One dolphin can call to another specific dolphin by mimicking the unique sounds of that other dolphin, they said.
"These whistles actually turned out to be names," researcher Randall Wells told CNN. "They're abstract names, which is unheard of in the animal kingdom beyond people."
Wells, of the Sarasota Dolphin Research Program in Florida, worked with U.S. colleagues and scientists from the University of St. Andrews in Scotland in a study of what they call "vocal copying" in dolphins.
"Each dolphin produces its own unique signature whistle that describes its individual identity," the researchers said in a University of St. Andrews release. "The new study suggests that in fact dolphins are mimicking those they are close to and want to see again."
Dolphins that were familiar with each other for a significant amount of time would mimic the whistle of another in that group when they were separated, the researchers said.
"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," St. Andrews researcher Stephanie King said in the release.
That ability puts dolphins' communication abilities closer to humans' than any other species, the researchers said.