Dolphins mimic the whistle of those closest to them as a way of saying they want to be reunited, new research suggests.
A team of marine biologists studied their vocal signatures and found certain dolphins copy those they share strong social bonds with.
Scientists, including researchers at the Walt Disney World Resort in Florida, only found mimicking present in mothers and their offspring, as well as in adult males who copied those they had long-term associations with.
Bottlenose dolphins are one of the very few species in the animal kingdom that use vocal learning to develop their own unique vocal signature early in life.
Each dolphin produces its own signature whistle which describes its individual identity.
Copying of signature whistles has been looked at before but scientists had been unsure whether it was an aggressive or friendly signal.
The new study at the University of St Andrews suggests that dolphins are mimicking those they are close to and want to see again.
The team of Scottish and American scientists analysed recordings from wild and captive dolphins to identify which animals copy one another's signature whistle.
They also found that dolphins introduce slight changes into copies, avoiding confusion for listeners.
Researchers said the copies were clearly directed towards the owner of the original signature whistle by being produced immediately after the owner of the whistle called first, a behaviour known as vocal matching.
Dr Stephanie King, from St Andrews, said: "Interestingly, signature whistle copying was only found in pairs of animals composed of mothers and their calves or adult males who form long-term alliances with one another.
"The fact that animals are producing whistle copies when they are separated from a close associate supports the idea that dolphins copy another animal's signature whistle when they want to reunite with that specific individual.
"Our next step is to use sound playbacks to see how dolphins respond to being matched with a copy of their own signature whistle.
"If they react, we would know that copying of signature whistles can be used to address dolphins."
The results of the study are to be published today in the journal Proceedings of the Royal Society B.