Since there is such an abundance of food in the sea,it is understandable that some of the efficient, highly adaptable, warm-blooded mammals that evolved on land should have returned to the sea.Those that did have flourished. Within about 50 million years -- no time at all, geologically speaking -- one of the four kinds of mammals that has returned to a marine environment has developed into the largest of all animal forms, the whale. A second kind, the seal,has produced what is probably the greatest population of large carnivorous mammals on Earth. This suggests that these "top dogs" of the ocean are prospering and multiplying. However, such has not been the case,at least not for the last 150 years.Trouble has closed in on these mammals in the form of equally warm-blooded and even more efficient and adaptable predators, humans. At sea, as on land, humans have now positioned themselves on the top of the whole great pyramid of life, and they have caused serious problems for the mammals of the sea. There is a simple reason for this. Marine mammals have the misfortune to be swimming aggregates of commodities that humans want: fur, oil and meat. Even so, they might not be so vulnerable to human depredation if they did not,like humans, reproduce so slowly. Every year humans take more than 50 million tons of fish from the oceans without critically depleting the population of any species. But the slow-breeding mammals of the sea have been all but wiped out by humans seeking to satisfy their wants and whims.
既然海洋中有如此丰富的食物，一些原本在陆地生存的高效率的，适应性强的暖血型动物返回海洋生存是不难理解的。 那些已经返回海洋的哺乳动物种族繁荣。 在大约 5 千万年间(这在地质学上不过是弹指一挥间)，返回海洋的4 种浦乳动物之一的鲸已成为体积最庞大的动物。 而另一种哺乳动物海豹，它的数量恐怕是地球上大型食肉哺乳动物中最大的。 这似乎表明这些海洋中的"大哥大"们人丁兴旺，一派繁荣景象。但至少在近 150 年以来，事实并非如此。 同样是暖血动物而且效率更高、适应性更强的捕食者，人类，成为这些哺乳动物的日益迫近的威胁。 在陆地和海洋，人类都已处在巨大的生物金字塔的最顶端，成为所有生物的主宰，并给海洋哺乳动物带来巨大的危胁。 原因很简单。 这些哺乳动物不幸成为人类所需要的一堆游动着的商品，如皮毛、油和肉。 尽管如此，如果它们不象人类一样繁殖如此缓慢，它们对人类的掠夺也不会如此无能为力。 每年人类从海洋中捕捞约 5 千万吨的鱼类，但这并未导致任何鱼种的灭绝。但繁殖缓慢的海洋哺乳动物却因为有了一心只想满足一已私欲的人类而濒临。