Pining bottlenose dolphins call out the "names" of their loved ones to show that they miss them, scientists have learned.
Every dolphin has a signature whistle that identifies it in a similar way to a human name.
Experts already knew that sometimes the creatures copy each other's whistles, but whether this is an act of aggression or friendliness has not been clear. The new research indicates that mimicking whistles is very much a friendly signal. In dolphin language, it means "I miss you".
Scientists from the University of St Andrews in Scotland and a team of American colleagues studied recordings of whistling by both wild and captive bottlenose dolphins.
Matching the sounds to different animals, they found that copying only occurred between mothers and offspring, and "bonded" adult males who had long term associations.
When dolphins were separated but within earshot of each other, copied whistles were produced immediately after the original call - a behaviour known as vocal matching.
The research is published in the latest issue of the journal Proceedings of the Royal Society B.
Lead scientist Dr Stephanie King, from the University of 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 scientists also found that slight changes are introduced into the copied versions of whistles. This is thought to be to avoid confusing any other dolphins who might be listening.