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 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.
With more work, the researchers were able show that blue whales worldwide are using deeper voices lately. Some have dropped their calls by only a few tones, but all showed a steady decline. "It was baffling," Hildebrand said.
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, Hildebrand said.
Their song is barely audible to the human ear -- a deep bass growl with very long wavelengths befitting very long whales.
The tone is so deep that if played in a small room, it's hard to hear: The long-period sound waves extend beyond the walls. But play a recording very loudly, in a large auditorium, and "you feel it in your chest as much as you hear it," McDonald said. "It's awesome."
The researchers pondered possible causes. Warmer temperatures? More acidic seas? Such factors affect the way sound moves through water, but not enough to explain the change, Hildebrand said.
The rumble of shipping traffic is thought to affect marine mammals. But the researchers argue that if whales were just trying to be heard above the fray, they would adopt higher, not lower, voices.
It's also possible that the low voice is just a fad. Biologists talk about whale "culture," and blue whales tend to be conformists. But researchers have said they doubt that a random, learned behavior could spread all over the globe.
So they put themselves in the whales' shoes. McDonald surmised that whales would rather not sing in higher voices if they didn't have to. They prefer deep and manly -- "a lower, sexier frequency," he said.
Among whales, he said, depth of voice may bespeak more desirable mates with larger bodies. It's useful shorthand, since it's hard to get a good look at one's suitor if he is 80 feet long and swimming in murky water.
After the whales were hunted nearly to extinction, they may have been spread so thin that they could no longer find one another easily, prompting them to raise their pitch.
Efforts to restrict whaling beginning in the late '60s helped populations rebound. With increased numbers, the whales may not have needed to shout and may have gradually returned to their deep tones.
"This hints that some of these great whales are recovering; it's not all doom," said co-author Sarah Mesnick, ecologist at the National Oceanic and Atmospheric Administration Fisheries Service.
If whale songs are related to population density, they might aid efforts to count blue whales, Hildebrand said. They once numbered in the hundreds of thousands. Today, their population is thought to be 10,000 or so.
Oceanographer Jay Barlow, program leader at NOAA fisheries, cautioned that changes in the whales' pitch don't track closely with population changes. California blues, for example, recovered most strongly in the '70s and '80s, and their numbers may not have grown much since, he said.
But Barlow had no alternate theory for the deeper songs, which he sometimes plays on his home stereo. The sound makes his floor shake and upsets his cats.
David Mellinger, a marine mammal bio-acoustician at Oregon State University, said that, whatever the reason, the finding "is astonishing." It recalled to him the first time he heard a blue whale sing.
He was on a boat, using headphones, and one passed. "It was a defining moment in my life," he said. "It made a visceral impression on me. Just this huge animal. I could hear the hugeness of it."