Tweets, Quacks & Chirps
Tabin Explores How Birds Develop Voices
“All the little birds on Jaybird Street |Love to hear the robin go tweet tweet tweet”
Notwithstanding Bobby Day’s 1958 chart-topping Rockin Robin, scientists have been pondering bird song since the 1700s. In fact, longtime NIH grantee Dr. Cliff Tabin and his laboratory have collaborated with paleontologist colleagues to delve back even farther for clues—into the Cretaceous Age of dinosaurs.
Over the course of more than 30 years, Tabin’s work in bone, limb and organ formation has been funded by several institutes including NIAMS, NHLBI, NIDDK, NHGRI and NICHD. In a Wednesday Afternoon Lecture Series talk titled “How did birds evolve the capacity to vocalize?,” he shared some of his group’s research. Their effort to document and explain the whens, hows and whys of birds gaining voice opens fascinating potential investigations regarding not only voice but also other functional and structural developments in human health and in the animal kingdom at large.
The George Jacob and Jacqueline Hazel Leder professor of genetics and chair of the department of genetics at Harvard Medical School, Tabin studies the genetic basis of structure and form during embryo development and over evolutionary time.
Birds “sing” using a unique vocal organ called the syrinx—an example of the evolution of a novel structure. Tabin’s lab examined the genetic pathways that lead to formation of the syrinx in order to understand its morphogenesis and how the adaptation arose.
He said most tetrapods (four-limbed animals) such as amphibians, mammals and reptiles use their larynx—voice box with pulsating vocal chords in the throat—to speak. Birds are the exception, though. While they also have a larynx, theirs doesn’t come with the vibrating “flaps” that produce sound. A separate organ, the syrinx, located in a bird’s chest, enables our feathered friends to make sounds.
Syringes, Tabin noted, are shaped dramatically different in individual bird species, which accounts for their unique “songs.” The wood thrush, for example, can harmonize with itself, producing a complex song using two sets of vocal chords.
Unique to Physique
Why does the wood pigeon sound different than the white-throated sparrow?
“The syrinx is a beautiful, diverse organ,” said Tabin, playing recorded “songs” of both birds. His lab now is studying the structure’s diversifications.
Both larynges and syringes are composed of cartilage. The variety of shapes in which the cartilage grows regulates muscles differently and locates a bird’s vocal folds in different areas. Think of the intricacies in the design of horn instruments, for example. Chambers and airways built into a clarinet’s body allow it to produce sounds different from those of a tuba or oboe. In this analogy, the vocal folds of vibratory tissue would be the equivalent of the reed for the oboe.
Somewhere on the evolutionary spectrum, birds lost the ability to use their larynx to make sound and developed the syrinx for that function instead. Tabin’s group and paleontologists at the University of Texas-Austin wanted to learn how and why that happened.
Premise for Peeps
Researchers analyzed the airway RNA sequences of alligators (which form larynges) alongside those of chickens (which develop syringes in addition to larynges). Turns out that something called the hedgehog signaling pathway (Shh) acts as a directional guide for forming parallel cartilage rings in the airway of tetrapods.
In birds, Shh has taken on a second important role as a timing mechanism, allowing the syringeal cartilage to form in a distinct shape prior to the formation of other airway cartilages, which would otherwise interfere with them.
Tabin’s group has also been able to draw strong parallels between birds’ genetic development of the vibratory tissue of the syrinx and genetic formation of the vocal cords in the larynx of other animals.
“We argue that the syrinx developed from the larynx by co-opting some of the regulatory networks that were already in place,” he said.
Thinking back farther in time, Tabin considered where the laryngeal gene networks may have come from. Scientists know that the larynx evolved in early air-breathing tetrapods to protect the airway. It’s thought that laryngeal valves—the vibrating flap-like vocal folds that produce sound—then formed, enabling animals to vocalize.
The larynx networks, Tabin speculated, which emerged prior to the syrinx, may be related to a body’s other “floppy, movable tissues,” similar to those found in heart valves, which developed even earlier in evolution.
“Birds built a new vocal organ from scratch,” Tabin concluded.
Its formation depended on altering the timing of the formation of the novel voice box relative to the development of the rest of the airway. And, although the larynx doesn’t function the same in fowl, he said, birds were able to mimic some of the organ’s structural features and create a novel instrument—the syrinx—that makes birds’ music uniquely their own.
Watch Tabin’s archived lecture in full at https://videocast.nih.gov/watch=44216.