They also think that toothed whales might communicate in the same way. Madsen and the team suggest the most likely tissues for producing the sounds are the 'phonic lips' in the nasal air cavities. He said this makes sense because using tissue vibrations would allow the dolphins to communicate more effectively at depth. They found that the sounds did not change pitch when the dolphin was breathing Heliox.ĭr Madsen said the results of the analysis suggest the sounds were not made as whistles at all (which would be made by expelling air out rapidly) but were the result of 'pneumatically induced tissue vibrations,' and this would explain why the sounds did not change in the presence of Heliox. Now, with the benefit of digital technologies, Madsen's team were able to digitize the old recordings and use advanced computing and visualization scripts to analyze them for the harmonics and frequencies of each recorded whistle. The data gathered by the Navy team could not be fully analyzed because at the time an analysis of a single whistle would have taken several hours. If that were true, the pitch of the sounds would change as the dolphin moved deeper, since the increased pressure in the nasal air cavities would also raise the pitch of their sounds. The scientists at the time thought the dolphin sounds were made by resonance of air in their nasal cavities. The aim of using Heliox was to find out if the dolphin sounds would rise in pitch in the presence of helium, as the human voice does (since the speed of sound in heliox is 1.74 times faster than in air). The Heliox was delivered to the dolphin via a mask over the animal's blowhole. They recorded the sounds made by the dolphin, which they interpreted as whistles, while the animal was breathing air and while breathing Heliox, which is a mixture of helium (80%) and oxygen (20%).

The researchers, Sam Ridgeway and Don Carder, were studying a trained bottlenose dolphin (Tursiops truncatus). Scientists at Aarhus University in Denmark, led by Peter Madsen, analyzed data gathered in 1977 by scientists working with the US Navy Marine Mammal Program.

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