LOS ANGELES -- 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.

At first, the researchers thought it was a quirk. But after a couple of years of adjusting for lower frequencies, "we knew there was something strange going on," said John Hildebrand, an oceanographer at Scripps Institution of Oceanography in San Diego and co-author of the study published recently in Endangered Species Research.

So the researchers scoured military data and seismograph readings for clues about what blue whales used to sound like.

A retired Navy scientist directed Dr. Hildebrand to a trove of tapes stored at Sea World. The delicate old reels were the size of dinner plates. It turned out they contained snippets of blue whale songs from 40 years ago.

The tapes eliminated all doubt: In the Beach Boys' era, blue whales' voices, while nowhere near falsetto, had been distinctly higher pitched.

Blue whales are shrouded in mystery as it is. Sleek, mottled and silvery, they are rare and don't reveal much. They don't leap on the surface as much as humpback whales do. They might, if really flustered, slap their tails on the water. More often, they quietly sink, Dr. Hildebrand said.

Their song is barely audible to the human ear -- a deep bass growl with very long wavelengths befitting very long whales.

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