Wild dolphins greet each of their pals using individual whistle signatures.
Until now this sort of behaviour has only been found in birds and humans.
Previous research with captive dolphins shows that each one has a unique whistle and can mimic another dolphin's whistle perfectly after hearing it just once.
Biologist Dr Vincent Janik at the University of St Andrews in Scotland decided to investigate how bottle nose dolphins interact in the wild.
He recorded nearly two thousand whistles from dolphin colonies off the Scottish coast.
So as not to disturb the dolphins with noisy boats, he used six underwater microphones and a computer-based method for locating individual vocalists. Human listeners then identified matching whistles.
First step to language
Dr Janik concluded that the dolphins were responding to each other by mimicking an individual's call back. Such interactions with learned signals are thought to be a first step toward the evolution of real language.
Communication between dolphins seems to be quite sophisticated yet no one really knows what they say to each other.
In his report published in the journal Science Janik said that the dolphin's greetings might not necessarily be a simple friendly "hello"; they could equally be an aggressive warning.