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.TOEFL Listening Comprehension TranscriptNarrator
Listen to part of a university lecture by a professor of Animal Behaviour.
Good morning, class. It''s a beautiful spring day outside, isn''t it? We''ll soon be seeing the first robin of spring- and so, it''s a perfect day to begin talking about migration. Migration is the main strategy that animals have for avoiding adverse environments and taking advantage of rich environments. Of course, there are other strategies, too- hibernation, for example- but far and away the most common way for animals to escape poor conditions and get to better ones is by migration- a mass journey from one place to the other.
Now there are all kinds of migrations, but the most familiar one is the sort that our robins will be experiencing- a seasonal, latitudinal migration. In the fall, the birds fly south, and in the spring they fly north again. In the southern hemisphere, of course, this works in the opposite direction. In both hemispheres, migrants move toward the equator when the earth chills and toward the poles when it warms. This is the way that species have been able to colonize, to use those subpolar resources that are seasonally difficult to access, that are unavailable to many living things for half the year.
Birds are certainly the most conspicuous latitudinal migrants, and they''re also the most awesome. Most famously, the Arctic Tern, which is a small seabird, migrates from one pole to the other, all the way from the arctic subpolar region to the antarctic subpolar region- and back again- annually. These birds travel roughly 70,000 kilometers a year!
Another kind of seasonal migration is altitudinal migration, which is where animals move vertically rather than to a different region of the earth. They go up the mountain in the spring and down it in the winter. The Dall Sheep of the American Rocky Mountains are good examples of this. In the spring, they follow the melting snow up to higher elevations, where they can feed on fresh plant growth and be safer from enemies, and then in the fall, when the snow begins to cover the mountain tops, they work their way back down to lower areas that are free of snow and more protected from the elements.
In regions with irregular climate patterns, some animals are nomadic- that is, they just kind of wander around from one area to another as, for instance, the rains bring fresh plant growth to these different places. The gnus, antelopes and other grazing animals of east Africa do this, wandering around the famous Serengeti Plain to where the grass is greenest.
Now most such migrations are undergone in search of food and water, but there are also reproductive migrations, where the purpose is to find a habitat that is safe and secure for the young. This accounts for the migration, for instance, of Grey Whales, who leave the food-rich waters of the north Pacific and travel to the protected waters of the Gulf of California to give birth.
Some animals, like our Arctic Tern, travel thousands of kilometers a year, and the most amazing thing is that many of them arrive in very specific places, in almost the same place, year after year. The Pacific salmon return after four or five years of wandering around the open ocean, swim up the rivers they originally emerged from, and arrive back at the same little branch of the same stream that they hatched in. How do animals do that, how do they navigate great distances so accurately- and with just the right timing?
Well, the answers seem to be multiple. Some migrants just seem to follow coastlines and mountain ranges and other landmarks- which is what many of the North American songbirds seem to do, following well-established ''flyways'' down both coasts and along the Rocky Mountains, and along the Mississippi River- and they either remember them, or they are genetically programmed to recognize them, or more likely a bit of both. Other animals use their other senses to help them get where they''re going. Research has revealed that our salmon are able to recognize the fine differences in the composition and concentration of the waters flowing from different sources, so that for the salmon, each little stream has its own unique flavour, and they can follow that flavour all the way up the river to their birthplace.
Other migrants have evidently evolved very sophisticated navigation systems that use the sun and the stars, or use day length or polarized light, or even use the Earth''s magnetic field, as timers and direction finders. Studies of loggerhead turtles, for instance, have shown that their hatchlings can sense the strength and direction of this geomagnetic field and use it when they first put to sea to follow the traditional routes of their parents.
But we still have a lot more to learn about these mechanisms, and about the evolution that has created these ways of species success in a harsh world. The earliest recorded observations of animal migration were over three thousand years ago, in the works of Aritotle and Hesiod, but we have still just begun to fully understand the migratory urge.
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