Vestigial Vocal Organ Muffles Human Speech

All Over But the Shouting...

All four great apes -- humans, chimps, gorillas and orangutans -- have vocal tract air sacs evolved for calling out to others over long distances. In humans, new research suggests, the anatomical structures have shrunk, leaving us with the vestiges of the sacs and much quieter voices as a result.

The find highlights how human evolution sacrificed volume for a better ability to speak with others, one on one.

Such "private talk" allows an individual to exclude unwanted listeners, such as eavesdropping prey in the wild or business rivals in modern life.

For many animals, explained lead author Tobias Riede, "an amplifying device is helpful."

"Unfortunately, it comes with a cost," added Riede, who is a researcher at the National Center for Voice and Speech in Denver. "You have to fine-tune it in order to keep the voice from breaking."

He and his colleagues came to that conclusion after studying models of mammalian air sacs, made "Myth Busters"-style out of PVC pipe, an inflatable urinary bladder from a pig, and other items. All experiments were conducted at the Japan Institute of Science and Technology, where Riede was a visiting researcher.

The scientists found that the larger air sacs in our ape ancestors, as well as certain other mammals, accomplish three things when an individual vocalizes. First, they make sounds louder.

"This happens if the acoustic resonance frequency of the air sacs meets the vibration frequency of the vocal folds," explained Riede.

Second, they change the spectral characteristics of sounds, meaning that the timbre or pitch can vary among individuals.

Finally, the air sacs cause vocalizations to break at times, giving the voice a hoarse, uncontrolled quality, not unlike a singer trying to reach a high note whose voice instead cracks.

The findings are published in this month's Journal of the Acoustical Society of America.

If humans had maintained a large vocal sac, our voices would be particularly prone to cracking, said Reide. Men's voices tend to fall within the 80 to 120 hertz spectrum, while women's voices fall between 180 to 250 hertz.

"These numbers are identical to the frequency of vocal fold vibrations," he said, adding that air sacs would be a disadvantage since "our voices would break all of the time if the air sac resonance [would] come near the frequency of the vocal fold vibrations."

Humans instead have what appear to be relics of the larger air sacs. These are the laryngeal ventricles, located in the voice box.

Chuck Brown, a professor of psychology at the University of South Alabama who specializes in the biology of acoustic communication, told Discovery News that prior to the research, he did not know that other primates may use the air sac effect "to heighten the possibility of communication in some settings."

"In speech, instabilities in voicing are usually regarded as undesirable phenomena that tend to impair communication," Brown said. "Yet, in many of our closest primate relatives, the presence of air sacs and other features of their vocal anatomy appear to be designed to expand the opportunities" for communication.

Riede and his team have already confirmed their findings by studying how air sacs work in Siamang gibbons, but they hope to conduct more studies on animals with air sacs -- big and small -- in the future.