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(2010-12-06 16:50:52)



Text 17 How Homing Pigeons Find Home ?
1. Science seems to be getting closer to answering a very old mystery. Homing pigeons can be taken hundred of miles from their home. When they are let go to fly again, they find their way home. Because of this special ability to find home, pigeons have been used as messengers for hundreds of years. 科学似乎快要找到这一非常古老神秘的问题的答案了.人们可以把信鸽带至离家数百英里的地方.当再将其放飞时,它们能找到回家的路.因其这一特殊的找家本领,鸽子几百年来一直被人们当作送信使者.
2. Today people even keep homing pigeons for racing as a sport. The birds are shipped to some chosen place a few hundred miles away. Then all of them are let go together. The winner is the bird that goes home first. A good racer can make it home 500 miles away in a single day.
3. The mystery of the homing pigeon is in how it navigates and how it finds home. It may be taken away in a covered-up cage, even a cage that is turned round and round to purposely mix up any sense of direction. To get home, it must fly over country that it has never seen before.
4. Suppose this were to happen to you? What would you need to find your way home (besides a good pair of legs)? I think I would ask for a compass, which always points north, to help find direction. I would also want a map. If a map shows where my home is, then I can use the compass to point me in the direction toward home. What we are talking about shows the two parts of the problem of the homing pigeon. Much of the study of homing pigeons leads to the idea that pigeons need the same kinds of information. They need to know how to tell direction and they need something like a map to tell which direction is toward home.
5. The first part seems to be pretty well answered, and we know of two ways that pigeons tell direction. First, they use the sun. Just getting rough direction from the sun is easy. It rise somewhere toward the east and sets somewhere toward the west. Getting accurate directions from the sun takes more care. You need to pay attention to the time of the year. Then you need to watch the path of the sun closely at each hour of the day. To tell direction accurately from the sun, a person needs to know the exact time.
6. Plants and animals that have been studied carefully (including the human) seem to have built-in clocks. These biological clocks, as they are called, usually are not quite exact in measuring time. However, they work pretty well because they are "reset" each day, maybe when the sun comes up.
7. Do pigeons use their biological clocks to help them find direction from the sun? We can keep pigeons in a room lighted only by lamps. And we can time the lighting to make their artificial "days," start at some different time from the real outside day. After a while we have shifted their clocks. Now we take them far away from home and let them go on a sunny day. Most of them start out as if they know just which way to go, but choose a wrong direction. They have picked a direction that would be correct for the position of the sun and the time of day according to their shifted clocks.
鸽子是否利用生物钟来靠太阳帮助确定方向呢?我们可以把鸽子放在一间只用灯光照明的房间里。 我们可以安排光线出现我时间,从不同于外面真实的时间开始,使其成为人工的白昼。过了一段时间,我们已经改变了其生物钟。现在我们带它远离家门,在一个阳光明媚的日子将它们放飞。它们大多立刻启程,仿佛它们明白该走哪条路,不过却飞错了方向。它们选择了根据太阳的位置本来应该是正确的方向,同时根据变化了的生物钟选择了时间。
8. We have talked about one of the more complex experiments that lead to the belief that homing pigeons can tell directions by the sun. What happens when the sky is darkly overcast by clouds and no one can see where the sun is? Then the pigeons still find their way home. The same experiment we talked about has been repeated many times on sunny days and the result was always the same. But on very overcast days those clock-shifted pigeons are just as good as normal pigeons in starting out in the right directions. So it seems that pigeons also have some extra sense of directions to use when they cannot see the sun.
9. Naturally, people have wondered whether pigeons might have a built-in compass -- something that would tell them about the direction of the earth's magnetic field. One way to test that idea would be to see if a pigeon's sense of direction can be fooled by a magnet attached to its back. With a strong magnet close by, anything like a magnetic compass can no longer tell about the earth's weak magnetic field.
10. To test the idea, one group of ten pigeons had strong little bar magnets attached to their backs. Another group carried little brass bars which were not magnetic. Let's call those two groups the magnet-pigeons and the brass-pigeons. In a number of experiments, both groups were taken away from home and let go. On sunny days none of the magnet-pigeons were fooled. They were just as good as the brass-pigeons in starting out in the right direction toward home. How about cloudy, overcast days with no sun? The brass-pigeons chose the right direction. But the magnet-pigeons were in trouble. They started out in different directions and acted completely lost.
11. What is the easiest way to understand that experiment? Maybe you would like to think about it some more. Here's what the scientists decided after they repeated the experiment many times. When pigeons can see the sun, they use it as their main means of direction finding. When they cannot see the sun, they use some special way to sense direction from the earth's magnetic field.
12. These ideas have been tested further by many different kinds of experiments. One experiment is to put a film of gelatin over a pigeon's eyes to blur its vision. Another is to put a little coil of wire on top of a pigeon's head and use an electric current from a small battery to make an electro-magnet. Enough experiments have been done to decide that homing pigeons (and maybe other birds) seem to have a build-in clock. The big questions is how these work and where they are located in the bird's body.


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