A new study showns that bottlenose dolphins could be using signature whistles to identify themselves and call out specific individuals, possibly making it the equivalent of names among humans.
It was already known that dolphins are taught signature whistles by their mothers, but their function was not entirely clear (and still isn't, but we're getting closer to cracking that nut). The authors of the study analyzed recordings made over decades by arrays of microphones in St. Andrews Bay to figure out how these whistles were used.
In cases where dolphin pods joined and swam together, the researchers found, such meetings were preceded by one dolphin in the group producing a signature whistle and another dolphin in the second group answering. When dolphin groups swam by one another and didn't join, these meet-and-greet whistles were absent.(source)
Is it thought that there might not be a rigid procedure for how dolphins from different pods meet, but rather, whoever wants to say "hello" just calls out the signature whistle of the individual they are looking for.
All pairs of animals that produced signature whistle copies were close associates, with only one pair having a low CoA for the year prior to recording. However, these two males were each other's closest male associate in the 4 year period prior to the recording. Many of the copiers were mother–calf pairs, with both mothers and calves likely to copy one another. While most female calves' signature whistles are distinct from their mothers', males sometimes do sound like their mothers. The signature whistles of the male calves in this study, however, did not resemble those of their mothers [...]. Signature whistles of male alliance partners also tend to become more alike over time. [...] We found no evidence for a deceptive function of signature whistle copies. (source)
The study is publicly accessible without a pay subscription, so if you're curious, check it out: Vocal copying of individually distinctive signature whistles in bottlenose dolphins.