A quick glimpse of a rapidly moving dorsal fin is what people usually associate with the sight of a porpoise. Although there are as many as 100,000 porpoises in Danish waters, we know surprisingly little about the small whale. This is mainly because it is very shy and because it only has limited social behaviour near the surface.

Other species of whale, such as the charismatic bottlenose dolphin, live in social groups, where individuals are able to communicate, identify and find each other by means of a distinctive whistling sound used specifically for communication.

Porpoises, however, can only produce one sound: a short, high-frequency sound, which is primarily used for echo-location. The porpoises emit the sound and listen for the returning echo in order to find their prey. Now, researchers from Aarhus University have thrown completely new light on the social life of the harbour porpoise and its sound communication by attaching small sound recorders to their backs. The study has revealed that porpoises use their echo-location click, normally used to search for food, to compose special social calls as well.  

Pernille Meyer Sørensen, who has had these findings from her Master's thesis published in the prestigious Scientific Reports, is very excited about the new findings, which provide another piece in the puzzle of the porpoise’s mysterious life.

"I already had a suspicion that porpoises could communicate with sound in one way or another. Sound is important for communication for virtually all other marine mammals, and porpoises are dependent of social behaviour, for example in connection with mating and bringing up their young. But that porpoises communicate with each other so much was a complete surprise," said Pernille Meyer Sørensen.

Their social calls consist of a number of clicks, and the researchers found that porpoises call out much more when there are other porpoises nearby. Closer investigation of the calls showed that porpoises can make at least two different types of call, and these may be used either to keep in contact with other porpoises, or to scare them away.

Finally, the porpoises often repeated the same call again and again, and calls were generally emitted with higher intensity than the clicks produced just before they catch their prey. Researchers from Department of Bioscience at Aarhus University suggest that this is because the porpoise’s clicks are at such high frequency that they are not transmitted very far, and not in all directions. Quite simply, porpoises send out their calls to increase the likelihood of their being heard. 

"It may perhaps seem rather illogical that porpoises haven’t developed a more low-frequency sound so that they can contact other porpoises over longer distances, as we see in many other whales. However, the porpoise has one major advantage from producing sound at such a high frequency: predators such as killer whales can’t hear sound at that frequency. The high-frequency sounds enable the porpoise to call out as much as it likes without being heard by predators with a taste for porpoise," said Pernille Meyer Sørensen.

The scientific article in Scientific Reports is on this link.

Contact:

Research assistant Pernille Meyer Sørensen
Department of Bioscience - Zoofysiologi
Aarhus University
Mail: pernille.soerensen@bios.au.dk
Mobile: +45 4117 0675