Dolphins seem to greet each other with signature whistles that act like names, according to a newly published study. That puts dolphins in a very small class of animals that can learn new sounds and use them to communicate.

“In bottlenose dolphins, the selective use of a signature whistle by one animal allows for the occasional copying of that whistle by another animal to be an effective way of addressing an individual,” University of St. Andrews marine biologists Stephanie King and Vincent Janik wrote in a paper that appeared in the Proceedings of the National Academy of Sciences on Monday.

For their study, King and Janik followed a group of wild dolphins that live off the eastern coast of Scotland. The researchers recorded dolphin whistles and played a variety whistles back to groups of wild dolphins to see how the animals would react. Some of the stimuli played were computerized versions of one dolphin’s whistle, others were the whistles of familiar dolphins from the same home group, and some were whistles of unfamiliar captive dolphins from Germany and Florida.

When dolphins heard their own signature whistle played at them, they responded with the same sound, almost as if returning a friend’s greeting. Dolphins did not respond to the whistles of the stranger dolphins, but when a recording of a familiar dolphin from the same group was played, the animals called back.  Strong social ties are thought to increase the likelihood of a dolphin copying another’s whistle.

Dolphins are likely respond to whistles from others because "there's a strong motivation to reunite with those individuals," King told LiveScience.

King and Janik published another finding on dolphin communication in February in Proceedings of the Royal Society B, this time in collaboration with researchers from the Woods Hole Oceanographic Institution, the Chicago Zoological Society and the Seas, an aquarium at Walt Disney World. In that study, the scientists sifted through data collected on the social relationships and vocalizations of wild bottlenose dolphins around Sarasota Bay in Florida between 1984 and 2009. When these wild dolphins are captured for study, they’re held in separate nets, unseen to each other but still able to communicate.

They found that dolphins copied each other’s signature whistles while being studied, almost as if searching for each other. The pairs that included the dolphins most likely to copy the others' whistles were males with close social ties, and mothers and calves, they found.

Whistle copying “represents an interesting parallel to humans and the apparent necessity for these vocal labels in maintaining group cohesion may lie at the root of the evolution of complex communication and cognition systems,” King and Janik wrote in their latest paper.

SOURCE: King et al. “Bottlenose dolphins can use learned vocal labels to address each other.” Proceedings of the National Academy of Sciences, published online 22 July 2013.