The whistling sounds that dolphins make are not true whistles, a new study, published online in Biology Letters on Sept. 7, found.
The term whistle has been used since a paper on dolphins was published in 1949, but the sounds are actually vibrations produced in the nasal cavity, probably using structures called phonic lips, which enable the dolphins to produce sounds across varied depths and pressures.
Led by Peter Madsen of Aarhus University in Denmark, a team of scientists digitized and reanalyzed recordings made in 1977 of a bottlenose dolphin breathing heliox (a mixture of helium and oxygen).
This gas was aimed at simulating pressure conditions during a deep dive because it raises the pitch of sound by 1.74 times that of normal air.
However, the dolphin’s whistles remained at the same tone and frequency. This means the dolphin’s pitch is not defined by the size of its nasal air cavities, that is, it is not whistling.
"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," Madsen said, according to Discovery News. "That is the same way that we humans make sound with our vocal cords to speak."
"It does not mean that they talk like humans, only that they communicate with sound made in the same way," Madsen told LiveScience.
"Because the frequency is changed by changing the airflow and the tension of the connective tissue lips in the nose, the dolphin can change frequency much faster than if it had to do it by changing air sac volumes," Madsen said, according to LiveScience.
"That means that there is a much bigger potential for making a broader range of sounds and hence increase information transfer."
The team believes this mechanism is present in all odontocetes (toothed whales), such as killer whales and porpoises, because their nasal anatomy is similar and they can all make sounds during deep dives.
As well as so-called whistles, dolphins can also make other complex sounds like chirps and click-trains (multiple clicks). They use these sounds to navigate via echolocation and to communicate.
Interestingly, dolphins can be trained to whistle, but Madsen believes they would not do this in the wild, because they can already make the same sound in a much more effective way.
Another team of researchers have developed a device called the CymaScope to decipher dolphin language by studying it pictorially as "CymaGlyphs," or patterns that represent individual words, rather like the hieroglyphs on the Rosetta Stone that were used to decode Egyptian hieroglyphics.
"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 acoustics engineer John Stuart Reid, according to Discovery News.
"The CymaScope provides our first glimpse into what the dolphins might be ‘seeing’ with their sounds."
