Bottlenose dolphins have whistles which they use to exclusively greet other members of their species, marine biologists in Scotland reported on Wednesday.
Using hydrophones, the researchers made recordings of dolphins swimming in St. Andrews Bay, off the northeastern coast of Scotland, in the summers of 2003 and 2004.
When groups of dolphins met up, they swapped whistles that outwardly sounded the same.
But forensic analysis showed the whistles were in fact individual signatures, for they were never matched or copied by other dolphins.
"Signature whistle exchanges are a significant part of a greeting sequence that allows dolphins to identify conspecifics [members of the same species] when encountering them in the wild," says the study.
The whistles are clearly important, as they were heard in 90 percent of the joinups, says the paper.
One particular signal came from what appeared to be the leader of a group, seemingly giving the OK to fellow dolphins in the team to join up with the other group.
Other whistles could be about agreeing roles to hunt for food or identifying individuals for socialising. Bottlenose dolphins operate in a "fission-fusion" society, meaning they live in groups that are fluid in numbers.
The study, by Vincent Janik and Nicola Quick of the University of St. Andrews, appears in the British journal Proceedings of the Royal Society B.
The discovery adds an intriguing footnote about the bottlenose dolphin (Tursiops truncatus), one of only very few species which can invent or copy noises.
Others are song birds, baleen whales, phocid seals and bats, but in these species, the learning trick is for reproduction, as it is the males who learn songs in order to attract females.
By using whistles for broadcasting identity and details of the environment, say the scientists, the dolphin shares similar skills with... the grey parrot.
Explore further: Can monogamy undermine intelligence?