But Vincent Janik, of the University of St Andrews, UK, now thinks that the calls of the bottlenose dolphin (Tursiops truncatus) may have another, more deadly purpose. Parts of these mammals' vocal repertoire of whistles, clicks, mews and brays could make up a sonar stun gun that makes easy prey of nearby fish.

Using three 'hydrophones' -- instruments that convert sound waves in water to electrical signals -- arranged in a triangle, Janik recorded and analysed wild dolphin vocalizations, as he describes in Proceedings of the Royal Society: Biological Sciences1. This allowed him to calculate the location of the calling dolphin. Dolphins surface frequently to breathe, so Janik's team could also track the direction and behaviour of individuals attracted by the calls.

Bottlenoses only make low pitch calls -- 'brays' -- when they are dining, Janik reports. These brays occurred during almost all the 'feeding events' he observed. In 71% of these cases other dolphins could be seen swimming rapidly towards the braying dolphin. When a caller used higher pitch vocalizations, such as whistling, other dolphins approached more slowly.

So why do dolphins bray when feeding instead of quietly pigging out by themselves? Are they simply calling for close relatives to come and enjoy the buffet, even though, as Janik points out, friends and acquaintances also turn up? Or does the presence of more dolphins stop the prey -- in this case salmon -- from escaping?

The real reason for the brays, Janik suspects, may be to subdue dinner into sticking around.

Hearing is a crucial sense in water, where vision is restricted. Ken Marten, director of research at Earthtrust's Project Delphis, an ongoing research project into aquatic mammals' cognition in Hawaii, has found that killer whales (Orcinus orca) may use sounds to deafen herring2. He thinks this is a "lock and key mechanism": the whale's feeding calls match the peak of the hearing curve of the fish.

By "overloading their hearing apparatus", fish are rendered effectively deaf, Marten explains. Thus disoriented, they are far easier to catch. As fish hear at low frequencies, he adds, the dolphins' low frequency brays may have evolved as an aid to catching the salmon.

"The brays could be used either for stunning prey or calling," says Marten. Now, to work out which it is, researchers should monitor the reaction of dolphins to ready-stunned fish, he proposes. Then the dolphins' vocal response to not needing to use their low frequency calls to disorientate prey might reveal their true nature: altruistic host or bottle-nosed assassin?