【正文】 。 they were　　looking for spices. And we know what important historical repercussions some of those　　voyages had.　　Listening6　　A：Well, it’s finally looking like spring is arriving. The last of the winter snow would be　　melting away in a few days. So before we close today, I thought I’d mention a　　biological event that’s a part of the transition from winter to spring, something you　　can go outside and watch if you have some patience. There is a small creature that　　livesin this area。 you’ve probably seen it. It’s the North American wood frog. Now the　　wood frog’s not that easy tosspot since it stays pretty to close to the ground, under　　leaves and things and it blends in really well with its background as you can see. But　　they are worth the effort because they do something very unusual, something you　　might not have eventhought possible. OK North American wood frogs live over a very　　broad territory or range. They’re found all over the northeastern United States and all　　through Canada and Alaska, even inside the Arctic Circle. No other frog is able to live　　that far and north. But wherever they live, once the weather starts to turn cold and　　the temperature starts to drop below freezing, as soon as the frog even touches an ice　　crystal or a bit of frozen ground, well, it begins to ...yes to me. You look a　　little bit takenaback.　　B: Wait,you mean it’s still alive but itfreezes,solid?　　A: Well,almost. Ice forms inall the spaces outside the cells but never within a cell.　　B: But… then how does its heart beat?　　A: It doesn’t.　　B: But…then how could it…….　　A: You are gonna do such a thing? Well, that first touch of ice apparently triggers a　　biological response inside the frog. That first of all starts drawing water away from the　　center of its body, so the middle part of the frog, its internal organs, its heart, lungs,　　liver, these start getting drier and drier while the water that’s being pulled away is　　forming a puddle around the organs just underneath the skin. And then that puddle of　　water starts to freeze. OK, up to known, the frog’s heart is still beating, right? Slower　　and slower but…and in those last few hours before it freezes, it distributes glucose, a　　blood sugar throughout its body, its circulatory system, sort of acts like an an　　antifreeze.　　B: Asolution of antifreeze like you put inyour car inthe winter?　　A: Well, you tell me. In frogs, the extra glucose makes it harder for the winter inside the　　cells to freeze. So the cells stay just slightly wet, enough so that they can survive the　　 that, the heart stops beating is that the same?　　B: Idon’t reallyknow, but how long dose itstay that way?　　A: Well,it could be days or months, all winter in fact but umm, see the heart reallydoesn’t　　needto do any pumping now because the blood is frozen too.　　B: Ijust, I guess I just don’t see how it isn’t, you know, clinicallydead.　　A: Well, that’s the amazing thing and how it revives is pretty amazing too. After months　　without a heartbeat, spring time came around again, the earth starts to warm up and　　suddenly one day, ping, a pulse, followed by another one, then another until maybe ten,　　twelve hours later,the animal is fullyrecovered.　　B: And does the thawing process have some kind of trigger as well?　　A: Well, we are not sure actually, the clearer thing is even though the sun is warning the　　frog up on the outside, its inside thaw out first, the heart and brain and everything. But　　somehow it all just happens that way every spring.　　B: Butafter theythaw does it affect them like their lifespan?　　A: Well, hmm, we really don’t know a lot about how long a wood frog normally lives,　　probably just a few years but there is no evidence its does have some other　　impacts though. In studies, we found that when it es to reproduction, freezing　　diminishes the mating performance of males. After they’ve been frozen and thawed of　　course, they don’t seem quite as vocal. They move slower and they seem to have a　　harder time recognizing a potential mate. So if the male frog could manage not to go　　through this freezingcycle, he’d probably have more success inmating. 源于：小馬過河相關(guān)推薦：2012年11月18日托福寫作真題解析2012年11月18日托?？谡Z真題解析2012年11月18日托福閱讀真題解析2012年11月18日托福聽力真題解析全國免費(fèi)咨詢電話：4000123267