OK, that's a boring title, but go with me. A lot of cetaceans can imitate each other or outside sounds. Luna, a wild orca of the Puget Sound area, became famous for his ability to imitate a motorboat (in a sad irony, he was kileld by a tugboat in 2006). Now comes the beluga (Delphinapterus leucas) who imitates bottlenose dolphins (Tursiops truncatus). The authors say, "Here we describe the changes in the vocal repertoire of a beluga whale that was housed with a group of bottlenose dolphins. Two months after the beluga’s introduction into a new facility, we found that it began to imitate whistles of the dolphins, whereas one type of its own calls seemed to disappear."
There's an awful lot going on in cetacean brains, and we don't have a handle on it all by a long shot. Safina's book Beyond Words describes how captive dolphins invent new moves or do things their trainers have only talked about, and how handlers get a little spooked by those abilities sometimes. I'm a believer in phasing out cetacean captivity, although the orcas are most important, as we can't come close to replicating their habitats and often don't try. But we can learn from them.