While there have been anecdotal reports of the whales, known as "canaries of the sea" for their high-pitched chirps, mimicking human-like speech, none had ever been recorded.
Researchers say Noc, a beluga whale who lived at the National Marine Mammal Foundation in San Diego following his capture as a juvenile in 1977, had been making unusual sounds since 1984.
When a diver at the foundation surfaced in Noc's tank saying, "Who told me to get out?" the researchers identified Noc as the culprit and made the first-ever recordings of the behavior, NewScientist.com reported Monday.
"We were skeptical at first," Sam Ridgway of the foundation said, so he and his colleagues analyzed Noc's sound waves. "They were definitely unlike usual sounds for a [beluga], and similar to human voices in rhythm and acoustic spectrum."
The vocal bursts averaged about three per second, with pauses reminiscent of human speech, the researchers said.
The frequencies were spread out into "harmonics" in a manner very unlike whales' normal vocalizations and more like those of humans, they said.
"Our observations suggest that the whale had to modify its vocal mechanics in order to make the speech-like sounds," Ridgway said.
About four years after Noc started talking like a human, he stopped. The whale continued to vocalize, but those sounds were just the typical beluga whistles and squawks. In 1999, Noc died.
"We never got his best speech imitation" on tape, Ridgway said.
"We do not claim that our whale was a good mimic compared to such well-known mimics as parrots or mynah birds," the researchers wrote in the journal Current Biology. "However, the sonic behavior we observed is an example of vocal learning by the white whale. It seems likely that Noc's close association with humans played a role in how often he employed his human voice, as well as in its quality."
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