By Katie Scott, Wired UK
A study in which recordings of dolphins made in the 1970s were re-analyzed has revealed that dolphins talk to each other in a manner very similar to human speech, using tissue vibrations.Aarhus University in Denmark concentrated on the dolphin’s whistle, which was believed to be produced by the resonance of air in the dolphin’s nasal air cavities. This would have implications for how dolphins communicate at depth — increased air pressure would affect the size of the nasal air cavities and therefore the pitch of the sounds they can make. Instead, the team discovered that the dolphin’s whistle isn’t in fact a whistle at all; but a sound produced by tissue vibrations.
The study centred upon recordings of a 12-year-old male bottlenose dolphin made in 1977 by Professor Sam Ridgway and Dr Don Carder, who were then working for the US Navy Marine Mammal Program. The dolphin, which had been trained for the study, was given a mixture of 80 per cent helium and 20 per cent oxygen (known as heliox) through a US Navy Fenzy breather, which was placed over its blowhole. It was then prompted to whistle and sound recordings taken. Recordings of the dolphin whistling whilst breathing solely air were also taken.
The hypothesis was that the pitch of the whistles would change as the sound speed in heliox is 1.74 times higher than in air — the dolphin would exhibit the same effect that helium has on humans. Specifically, the pitch of the whistle would be 1.74 times higher than when the dolphin had been breathing air. However, this was not so.
Peter Madsen, lead author of the paper, which has now been published in Royal Society Biology Letters explains that the US Navy team “couldn’t make sense of their findings” as analyzing just one whistle would take an entire afternoon. The Aarhus team digitized the recordings; and analyzed them using mathematical computing and visualization scripts. Speaking to Wired.co.uk, he explained that these scripts allowed the team to trace the harmonics in each whistle; and determine their frequency.
They found that the pitch did not change between the whistles produced in Heliox and the normal whistles. The dolphin’s whistle is therefore not a whistle but, states the paper, a sound produced by “pneumatically induced tissue vibrations analogous to the operation of vocal folds in terrestrial mammals and the syrinx in birds”. The team suggests that dolphins’ phonic lips are good candidates for the source of the sounds.
Madsen adds that this finding reveals why dolphins can “convey information and their identity regardless of the depth that they are swimming”. Specifically, the use of tissue vibrations made by recycling air allows them to change tone very quickly whereas simply blasting air out once, as humans do when we whistle, wouldn’t “be conducive for good communication in a social setting”. Madsen says that dolphins would have used tissue vibrations to communicate when they lived on land; but suggests that they may have lost this ability when they first moved into the oceans only to develop it over time as a more effective form of communication at depth.
Madsen also believes that the findings could have implications for all toothed whales. True whistles would also be less effective for them when swimming the depths of the oceans and so they too may have developed similar ways of communicating.
Audio: Journal of the Royal Society Biology Letters