Blue whales have changed their songs.

It's the same old tune, but the pitch of the blues is mysteriously lower -- especially off the coast of California where, local researchers say, the whales' voices have dropped by more than half an octave since the 1960s.

No one knows why. But one conjecture is that more baritone whales indicate healthier populations: The whales may be less shrill because they're less scarce and don't have to pipe up to be heard by neighbors.

The discovery was accidental. Whale acoustics researcher Mark McDonald was trying to track blue whales' movements using data from Navy submarine detectors. He had created a program to filter out the blues' songs from a din of ocean noise captured by these instruments.

But he kept having to rewrite the code. Each year, it seemed, the whales sang at a lower pitch.

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