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The blue whale is the largest animal that's ever existed—heavier than even the biggest of the dinosaurs.
蓝鲸是体积最大的动物—甚至比最大的恐龙还重。
But scientists have debated why whales are so huge for a long time.
但科学家们争论到，为什么许久以来，鲸鱼的体积都是如此巨大。
The prevailing idea is that if you live in water, buoyancy allows you to get bigger than if you lived on land.
常见的说法是，如果你生活在水里，浮力会让你变得比生活在陆地上时更大。
But it might not be quite that simple.
但是这或许并没有那么简单。
Marine mammals may actually have more constraints on their body size than land mammals—
或许海洋哺乳动物比陆地哺乳动物对于体型的限制更多—
and scientists now think they have to be big to conserve energy.
并且科学家们现在认为巨大的体型是为了存储能量。
Land mammals have slipped back into the water four separate times.
陆地哺乳动物四次倒退回水中。
Whales evolved from ancient hippo-like mammals; seals from a dog-like ancestor.
鲸鱼从远古类似河马一样的哺乳动物进化而来；海豹从类似狗类的生物进化。
Manatees and dugongs are a branch of the elephant family tree, and sea otters are basically just really big weasels.
海牛和儒艮是大象家族中的一支，海獭基本就是大型鼬鼠。
And until 2018, most scientists thought that their buoyancy in water gave marine mammals the freedom to grow larger,
直到2018年，多数科学家都认为水中的浮力为海洋哺乳动物体型的增长提供了自由，
because it kind of literally lifted the weight from their skeletons.
因为浮力减轻了它们骨骼的重量。
But when researchers looked for patterns of change in body size in almost 4,000 living and 3,000 fossil mammals,
但是当研究人员在将近四千种活的哺乳动物和三千种化石哺乳动物中寻找体型改变的模式时，
they found something more complicated.
他们发现了一些更复杂的情况。
In 3 out of the 4 marine mammal groups—whales, seals, and manatees—
在四分之三的海洋哺乳动物群体中—鲸鱼、海豹以海牛—
species converged on the same size: an average of around 500 kg or 1100 pounds.
这些物种有着相同的体型：平均500kg或1100磅左右。
And when they tested various hypotheses to explain that, they found while buoyancy might allow marine mammals to get big,
并且当他们对各种假设进行测试时，他们发现虽然浮力让海洋哺乳动物变得更大，
it's not the reason they grow. That probably has more to do with keeping warm.
但这并非它们增长的原因。这或许更多的是为了保暖。
Ocean waters are colder than most mammals' ideal body temperature, and water is better than air at conducting heat away from you.
海水比多数哺乳动物的理想体温更低，相比空气，水更擅长传到热量，导走你身上的热。
Since larger and rounder bodies have less surface area in relation to their volume, they lose heat less rapidly even in chilly seas.
更大更圆的身体与体积相关联的表面区域更少，所以在寒冷的海水中，它们身体热量消失的速度更慢。
You'd think that means marine mammals could just keep growing bigger and bigger,
你以为这意味着海洋哺乳动物的体积可以一直增长，
but the researchers found that if they get too big, there's no way they could eat enough to feed all those cells.
但是研究人员发现如果它们的体积过大，就没有办法吃到足够的食物维系所有的细胞。
500 kg seems to be just the right size to stay warm but still be able to eat enough to maintain a huge body. But there are outliers.
500kg似乎正好，既能保持温暖又能有充足食物维持巨大的身体。但是也有例外。
Baleen whales like blue whales managed to hack the equation a bit when they developed those fibrous baleen bristles,
须鲸类，比如蓝鲸，当它们长出那些纤维鲸须时，蓝鲸会努力达到这种等式，
because filter feeding is a very efficient way to get lots of calories while spending very little energy.
因为滤食性摄食是在消耗较少能量时，获得大量卡路里的一种非常有效的方法。
And sea otters were the oddball that didn't converge on that 500kg size.
海獭属于奇怪的那一种，它们的重量没有500kg。
They're bigger than their weasel ancestors, but not as big as other marine mammals.
它们比鼬鼠祖先要更大，但是却没有其他海洋哺乳动物大。
That might be because there's something special about weasels and their relatives that limits their size, or allows them to stay trim.
这或许是因为鼬鼠和它们亲戚的特别之处限制了它们的体型或是让它们保持整齐。
Or, it's possible sea otters are still growing, and that milennia from now, they'll be huge.
或者可能是海獭仍在生长，并且从现在开始往后千年，它们的体型也会变得巨大。
Wait—would sea otters the size of polar bears still be ridiculously cute, or utterly terrifying? I'm going with terrifying.
等等—要是海獭长成北极熊那样的体型是可爱还是恐怖？我觉得是恐怖。
Anyhow, the findings have got scientists rethinking what they thought they knew about the life in the seas.
总之，这些发现让科学家们重新思考他们对海洋生物的认识。
And they just go to show that if you keep asking good questions, you might find the answers aren't as simple as you thought—
他们的经历也表明，如果你一直提出好问题，那么你或许会发现，答案并没有你想的那么简单—
but they're also much cooler.
但是却更酷。
Thanks for watching this episode of SciShow, a Complexly Production.
感谢收看本期《科学秀》，本期《科学秀》由Complexly出品。
If you find marine mammals fascinating, you might like the episode of our sister channel, Eons,
如果你对海洋哺乳动物感兴趣，可以登录我们的姊妹频道Eon
where they delve into how whales made the transition from land to sea.
深入研究鲸鱼如何从陆地迁移至海洋。
