Bottlenose dolphins, the species which starred in the 1960s television series "Flipper", are the only animals besides humans that use sounds to identify themselves and recognise members of their group, researchers have found.
Scientists have speculated for years that bottlenose dolphins' whistling transmitted more identity information than the communications of other animals, which can often express species and group identity.
But in the study published in the Proceedings of the National Academy of Sciences, scientists said they found dolphins communicate in a more sophisticated way than other animals, expressing and understanding specific "names" more like humans do.
The scientists, led by Vincent Janik of the University of St. Andrews in Scotland, wanted to test whether dolphin whistles contained an expression of identity that was separate from the dolphin's personal "voice".
Synthetic names
Working with a group of bottlenose dolphins in Sarasota, Florida, they synthesised the animals' signature whistles, cutting any other intonation and sound that were features of the dolphin's voice while leaving the frequency modulation shape.
The approach could be compared to isolating the pure syllables of a human's name from the accents and other voice characteristics which might identify the speaker.
To test whether the dolphins recognised "names" reproduced in this way, the scientists played dolphins the synthesized whistles of a close relative to see if they reacted.
Usually the close relatives were mothers or independent offspring. In nine out of 14 cases, the dolphins reacted to the "name" of the close relative.
Scientists suggested the evolved method of identification could be necessary in the dolphins' environment, where water pressure changes can distort voice recognition.
"This finding does not mean that dolphins do not have individually specific voice features. However, sound production is greatly affected by changing water pressure, which would make voice recognition more difficult than in air," the study said.
"In such conditions, signature whistles can facilitate contact between
individuals."
The scientists said five dolphins in the study did not react to the synthesised "names" broadcast through the speakers.
But they guessed it involved "a motivational issue", such as the relationship between the animal being tested and the name being broadcast.