Dolphin whistles aren't just random sounds for other dolphins but are a way the animals "talk" to each other, according to a new study.

Scientists from Europe and the U.S. found that while many dolphin calls sound like whistles to humans, the sounds produced by tissue vibrations are similar to the operation of vocal folds by humans and other land-based animals.

"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 the journal, 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 and his team observed how dolphins communicate by digitizing and reanalyzing a recording of a trained dolphin who had breathed in a "heliox" mixture consisting of 80 percent helium and 20 percent oxygen. The mixture ensures that any sound made will be 1.74 faster than any made with normal air.

"To our surprise, we found that the whistles have the same frequency in air and in heliox," Madsen said. "That means that dolphins do not actually whistle, but produce tonal sounds using tissue vibrations as do humans when we speak."

Dolphins have a rich acoustic repertoire including clicks, burst pulses and tonal calls for communication and to locate each other. It is known that dolphins relay information about each other through this form of communication and enables them to stay connected even while they separated in vast bodies of water.

Another step in deciphering the meaning of dolphin language is in a new instrument called the CymaScope that allows sound to be studied pictorially.

"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," John Kassewitz of the organization Speak Dolphin who co-created the Cyma scope said. "The CymaScope provides our first glimpse into what the dolphins might be 'seeing' with their sounds."