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Songbirds can precisely control single vocal muscle fibers while singing

Zebra finches and other songbirds can control single muscle fibers during singing. Photo by Jim Bendon/Wikimedia Commons
Zebra finches and other songbirds can control single muscle fibers during singing. Photo by Jim Bendon/Wikimedia Commons

June 4 (UPI) -- Most studies of songbird singing has focused on the neurological pathways involved in the learning and performance of their intricate songs, but new research has offered unique insights to the actual mechanics of songbird vocalization.

The new study, published Friday in the journal Current Biology, suggests songbirds have precise control over their singing muscles.

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The complex melodies produced by zebra finches and other songbirds are made possible by the syrinx, a vocal organ unique to birds.

The syrinx is located at the base of the trachea and is surrounded by muscles that vibrate at extremely high frequencies.

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"We found that songbirds have incredible fine control of their song, including frequency control below one Hertz," lead study author Iris Adam, assistant professor of biology at the University of Southern Denmark, said in a press release.

Using already established data on neuronal singing patterns, researchers used sophisticated mathematical models to decipher the relationship among motor control neurons, nerve fibers and single muscles in zebra finches, which allowed scientists to predict how many fibers single motor neurons control.

Their analysis showed most of the motor neurons involved in singing control a small number of fibers. Many control just a single fiber.

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"Motor units vary in size from several hundreds or thousands of muscle fibers in our leg muscles down to only 5 to 10 in the muscles controlling eye position and the muscles in the larynx" senior study author Coen Elemans said in the release.

"In zebra finch song muscles our models predicted that 13% to 17% of the motor neurons innervates a single muscle fiber," said Elemans, head of the Sound Communication and Behavior group at the University of Southern Denmark.

The new analysis technique enabled scientists to measure the movements of individual fibers within a single muscle.

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"Our new method allowed us for the first time to activate single motor neurons and visualize and record the activity of all responding muscle fibers simultaneously," Adam said.

To better understand how this level of precision influences song performance, researchers measured the stress of a single fiber on the frequency of a vibrating muscle.

The analysis showed a single muscle fiber can put enough stress on muscle to alter its frequency.

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"To be able to study how changes in muscle force alter the sound made by the birds' vocal organ, the syrinx, we had to invent a new setup" Elemans said. "This setup blows air through the syrinx while we can control the muscles with small motors."

These tremendous levels of vibrational speed and motor control precision involved have been inspired by the songbird's 40-million-year-long quest to woo potential mates.

Previous studies have shown that the tiniest of differences in song performance can affect a female's mating choice.

This precise control can also help tell the story of songbird diversification. As songbirds evolved greater control over their singing muscles, they were able to develop a greater diversity of songs and differentiate themselves.

"We think that next to a special syrinx and their amazing ability to imitate sounds, the fine gradation of the song features such as pitch has increased the amount of different sounds a bird can make," Adam said. "We suggest that the fine graduation of sound has contributed to the radiated of songbirds."

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