The bird voice box is one of a kind in the animal kingdom
The melodious call of many birds comes from a mysterious organ buried deep within their chests: a one-of-a-kind voice box called a syrinx. Now, scientists have concluded that this voice box evolved only once, and that it represents a rare example of a true evolutionary novelty.
“It’s something that comes out of nothing,” says Denis Dubuole, a geneticist at the University of Geneva in Switzerland who was not involved with the work. “There is nothing that looks like a syrinx in any related animal groups in vertebrates. This is very bizarre.”
Reptiles, amphibians, and mammals all have a larynx, a voice box at the top of the throat that protects the airways. Folds of tissue there—the vocal cords—can also vibrate to enable humans to talk, pigs to grunt, and lions to roar. Birds have larynxes, too. But the organ they use to sing their tunes is lower down—where the windpipe splits to go into the two lungs. The syrinx, named in 1872 after a Greek nymph who was transformed into panpipes, has a similar structure: Both are tubes supported by cartilage with folds of tissue.
The oldest known syrinx belongs to a bird fossil some 67 million years old; that’s about the same time all modern bird groups became established. To figure out where the bizarre organ came from, Julia Clarke, a paleontologist at the University of Texas in Austin, who made the syrinx discovery in 2013, assembled a team of developmental biologists, evolutionary biologists, and other researchers. The group combed the literature and compared the anatomy, genetics, and development of bird syrinxes and larynxes from a range of modern reptiles. The organs are quite different—even more so than early biologists believed—they discovered. To work the vocal cords, larynxes depend on muscles that attach to that organ’s cartilage. But the syrinx relies, at least in part, on muscles that in other animals extend from the back of the tongue to the bones that connect the arms to the body.
The two organs also appear to develop differently. The larynx is made from a mix of mesoderm and neural crest cells, which make up muscles and some facial bone and nerve cells, respectively. But the syrinx is made of just mesoderm cells—there are no neural crest cells involved. “It presents a rather interesting rare case of how new structures and developmental programs evolve,” Clarke says. These differences, however, still resulted in organs with the same function.
Clarke and her colleagues suspect the ancestors of modern birds also had a larynx. Then, at some point before birds became birds, the cartilage in the windpipe just above the lungs expanded to form the syrinx. This expansion may have initially provided additional support for the split in the windpipe; eventually, it developed rings of muscle that enabled the complex avian sound repertoire heard today. Over millions of years, the syrinx took over sound production from the larynx, possibly because the syrinx was more versatile at producing a wide variety of sounds
This means that the syrinx is an evolutionary novelty, Clarke and her colleagues reported last week in the Proceedings of the National Academy of Sciences. True novelties in evolution are hard to come by. They are innovations—new traits or new structures—that arise without any clear connections to existing traits or structures. Most previously suspected novelties, such as fingers and toes in land animals, have turned out to be the result of evolution tinkering with something that already exists, like fish fins in the case of fingers and toes.
Such new innovations can “trigger further evolutionary steps,” says Johannes Müller, a paleozoologist at the Museum of Natural History in Berlin. By enabling songs to get more complex, he adds, the syrinx could have prompted birds with new variations of their songs to split into new species.
And the study may have implications beyond avian crooners. Behavioral ecologist Richard Vogt from the National Institute of Amazonian Research in Manaus, Brazil, says it gives him a starting point to search for the structures that make sounds in turtles. Since 2008, Vogt and conservation biologist Camila Rudge Ferrara of the Wildlife Conservation Society in Manaus have shown that turtles, particularly social species, make a variety of sounds, even in their egg cases. It’s currently unclear whether they are using their larynx or generating these noises in just their mouths.
Neither turtles nor the later-evolving crocodiles has a syrinx, says Nicolas Mathevon, an ethologist at the University of Lyon/St. Etienne in France, who studies the sounds crocodiles and their relatives—the only modern representatives of dinosaurs apart from birds—make. Crocodiles diverged from birds 240 million years ago, and many are famous for their calls.
But crocs have a very basic larynx, with structures that vibrate in airflow. If the syrinx evolved as birds came into existence, then “some dinosaurs may have had two sound sources,” Mathevon says. “Maybe one day we will find a fossil of a dinosaur with a larynx together with a syrinx.”