A caixa de voz de pássaros é única no reino animal
“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.
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
Thrush song
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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.
Turtle sounds
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Caiman call
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doi:10.1126/science.aav6415
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