Until now, the scientific community had thought that whistles were the main means to communicate , and were unaware of the importance and use of burst-pulsed sounds. Researchers from the Bottlenose Dolphin Research Institute (BDRI), based in Sardinia (Italy) have now shown that these sounds are vital to the animals' social life and mirror their behavior.
Although lacking vocal cords, dolphins emply the sphincter muscles within the blow holes to communicate by producing a complicated system of whistles, squeaks, moans, trills and clicks of varying frequencies. The clicks are also a device for echolocation, or sonar, the clicking sounds bouncing off objects with the echoing sound waves sensed by the dolphin's bulbous forehead and lower jaw. These communicative mechanisms enables them to determine the distance, size and shape of objects.
"Burst-pulsed sounds are used in the life of bottlenose dolphins to socialise and maintain their position in the social hierarchy in order to prevent physical conflict, and this also represents a significant energy saving," says Bruno Díaz, lead author of the study and a researcher at the BDRI.
The study presents the most complete repertoire ever of these burst-pulsed sounds and whistles, gathered using bioacoustics since 2005 in the waters off Sardinia.
According to the experts, the tonal whistle sounds (the most melodious ones) allow dolphins to stay in contact with each other (above all mothers and offspring), and to coordinate hunting strategies. The burst-pulsed sounds (which are more complex and varied than the whistles) are used "to avoid physical aggression in situations of high excitement, such as when they are competing for the same piece of food, for example," explains Díaz.
Bottlenose dolphins, according to Díaz, make longer burst-pulsed sounds when they are hunting and at times of high aggression: "These are what can be heard best and over the longest period of time," and make it possible for each individual to maintain its position in the hierarchy.
The dolphins emit these strident sounds when in the presence of other individuals moving towards the same prey. The "least dominant" one soon moves away in order to avoid confrontation. "The surprising thing about these sounds is that they have a high level of uni-directionality, unlike human sounds. One dolphin can send a sound to another that it sees as a competitor, and this one clearly knows it is being addressed," explains the Spanish scientist.
Casey Kazan via Phys.org and Plataforma SINC (2010, June 9). Dolphins use diplomacy in their communication, biologists
Image credit: With thanks to SeaPics.com