Listen to part of lecture in a marine biology class.
Professor.I want to continue our discussion about whales. Specifically, today, um,l want to talk about whale migration-um, why whales head south for the winter. Or really why whales in the cold water of the Northern Hemisphere head south for the winter. Now, not all kinds of whales migrate, but most baleen whales do.
And interestingly enough, we still dont really know why the baleen whales migrate. We do have several theories, however, which I'll discuss today. Uh, can anybody name one reason why baleen whales might migrate south, to the warm tropical water?
Male student: Uh, for food? You know, the whales move to warmer water in order to find a good area to feed.
Professor: Good guess.That should be an obvious reason-after all, most animals that migrate do so for the purpose of finding food. But, uh, that doesn't seem to be the case with baleen whales. To understand why, you need to know something about water temperature. There are a lot of technical reasons that I'm not going to go into right now.
But let's just say that nutrients don't rise to the surtace of tropical water like they do in other kinds of water. Tropical water simply never gets cold enough. So ...well, what this means, uh,is that tropical water doesn't have much of the plankton that most whales feed on.
Male student: l don't understand-if there's no plankton, how do the whales survive through the winter?
Professor: Right. How do they survive? You see, they dont have to eat anything, because they've stored up so much fat during the summer feeding season that they can just survive off of that. So if they dont need to eat anything, were back to our oniginal question. Why do baleen whales migrate?Any theories? No?
Well, there's one idea out there that a lot of people believe. In fact, uh, you could say its the most popular theory we have about whale migration. Basically, the argument is that for baleen whales, migration is a kind of balancing act. Let me explain. On one hand, whales need to take advantage of the summer months by eating as much food as they can.
And that's what they can do best in the northern seas. This allows them to build up a lot of fat. But in the winter, food is scarce even in thenorth, so what the whales need to do is save energy. And that's what migrating south can help them do. Amanda, you have a question?
Female student: Yes, Um , the balancing-act theory doesn't make sense to me. Maybe whales might need to save energy during the winter, but wouldn't moving all the way down to tropics make them lose energy?
Professor. Thats a good point, and its one reason why this isn't a perfect theory. lt does cost the whales energy to migrate, but it's easierfor whales to save energy in warm water than it i to save energy in cold water, so there might still be, you know, a good reason to move south for the winter. OK?
Now, before moving on to the next chapter I want to briefy discuss how the baleen whale manages to navigate. lts pretty remarkable, because the whales manage to return to the same places year after year, and have to travel over an enormous area of ocean in order to do it. I mean, it's not like whales can just look at a map, right? So exactly how do they do it?
Well, a lot of experimental work still needs to be done, but we have been able to figure out at least three ways the baleen whale navigates without getting lost. The first is the ability to use Earth's magnetic field like it was a map. That sounds strange, but we know that many birds use that method, use the magnetic field, and it's possible that whales have the biological ability to do the same thing.
Another theory is that if they stay close to the coast, whales might be able to find familiar landmarks and use those as guides. But we don't really know if a whale's eye-sight is good enough to be able to do that, so that's not a perfect theory.
And finally, we know that many whales make very loud sounds that can travel literally hundreds of miles underwater. Through a process called echolocation, its possible that these whales hear the sounds bounce off of isands or other pieces of land and use those echoes as clues to help them find their way.
This marine biology class 提到了baleen whales 须鲸，这种鲸鱼有一种冬天向南迁移的习性。海洋生物学家们并没有figure out why，所以提出了一些假说。其中最著名的一个假说是为了去赤道保存体力，因为温暖的水温能够减缓他们fat消耗。You know，鲸鱼在过冬之前一般会贮存特别多的脂肪用来过冬，而去赤道的迁徙过程中虽然理论上会耗费大量体力，但是这对于鲸鱼来说很容易。浮游生物不是它们的主要理由，因为赤道的海洋温度不够冷，所以浮游生物一般不会在surface。而北方的浮游生物在冬天也是非常稀少的...
- baleen whales 须鲸
- scarce 稀缺
- remarkable 非凡的;奇异的;显著的;引人注目的
- enormous 巨大的
- magnetic 磁的;磁性的;
- literally 照字面上说的；不夸张的
- echolocation 回声定位
- bounce 弹跳
- bounce off 反弹
- name one reason 列举一个原因