Dolphins mimic those closest to them as a way of saying they miss them, according to the latest research.
The new study by marine biologists at St Andrews University shows that certain dolphins only copy those they share strong social bonds with.
The team of Scottish and American scientists studied dolphin whistles vocal signatures to find out why they appear to copy each other.
Bottlenose dolphins are one of the few species using vocal learning to develop their own unique vocal signature.
The copying of signature whistles has been noted in previous studies but scientists have been unsure whether it is an aggressive or friendly signal.
The research was carried out by Dr Stephanie King and Dr Vincent Janik from St Andrews with Dr Laela Sayigh (Woods Hole Oceanographic Institution), Dr Randall Wells (Chicago Zoological Society) and Dr Fellner (Walt Disney World Resort).
They analysed recordings from wild and captive dolphins to identify which animals copy one another’s signature whistle.
Dr 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.”
The team also found that dolphins introduce slight changes into copies, thus avoiding confusion for listeners.
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 King said: “Our next step is to use sound playbacks to see how dolphins respond to being matched with a copy of their own signature whistle.”