Communicating underwater is challenging.
在水里面沟通极具挑战性。
Light and odors don't travel well, so it's hard for animals to see or smell.
光线和气味不易在水里传播，因此水下的动物很难看到或闻到东西。
But sound moves about four times faster in water than in air,
但是，声音在水中传播的速度是在空气中的四倍，
so in this dark environment, marine mammals often rely on vocalization to communicate.
因此，在水下这片黑暗的世界中，哺乳动物通常依靠声音进行交流。
That's why a chorus of sounds fills the ocean.
海洋中遍布了声音的合唱队。
Clicks, pulses, whistles, groans, boings, cries, and trills, to name a few.
卡嗒声，砰砰声，啸啸声，呻吟声，啵嘤声，哭泣声，还有少数颤声。
But the most famous parts of this underwater symphony
但这海底交响乐最旖旎的部分，
are the evocative melodies, or songs, composed by the world's largest mammals, whales.
是那最令人回味的旋律或歌曲，出自全世界最大哺乳类动物，鲸鱼。
Whale songs are one of the most sophisticated communication systems in the animal kingdom.
鲸鱼之歌是动物王国里最复杂的沟通系统之一。
Only a few species are known to sing.
只有少数几个物种会唱歌。
Blue, fin, bowhead minke whales, and of course humpback whales.
蓝鲸、鳍鲸、弓头鲸、小须鲸，当然还有座头鲸。
These are all baleen whales which use hairy baleen plates instead of teeth to trap their prey.
它们都属于须鲸亚目，用鲸须板取代牙齿来捕捉猎物。
Meanwhile, toothed whales do use echolocation,
与此同时，齿鲸利用回声定位，
and they and other species of baleen whales make social sounds, such as cries and whistles, to communicate.
齿鲸和其他种类的须鲸会发出声音来社交，例如以呼喊声和口哨声来沟通。
But those vocalizations lack the complexity of songs. So how do they do it?
但那些声音不似歌曲般复杂。它们如何做到的？
Land mammals like us generate sound by moving air over our vocal cords when we exhale, causing them to vibrate.
像人类这样的陆上哺乳类动物，以呼出空气振动声带来发声。
Baleen whales have a U-shaped fold of tissue between their lungs and their large inflatable organs called laryngeal sacs.
须鲸有个U型皱折组织，介于肺与大型可充气的喉气囊间。
We don't know this for sure because it's essentially impossible to observe the internal organs of a living, singing whale,
虽然我们不确定这一点，因为根本不可能观察歌唱中的活鲸鱼内部的器官，
but we think that when a whale sings, muscular contractions in the throat and chest
但我们认为鲸鱼唱歌时收缩喉咙和胸部的肌肉，
move air from the lungs across the U-fold and into the laryngeal sacs, causing the U-fold to vibrate.
使肺部的空气通过U型皱折来到喉气囊，振动U型皱折。

The resulting sound resonates in the sacs like a choir singing in a cathedral
声音在喉气囊里共鸣，如同合唱团在大教堂内唱歌，
making songs loud enough to propagate up to thousands of kilometers away.
使音量响亮，足以传到数千公里外。
Whales don't have to exhale to sing.
鲸鱼毋需呼气来唱歌。
Instead, the air is recycled back into the lungs, creating sound once more.
而是将空气重新循环入肺里，又再次发出声音。
One reason whale songs are so fascinating is their pattern.
鲸鱼的歌曲如此地令人着迷原因之一是其样式。
Units, like moans, cries, and chirps are arranged in phrases.
把呻吟、呼喊和啁啾之类的声音单元排成乐句。
Repeated phrases are assembled into themes.
重复的乐句组成主旋律。
Multiple themes repeated in a predictable pattern create a song.
多个重复的主旋律依可预测的样式编成歌曲。
This hierarchical structure is a kind of grammar.
这样阶级式的结构是种语法。
Whale songs are extremely variable in duration, and whales can repeat them over and over.
鲸鱼的歌可长可短，差距颇大，鲸鱼能不断地重复歌唱。
In one recorded session, a humpback whale sang for 22 hours. And why do they do it?
某一只座头鲸长达22小时的歌声曾被录下来。它们为何唱歌？
We don't yet know the exact purpose, but we can speculate.
我们不确知其目的，但能推测。
Given that the singers are males and they mostly sing during the mating season, songs might be used to attract females.
由于歌唱的是雄鲸，多半在交配季节唱歌，它们可能是用歌声来吸引雌鲸。
Or perhaps they're territorial, used to deter other males.
或是用来宣示地盘，威慑其他雄鲸。
Whales return to the same feeding and breeding grounds annually, and each discrete population has a different song.
鲸鱼每年回到同样的觅食地和繁殖地，每个聚落有不同的歌曲。
Songs evolve over time as units or phrases are added, changed, or dropped.
歌曲随着时间衍变，单元与乐句或增、或修改、或删除。
And when males from different populations are feeding within earshot, phrases are often exchanged,
当听力范围内有不同聚落的雄鲸觅食时，彼此之间经常交换乐句，
maybe because new songs make them more attractive to breeding females.
也许新歌会使它们对育龄的雌鲸更具吸引力。
This is one of the fastest examples of cultural transmission,
这是文化传播最迅速的例子之一，
where learned behaviors are passed between unrelated individuals of the same species.
习得的行为在同一物种但无亲属关系的个体间传递。
We can eavesdrop on these songs using underwater microphones called hydrophones.
我们能用水中受波器来聆听这些歌曲。
These help us track species when sightings or genetic samples are rare.
这些有助于我们追踪罕见或罕有基因样本的物种。
For example, scientists have been able to differentiate the elusive blue whale's populations worldwide based on their songs.
例如，科学家已能分辨分布世界各地、难以寻找的蓝鲸聚落，凭借的就是它们的歌曲。
But the oceans are getting noisier as a result of human activity.
但是人类的活动使得海洋变得愈来愈嘈杂。
Boating, military sonar, underwater construction, and seismic surveys for oil
行船、军用声纳、水下建造、用震波探测石油
are occurring more often which may interfere with whale's communication.
正变得愈来愈频繁，这些都可能干扰鲸鱼间的沟通。
Some whales will avoid key feeding or breeding grounds if human noise is too loud.
如果人为的声音太吵杂，有些鲸鱼会避开重要的觅食或繁殖地。
And humpback whales have been observed to reduce their singing in response to noise 200 kilometers away.
据观察，200公里外的噪音会减少座头鲸的歌唱。
Limiting human activity along migratory routes and in other critical habitats,
限制在鲸鱼迁徙行经地路线和其他重要栖息地的人为活动，
and reducing noise pollution throughout the ocean would help ensure whales continued survival.
以及减低整个海洋的噪音污染，有助于确保鲸鱼能继续生存。
If the whales can keep singing and we can keep listening, maybe one day we'll truly understand what they're saying.
如果鲸鱼能继续歌唱，而我们能继续聆听，也许有一天我们会真正地明白它们在说些什么。