All around the world, blue whales aren't singing like they used to, and scientists have no idea why.

The largest animals on Earth are singing in ever-deeper voices every year. Among the suggested explanations are ocean noise pollution, changing population dynamics and new mating strategies. But none of them is entirely convincing.

"We don't have the answer. We just have a lot of recordings," said Mark McDonald, president of Whale Acoustics, a company that specializes in the sonic monitoring of cetaceans.

McDonald and his collaborators first noticed the change eight years ago, when they kept needing to recalibrate the automated song detectors used to track blue whales off the California coast. The detectors are triggered by songs that match a particular waveform. Every year, McDonald had to set them lower.

Since then, he and Scripps Institution of Oceanography researchers Sarah Melnick and John Hildebrand have gathered thousands of blue whale recordings made since the 1960s, spanning populations from the North Atlantic to the South Pacific to the East Indian Ocean. Their analysis, published in October in Endangered Species Research, shows that the songs' tonal frequency is falling every year by a few fractions of a hertz.

"It's a fascinating finding," said John Calombokidis, a blue whale expert at the Cascadia Research Collective. "It's even more remarkable, given that the songs themselves differ in different oceans. There seem to be these distinct populations, yet they're all showing this common shift."

According to McDonald, the first explanation to come to mind involved noise pollution caused by increased shipping traffic. Ambient ocean noise has increased by more than 12 decibels since the mid-20th century. But if whales were trying to be heard above the din, they'd sing at higher rather than lower pitches, said McDonald.

It's also possible the whales are responding to changing dynamics in how sound travels through water that's become warmer as Earth heats up, absorbing more carbon dioxide and growing more acidic than before. "But those factors are so small, and this is such a huge shift in frequency," said McDonald.

Another explanation involves the recovery of blue whale populations, which were nearly hunted to extinction during the first half of the last century. It's only since hunting ceased that they've been recorded. Maybe songs were higher-pitched when recording started, because the whales had to sing extra-loud in order to reach their scattered brethren. Now that there are more, they can lower their voices and their pitch.

But even in populations that escaped the carnage relatively unscathed, where population densities have remained steady, songs are getting lower.

"That's the first place to look for an answer, but it doesn't fit more-localized patterns. The population of blue whales off the U.S. west coast hasn't shown a dramatic upwards trend in numbers, but its pitch is declining," said Calombokidis.

Those whales are the best-studied of all blue whale populations, and their song pitch has dropped by 31 percent since the late 1960s.

Because only male blue whales sing, the answer may involve mate choice and sexual selection. The researchers hypothesize that as larger, ostensibly more virile whales tend to produce deeper songs, other males may be trying to emulate them, just as human guys might lower their voices when trying to impress a woman.

That the largest animals in the world could feel the need to inflate their size is an appealing idea, but Calombokidis warned that very little is known about how blue whales use their songs, or how social dynamics could affect them. "We need a better understanding of the songs, and a better understanding of their reproductive habits," he said.

Hal Whitehead, a Dalhousie University biologist who specializes in cetacean communication, emphasized that whale song is a cultural affair. Humpback whales are known to learn from each other, and whales have extraordinarily large and complex brains. They appear to share many social and cognitive traits with people.

"The exciting possibility, I think, is that they're all listening to each other," said Whitehead. "This is a worldwide cultural phenomenon, and that's very cool."

*Images: 1) NOAA

2) Historical graph of song frequencies/*Endangered Species Research 3) Mark McDonald

Listen to a blue whale song.

See Also:

Citation: "Worldwide decline in tonal frequencies of blue whale songs." By Mark A. McDonald, John A. Hildebrand, Sarah Mesnick. Endangered Species Research Vol. 9 No. 1, published online Oct. 23, 2009.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.