|Photo Credit: NASA (Public Domain image, via Wikimedia Commons)|
Dolphins produce different sounds such as squeaks, moans, trills, grunts, and so on. They're most famous for their whistles, and they even have what we call "signature whistles" that they use to identify each other. These whistle sounds were a mystery to researchers for a long time, because dolphins are able to produce the sounds underwater, and a true whistle is produced by a stream of forced air. There's also the fact that there's compression when a dolphin dives, which should change the frequency of the sound, but the frequency remains the same.
Last year, researcher Peter Madsen and his colleagues at the Institute of Bioscience in Denmark re-examined a study from the 1970s in which scientists had dolphins breathe heliox (a mixture of helium and oxygen) because it should have mimicked the conditions during a dive by causing a change in frequency. However, the whistles were the same, whether the dolphin was breathing heliox or regular air. Madsen's group concluded that dolphins probably make the whistling sounds by vibrating tissues called "phonic lips" that are located in their nasal cavities. Dolphins can quickly change the frequencies of the sounds by changing the tension of the tissue and the airflow. They can whistle and click at the same time because they have two sets of "phonic lips" that can either work independently or together.