For example, a dolphin might be expected to recognize its name if called by its mother, but the new study found most dolphins recognized names -- their signature whistles -- even when emitted without inflection or other vocal cues.

More than that, two dolphins may refer to a third by the third animal's name, said Laela Sayigh, one of three authors of a paper published in the Proceedings of the National Academy of Sciences.

"They are known to produce these individually distinctive signature whistles, like names," Sayigh said in a telephone interview. She said the researchers wanted to know what information in the whistles helped dolphins identify each other's names.

The scientists already knew that dolphins responded to whistles, but wondered if something in the actual voice of the whistling dolphin was making the identity clear, or if the name itself was enough for recognition.

To find out, they studied bottlenose dolphins in Sarasota Bay, Florida. Instead of playing recordings of actual dolphins making signature whistles, the researchers synthesized signature whistles with the caller's voice features removed and played them to dolphins through an underwater speaker.

In nine out of 14 cases, the dolphin would turn more often toward the speaker if it heard a whistle that sounded like a close relative's.

"It's a very interesting finding that encourages further research, because they are using whistles as referential signals -- that's what words are," said Sayigh, of the University of North Carolina Wilmington. "Dolphins appear to be using these arbitrary signals to identify another dolphin."

She stopped short of saying dolphins might have a human-like language.