2018年6月大学英语六级听力常考时文：Marine Mammals
1.Why does the autnor think some mammals should have returned to the sea?Because there is ... in the sea.
2.What do humans want to get from marine mammals? Fur, ... and ...
3.Which mammals in the sea have almost been wiped out by humans? Those ...
mammals.
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.
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