While many dolphin calls sound like whistles, the study found the sounds are produced by tissue vibrations analogous to the operation of vocal folds by humans and many other land-based animals.
Communicating similar to the way that humans do solves what would otherwise be a major dolphin problem.
"When we or animals are whistling, the tune is defined by the resonance frequency of some air cavity," said Peter Madsen, lead author of the research appearing in Royal Society Biology Letters."The problem is that when dolphins dive, their air cavities are compressed due to the increasing ambient pressure, which means that they would produce a higher and higher pitch the deeper they dive if they actually whistle."
Madsen, a researcher in the Department of Biological Sciences at Aarhus University, and his team studied how dolphins communicate by digitizing and reanalyzing recordings made in 1977 of a 12-year-old male bottlenose dolphin.
The dolphin breathed in a "heliox" mixture consisting of 80 percent helium and 20 percent oxygen -- a concoction that causes humans to sound like, as the scientists put it, Donald Duck. The reason is because the mixture has a sound speed that's 1.74 times higher than normal air. If a person whistles after sucking in helium, the pitch of the tune will then be 1.74 times higher than if he or she whistles after breathing in just 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," Madsen said. "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," he added.
The researchers believe the finding applies to all toothed whales, since they have similar nasal anatomy and they "all face the same problem of making sound during deep dives."
In terms of what the dolphins are communicating, it's known they share information about their identity, helping them to stay connected even while traveling in vast bodies of water.
Acoustics engineer John Stuart Reid and Jack Kassewitz of the organization Speak Dolphin have created an instrument known as the CymaScope that reveals detailed structures within sounds, allowing their architecture to be studied pictorially.
Similar to deciphering Egyptian hieroglyphs, the researchers may then be able to figure out the meaning of dolphin calls. In addition to the whistle-like sounds, dolphins produce chirps and click trains, suggesting they engage in very complex and sophisticated social interactions.
"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," Kassewitz said. "The CymaScope provides our first glimpse into what the dolphins might be 'seeing' with their sounds."
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