PINING bottlenose dolphins call out the "names" of their loved ones to show that they miss them, scientists say.
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.
When dolphins were separated but within earshot of each other, copied whistles were produced immediately after the original call.
Lead scientist Dr Stephanie King 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."