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Blue whales singing in deeper voices every year

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, and 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.

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