大得有理由
n general, aquatic mammals tend to be larger than their closest land-bound relatives.
一般来说，海洋哺乳动物往往比它们陆地上的近亲庞大不少。
The largest sea lions are twice as big as the largest bears, for example, and manatees outweigh their cat-size hyrax cousins by nearly 500 kilograms.
举个例子，最大的海狮是最大的熊的两倍那么大，而海牛则比它们像猫一样大的亲属岩狸重500公斤。
To find out why, researchers looked at the sizes of four different evolutionary groups of sea mammals—roughly 4000 living and 3000 fossil species—beginning with the time they diverged from their terrestrial relatives.
为了找到原因，研究者们查看了四个不同进化分支的海洋哺乳动物—大概4000个现存和3000个化石物种—就从它们开始从陆生亲属分叉开去的时间。
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Once they began an aquatic life, these mammals all evolved toward larger sizes over the past 60 million years or so.
在开始海生生活之后，这些哺乳动物在以后六千万年左右的时间里全都朝更大型的方向进化。
Their land relatives, on the other hand, didn’t trend toward any particular size, the team reports today in the Proceedings of the National Academy of Sciences.
另一方面，它们的陆生亲属却没有朝某个体型方向进行进化，该研究团队在《美国国家科学院院刊》上发表了报告。
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The scientists think this is because it’s too chilly to survive in the ocean as a warm-blooded animal without sufficient size.
科学家们认为这是因为想要作为温血动物在海中生存，没有足够的体积的话就会感觉太冷了。
The amount of heat your body can generate depends on how many cells you have, and small animals simply don’t have enough to replace the heat they lose to the water.
你的机体能产生热量的数量取决于体内有多少细胞，而小型动物就没有足够的细胞来弥补它们在水中损失的热量。
On top of that, a small body means a lot of surface area where heat can be lost relative to their overall body mass.
此外，小的身体意味着散热的面积跟总的身体表面积的比值更大了。
That means in the frigid sea, it’s better to be bigger.
也就是说在寒冷的海洋里，长大点更好。
There’s a limit on how massive any creature can get, of course.
当然了，一个生物能长到多大也是有限制的。
Your maximum size generally depends on how you eat.
你能达到的最大体型一般取决于你进食的方式。
Smashing shellfish as a lifestyle, as sea otters do, might not be enough to sustain 100 metric tons, but baleen whales like the blue whale can take advantage of krill, a rich food source.
假如生性爱嚼贝类，就像海獭那样，恐怕不足以支持一百吨的体重，而须鲸比如蓝鲸就可以利用磷虾，这是一种优质的食物资源。
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