Within the first year, a young bottlenose dolphin develops a signature whistle, a frequency contour it produces far more often than any other. The shape is stable for decades and is not simply a by-product of the animal's voice, since the same dolphin can produce other whistle types.
Dolphins copy each other's signature whistles, usually those of close associates, and they do so most often when animals are separated. In playback work off Scotland, wild dolphins were exposed to synthetic copies of their own signature whistle, stripped of voice cues, alongside copies belonging to other group members. They responded by calling back to their own and largely ignored the others.
The usual description is that dolphins have names, which is close but imprecise. A signature whistle is invented by its owner rather than assigned by others, and there is no evidence that dolphins use these labels to refer to absent individuals in the way human names do.