is encoded in its so-called signature whistle, according to a new study of bottlenose dolphins. The dolphins develop individually distinctive whistles that researchers have long suspected contain identity information. Previous work had shown that dolphins recognize the calls of their relatives. But it was unclear whether the animals were responding to the caller's voice features, or the frequency modulation of the whistle. The new study indicates that it is the shape of the signature whistle--not the voice features--that carries the identity information, which would facilitate communication underwater, where voice recognition is difficult. Signature whistles may therefore be used as referential signals, either addressing other dolphins or referring to them--much as humans use names--the scientists observe. A paper detailing the findings is being published online this week by the Proceedings of the National Academy of Sciences.