Past studies have shown that individual dolphins have a unique whistle, called a "signature whistle," that they often use in big group settings, like when several pods of dolphins meet at sea.
The idea that dolphins have a name in the form of a whistle has been around since the 1960s, and studies of captive dolphins have shown that the animals are responsive to the whistles of dolphins they know.
But a new study takes the theory a step further by asserting that a dolphin will respond when it hears the sound of its own signature whistle, repeating that whistle back in a way that seems to say, "Yup, I'm here—did you call my name?" explained Whitney Friedman, a dolphin-behavior expert at the University of California, San Diego.
It's "compelling evidence" that the dolphin indeed uses the sound as a name, according to the study, published July 22 in the Proceedings of the National Academy of Sciences.
The research was performed by a group of scientists on a boat off eastern Scotland who joined up with a group of wild dolphins. When one of the dolphins announced itself with its signature whistle—the equivalent of "Joey!" for instance—the researchers recorded that sound.
Later, the team played that same "Joey!" call back to the dolphins, and a significant portion of the time, the dolphin they called Joey responded with the same call—as if Joey was saying, "Yup, I'm here."
The dolphins responded a little when the scientists played recordings of whistles of familiar dolphins from the same population, but did not respond at all to unfamiliar dolphins from a different population. (Watch video: "Dolphin Talk Decoded.")
The finding shows that signature whistles aren't just noises dolphins produce, but also signals to which they respond—a "fantastic contribution" toward understanding how dolphins communicate, said Friedman, a National Geographic grantee who was not part of the new study.
The finding is more evidence that identity is important for dolphins, which form complex relationships within tight-knit communities.
For instance, being able to call a dolphin by name could be crucial in a situation where a competition erupts between two groups of male dolphins and a member of one group is missing. One of the dolphin groups could call the missing dolphin's signature whistle to request that he help out, noted Friedman.
What's more, the ocean is noisy and visibility is poor, so there's a greater need for sounds that indicate an animal's identity, said Jeremy Karnowski, also a UCSD dolphin-communication expert who was not involved in the new study.
"Having sounds that express identity might be a way for the group to know where each group member is at a given time, and the specific identity of that dolphin," he said.
Building on this study, scientists could look at the two other types of dolphin vocalizations, Karnowski said by email.
One is the "clicks" dolphins make during echolocation—the use of built-in sonar that helps dolphins find prey—and the other is "burst-pulse" calls that sound like buzzing and squeaking. (See "The Secret Language of Dolphins.")
These "other vocalizations might have meanings," he said.
The study also raises other questions, like exactly when and how often dolphins address each other, Karnowski said. Perhaps most intriguing: Do dolphins use these names to talk about each other behind their backs?