纸质版TPO4 Lecture2（TPO Extra 1）

纸质版TPO4 lecture2 biology Narrator Listen to part of a lecture in a marine biology class. Professor I want to continue our discussion about whales. Specifically, today, um, I 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 don't really know why the baleen whales migrate. We do have several theories, however, which I'II discuss today. U h, 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 surface 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 I 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 don't 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 don't need to eat anything, we're back to our original 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 it's 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 the north, 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 That's a good point, and it's one reason why this isn't a perfect theory. It does cost the whales energy to migrate, but it's easier for whales to save energy in warm water than it is 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 briefly discuss how the baleen whale manages to navigate. It's 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, it's possible that these whales hear the sounds bounce off of islands or other pieces of land and use those echoes as clues to help them find their way. Narrator Listen again to part of the lecture. Then answer the question. Professor 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 surface of tropical water like they do in other kinds of water. Narrator What does the professor mean when she says this: Professor There are a lot of technical reasons that I'm not going to go into right now. Narrator Listen again to part of the lecture. Then answer the question. Professor It's pretty remarkable, because the whales manage to return to the same places year after year, and have to trawl 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? Narrator What point does the professor make when she says this: Professor I mean, it's not like whales can just look at a map, right?