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This is scientific American 60 second science, I’m Christopher Intagliata.

Blue whales are the largest animals ever to exist on Earth. But they’re still tough to track. Because they live underwater where we can’t easily see them–and often in remote areas, like the southern ocean. But the whales’ songs can travel hundreds of kilometers underwater… so scientists often listen for them instead. By the way that’s sped up 10 times so you can hear it.

But these eavesdropping biologists have noticed a mysterious trend: that certain blue whale calls are gradually lowering in pitch, over time. For example, here’s a call from 2002, followed by one from 2017.

“It has been observed in blue whale populations worldwide. So this phenomenon has to have a worldwide explanation.” Emmanuelle Leroy is a bioacoustician at the University of New South Wales in Australia, who did the work at the University of Brest in France.

Her team confirmed the phenomenon holds true for populations of blue whales in the southern Indian Ocean, too. And they suggest one reason may be that whale numbers have rebounded from the days of Captain Ahab. More whales means individuals don’t have to shout as loud to be heard by other whales. And because of an anatomical peculiarity in the way whales sing, the softer they sing, the lower the pitch.

The scientists have another theory, too, which may be acting in concert with the first: which is that whales are singing more softly—and therefore more deeply—because increasingly acidic ocean waters carry their calls farther.

The full write-up is in the Journal of Geophysical Research: Oceans.

Leroy and her team noticed one other short-term trend: that southern blue whales’ songs actually get higher in pitch during the Austral summer. Perhaps in an effort to be heard over cracking icebergs.

“Like when you put an ice cube in your drink, you can hear it crack. So it’s the same for an iceberg - so it will be really loud and you can hear it across an oceanic basin.”

To solve these mysteries more definitively, the scientists say that we’ll need to keep listening, and monitor the changing chemical and acoustic properties of the oceans. To see which of these ideas are borne out. And which don’t hold water.

Thanks for listening for, scientific American 60 second science, I’m Christopher Intagliata.