Every dolphin has a unique whistle which is used as a signature call and stays with them unchanged throughout life. These identifying noises are even more reliable than the facial features used by people to recognize each other, which notoriously change over time.
Now a study has shown that when dolphins have been separated for 20 years or more they are still able to distinguish the whistle call of a former close companion from a host of other calls emitted by complete strangers, scientists said.
Source: independent.co.uk