It's called a syrinx, and no other organism on Earth has one. It's something of an evolutionary mystery why not, actually. After all, other animals need to be able to communicate, just like both cephalopods and mammals need to be able to see what they're doing. In fact, the syrinx is arguably much weirder than the high-acuity, camera-style eye we share with the octopus because it came out of nowhere, evolutionarily speaking.

It could be argued that the syrinx is a bit redundant in bird anatomy, as they also possess larynxes — the voice box structure they share with mammals and some amphibians and reptiles (though it only functions very well in mammals and a few lizards). The larynx is what enables cows to moo, dogs to bark and babies to cry, and it's situated at the top of the throat. However, birds use their syrinx to make their flutey vocalizations — it's similar to the larynx in that it's made of folds of vocal membranes supported by cartilage; however the syrinx is buried deep in a bird's chest, right above where their tracheobronchial tube splits off into their lungs.

In 2016, paleontologists from the University of Texas in Austin reported that, based on fossil evidence, the structure we know as the syrinx is around 67 million years old. Since then, the research team has been comparing the larynx and syrinx anatomy, genetics and development of birds to those of modern reptiles, and have discovered the evolution of the syrinx is even stranger than previously thought.

In a new paper published in the Sept. 24, 2018 issue of the Proceedings of the National Academy of Sciences, the research team reports the function, form and development of the syrinx and the larynx are actually quite different. For starters, the vocal cords of the larynx are manipulated by muscles attached to the cartilage that supports them. The syrinx, on the other hand, partly attaches to muscles that, in other animals, fasten the tongue to the bones connecting the arms to the rest of the body. Secondly, while baby birds and baby lizards are developing, different types of cells form their respective voice boxes: Larynxes are made of a mixture of mesoderm and neural crest cells, while the syrinx is created exclusively using mesoderm cells.

So, at some point, the ancestors of modern birds just started making a new voice box, which eventually took over the job of the larynx. As a result, now birds can sound like this.

This research is exciting because, though convergent evolution is a really cool thing for scientists to wonder over, a true evolutionary outlier is arguably cooler.

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