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科学美国人：Blue Whales Have Changed Their Tune
60秒科学节目（SSS）是科学美国人网站的一套广播栏目，英文名称：Scientific American - 60 Second Science,节目内容以科学报道为主，节目仅一分钟的时间，主要对当今的科学技术新发展作以简明、通俗的介绍，对于科学的发展如何影响人们的生活环境、健康状况及科学技术，提供了大量简明易懂的阐释。
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. <<blue whale call>> (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. <<2002-2017 call>>
"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 further. The full write-up is in the Journal of Geophysical Research: Oceans. [Emmanuelle C. Leroy et al, Long‐Term and Seasonal Changes of Large Whale Call Frequency in the Southern Indian Ocean]
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.
[The above text is a transcript of this podcast.]
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