Muffled conversations initially had researchers at a San Diego aquarium baffled, but when a diver thought he was told to get "out" of the water they realised the chatter was coming from a beluga whale.

Noc was about a year old when he was captured off the Pacific coast of Canada in 1977. He was kept in an open-ocean pen at the US National Marine Mammal Foundation in San Diego, California.

After seven years at the foundation he began, spontaneously, to make unusual sounds, a report in the latest issue of the journal Current Biology reported.

"We interpreted the whale's vocalisations as an attempt to mimic humans.