Dolphins produce sound using tissue vibrations comparable to how human vocal folds work

A team of researchers from the Department of Biological Sciences at Aarhus University has discovered that dolphins "talk" very similarly to the way humans do instead of whistling, like many previously thought.

For years, it was assumed that dolphins communicate through whistles because that's what it sounded like. While dolphins do have the ability to whistle for fun, like humans do, they do not primarily communicate that way.

"When we or animals are whistling, the tune is defined by the resonance frequency of some air cavity," said Peter Madsen, study leader from the Department of Biological Sciences at Aarhus University. "The problem is that when dolphins dive, their air cavities are compressed due to the increasing ambient pressure, which means they would produce a higher and higher pitch the deeper they dive if they actually whistle."

Madsen and his team have found that dolphins produce sound using tissue vibrations comparable to how human vocal folds work.

The team was able to come to these conclusions by digitizing and analyzing recordings of a 12-year-old male bottlenose dolphin from 1977. By doing this, they found that a dolphin breathes in a "heliox" mixture that consists of 80 percent helium and 20 percent oxygen. This combination would make humans sound like Donald Duck, according to the researchers, because it has a sound speed that is 1.74 times higher than normal air.

So if a person whistles after sucking in helium, the pitch will be 1.74 times higher than if that person had breathed in regular air.

"We found that the dolphin does not change pitch when it is producing sound in heliox, which means that its pitch is not defined by the size of its nasal air cavities, and hence that it is not whistling," said Madsen. "Rather, it makes sound by making connective tissue in the nose vibrate at the frequency it wishes to produce by adjusting the muscular tension and air flow over the tissue. That is the same way that we humans make sound with our vocal cords to speak."

The team believes that this finding applies to all toothed whales because they have the same nasal anatomy.

As far as what the dolphins are actually "saying" to one another, it's known that they share identity-related information in order to stay together when traveling. But according to acoustics engineer John Stuart and Speak Dolphin organization member Jack Kassewitz, dolphins have a complex system of social interactions that include sounds like chirps and clicks.

Stuart and Kassewitz developed an instrument called the CymaScope, which pictorially reveals structures within sounds. The CymaScope can be used to understand the dolphin language better in the future.

"There is strong evidence that dolphins are able to 'see' with sound, much like humans use ultrasound to see an unborn child in the mother's womb," said Kassewitz. "The CymaScope provides our first glimpse into what the dolphins might be 'seeing' with their sounds. I believe that people around the world would love the opportunity to speak with a dolphin. And I feel certain that dolphins would love the chance to speak with us -- if for no other reason than self-preservation."

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