In the last few decades blue whale calls have been getting lower in pitch—and a rebound in their numbers may be the reason. Christopher Intagliata reports.

　　在过去的几十年中，蓝鲸叫声的音调越来越低 - 可能是因为它们数量的反弹。 Christopher Intagliata报道。

　　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.

　　蓝鲸是地球上有史以来最大的动物。 但他们仍然难以追踪。 因为他们生活在水下，我们无法轻易看到它们 - 而且通常是在偏远地区，如南部海洋。 但是鲸鱼的“歌曲”可以在水下传播至数百公里的地方......所以科学家经常用这些传播的声音代替蓝鲸现场发出的声音。

　　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>

　　但这些专注蓝鲸声音的生物学家已经注意到一个迷一样的的趋势：随着时间的推移，某些蓝鲸叫声的音调逐渐降低。 例如，这是2002年的一个声音样本，接着是2017年的一个声音样本。<2002-2017声音录音>

　　"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.

　　“在世界各地的蓝鲸种群中我们都观察到这种。所以这种现象是全世界共有的，有共同的原因。” Emmanuelle Leroy说到，他是澳大利亚新南威尔士大学的生物声学家，曾在法国布雷斯特大学完成这项工作。

　　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.

　　她的团队证实，这种现象也存在于印度洋南部的蓝鲸种群。 他们提出一个原因可能是鲸鱼数量从亚哈船长(小说《白鲸)里的人物)时代开始反弹。 更多的鲸鱼意味着单个鲸鱼不需要大声呼叫以被其它鲸鱼听到。 而且由于鲸鱼发声方式的在解剖学上的特点，他们发声越柔和，音高越低。

　　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. [Emmanuelle C. Leroy et al., Long‐Term and Seasonal Changes of Large Whale Call Frequency in the Southern Indian Ocean]

　　科学家们也提出另一种理论，它可能与第一种理论具有联系作用：鲸鱼更发声更柔和它的声音也因此更深入水下，因为越来越酸性的海水会将这些声音传播得越来越远。 相关研究的完整文章发表在“地球物理研究杂志：海洋”杂志上。 [Emmanuelle C. Leroy等人著，南印度洋大型鲸鱼叫声频率的长期和季节性变化]

　　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.

　　Leroy和她的团队注意到另一个短期趋势：南方蓝鲸的声音实际上在澳大利亚夏季变得更高。 也许是为了在冰山破裂发出巨大噪声时，鲸鱼想让自己的声音被听见。

　　"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."

　　“就像你在饮料中放一个冰块一样，你可以听到它裂开。所以冰山一样 - 所以它会非常响亮，你可以在海洋盆地中听到它。”

　　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.

　　为了更明确地解决这些谜团，科学家说他们需要继续倾听，并监测海洋不断变化的化学和声学特性。 以期了解哪些想法是可以证实的，而哪些站不住脚。