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本文

1  Not many other birds than Clark’s nutcrackers stay in the high mountains during winter time. It is cold and stormy up there. Food is also scarce. But nutcrackers are smart, and they have learned a trick to survive in the mountains during the cold season. The trick is pretty simple: They stock up.

 

2  Nutcrackers feed mainly on pine seeds, which are easy to find when cones ripen in summer and fall. But since the cone crop is seasonal, Clark’s nutcrackers must cache large quantities of pine seeds to eat in winter and spring. Amazingly, they depend completely on the stored seeds during this time.

 

3  In one fall season, a single nutcracker may store tens of thousands of pine seeds in as many as 5,000 different caches. Incredibly, hungry nutcrackers are able to locate most of their caches through the winter. Nutcrackers somehow remember exactly where thousands of different stores of seeds are buried without a single yellow sticky note or GPS. How do they do it?

 

4  Stephen Vander Wall, a graduate student at Northern Arizona University, figured that nutcrackers might relocate stored seeds in one of five ways: (1)The birds dig randomly and find buried seeds by chance; (2)They search randomly, but only in certain heavily cached areas; (3)They are able to smell the seeds; (4)They mark the ground surface in some way to show where pine seeds are hidden; or (5)They remember the exact locations as if marked on a mental map. Vander Wall decided to design an experiment to narrow down the list.

 

5  As following and observing nutcrackers in the wild would be impossible, he experimented with captive birds. He covered the floor of a large cage of about seventy-five square meters with a couple of inches of loose soil, and then arranged various rocks, logs, and other “landmarks” around the room. After two nutcrackers named Orange and Red had been trained to bury seeds inside the cage, Vander Wall began his tests.

 

6  First, he released Orange and Red inside the cage and watched in separate trials to see where they stored seeds. When both birds had made at least 150 caches in the soil, in alternate sessions so that they could not watch each other, Vander Wall sneaked in and made 100 caches himself and removed fifty of the birds’ caches. Then he let both birds back into the cage by turns to see which caches they would dig up. If the birds searched by remembering the locations of their own caches, Vander Wall reasoned, they would not be able to find each other’s stocks or his additional buried seeds.

 

7  And that is exactly what happened. Orange found sixty-three of his own caches, but none of Red’s or Vander Wall’s. Red found sixty-one of his caches, three of Orange’s, and none of Vander Wall’s. The birds were not searching randomly, which showed his first two hypotheses were false. Both birds also dug unsuccessfully for their personal caches that had been secretly removed by Vander Wall, which proved that his third hypothesis―the birds found food by smell―was also false.

 

8  He went on to a second experiment to find out whether the birds might be marking the soil. He let Orange and Red cache more seeds inside the cage, then raked half of its floor smooth to obscure any surface cues. Vander Wall thought that if they were looking for surface irregularities, then the birds would be able to find their caches only in the unraked half. But when the two nutcrackers were returned to their cage, they dug up seeds in both the raked and unraked areas, indicating that surface texture did not affect their ability to remember locations. Hypothesis 4 was now rejected.

 

9  Only one possibility remained, that nutcrackers were using spatial memory to relocate their caches. To test it, Vander Wall designed a third experiment. As in the first two tests, he kept Orange and Red inside the cage until they had made lots of seed caches in the loose soil. Then, as before, he removed the birds and made some secret changes. This time, he rearranged the landmarks in half of the room, moving the rocks, logs, and others exactly twenty centimeters in the same direction; in the other half, he left the landmarks alone. He thought that the nutcrackers would locate their caches only where the nearby landmarks had been left in place.

 

10  When the birds returned to the cage, they searched for their cached food. This time, they found only half of the caches―those in the undisturbed half. In the other half of the cage the birds dug about twenty centimeters away from the actual locations of their caches, almost exactly corresponding to the distance and direction that the local landmarks had been moved.

 

11  Vander Wall showed that Clark’s nutcrackers use spatial memory to remember where they store food by tying each cache to a landmark within their home territory, and make it through the winter.

 

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Comprehension

  1. What is the main idea of the text?
  2. Stephen Vander Wall conducted experiments to find the way that Clark’s nutcrackers remember the locations of their caches.
  3. Clark’s nutcrackers cache large quantities of pine seeds in fall to survive in the high mountains during the cold season.
  4. The two Clark’s nutcrackers were able to locate their caches only where the landmarks had been left in place.

 

  1. Complete the following table using the words in the box. Change the form of the words if necessary.

1,2,3 Clark’s nutcrackers (1.     ) large quantities of pine seeds in many different caches to survive in high mountains during the cold season, and are somehow able to (2.     ) most of them.

4,5 Stephen Vander Wall thought of five possibilities to explain how nutcrackers might locate stored seeds. He decided to do his experiments with (3.     ) birds.

6,7,8 The first experiment using two nutcrackers proved that the birds did not search (4.     ) and that they did not find food by (5.     ). The second experiment showed that the birds did not look for surface (6.     ) to find their caches.

9,10 The third experiment showed that the nutcrackers located their caches successfully only where the (7.     ) near the caches had been left in place.

11 Vander Wall showed that nutcrackers use (8.     ) memory to remember the locations of their caches.

 

captive  irregularity  landmark  locate  randomly  smell  spatial  store

 

 

  1. Write T for true or F for false for each statement.
  2. Nutcrackers feed mainly on pine seeds because they are easy to find during the cold season. (   )
  3. In the first experiment, neither of the two nutcrackers found any of Stephen Vander Walls caches of pine seeds. (   )
  4. In the second experiment, neither of the two nutcrackers found their caches in the raked area. (   )
  5. In the third experiment, neither of the two nutcrackers found their caches in the area where Vander Wall had moved the landmarks. (   )
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和訳

羽の生えた生き物

 

[1]  冬の間,ハイイロホシガラス以外に,高い山にとどまる鳥はあまり多くない。そこは寒くて荒れ模様なのだ。食べ物も乏しい。しかし,ハイイロホシガラスは利口で,寒い季節の間を山の中で生き残る秘訣を習得している。その秘訣はかなり単純だ。しっかり蓄えるのだ。

 

[2]  ハイイロホシガラスは主に松の種子を食べる。松の種子は松かさが夏と秋に実るときに見つけやすいのだ。しかし松かさの収穫は季節が限られているので,ハイイロホシガラスは冬と春に食べる大量の松の種子を隠し場所に蓄えなければならない。驚いたことに,ハイイロホシガラスはこの期間に蓄えた種子に完全に依存しているのだ。

 

[3]  たった1羽のハイイロホシガラスが,ひと秋に何万という松の種子を5,000もの異なる隠し場所に蓄えることもある。信じられないことに,腹をすかせたハイイロホシガラスは,冬の間じゅう隠し場所のほとんどを見つけ出すことができる。ハイイロホシガラスは何らかの方法で,異なる何千もの種子の蓄えがどこに埋められているかを,ただ1枚の黄色い付箋紙もGPSもなしで正確に覚えている。どのようにそうするのだろうか。

 

[4]  北アリゾナ大学の大学院生スティーブン・バンダー・ウォールは,ハイイロホシガラスが5つの方法の中の1つで,蓄えた種子の場所を再び見つけるだろうと考えた。(1) その鳥はでたらめに掘り起こして,埋められた種子を偶然見つける。(2)でたらめに探すが,ある特定の,隠し穴が密に存在する地域だけを探す。(3)(隠した)種子の臭いを感じることができる。(4) 松の種子が隠されている場所を示すために,ある方法で地表に印をつける。または,(5)あたかも脳内の地図に印がつけられているかのように,正確な場所を覚えている。バンダー・ウォールは,リストを絞り込むための実験の計画を立てることにした。

 

[5]  野生のハイイロホシガラスを追跡したり観察したりすることは不可能だろうから,彼は捕獲された鳥を用いて実験した。彼は約75平方メートルの大きな鳥かごの床をほぐした柔らかい土で2,3インチ覆い,それから部屋のあちこちにいろいろな岩,丸太,そしてその他の「目印」を配置した。「オレンジ」と「レッド」と名づけられた2羽のハイイロホシガラスが,鳥かごの内側で種子を埋めるように訓練された後で,バンダー・ウォールは実験を開始した。

 

[6]  まず最初に,彼は「オレンジ」と「レッド」を(1羽ずつ)鳥かごの中に放ち,それらがどこに種子を蓄えるかを調べるために,(1羽ずつ)別々の実験で観察した。両方の鳥が互いを観察することができないように,代わる代わる土の中に少なくとも150か所の隠し場所を作った後,バンダー・ウォールはこっそり歩み入って,自分で100か所の隠し場所を作り,鳥たちの50か所の隠し穴を取り除いた。彼はそれから両方の鳥がどの隠し場所を掘り起こすかを調べるために,それらを代わる代わる鳥かごに戻した。もし鳥たちが自分の隠し場所のありかを覚えていて探すのなら,互いの蓄えも彼が追加して埋めた種子も見つけることはできないだろう,とバンダー・ウォールは推論した。

 

[7]  そして,それがまさに起こったことだった。「オレンジ」は63か所で自分が隠した種を見つけたが,「レッド」のものとバンダー・ウォールのものはどれも見つけなかった。「レッド」は61か所で自分が隠した種を見つけ,また,「オレンジ」のものを3か所見つけたが,バンダー・ウォールのものはどれも見つけなかった。鳥たちはでたらめに探していたのではなく,それは彼の最初の2つの仮説が誤りだということを示していた。両方の鳥はまた,バンダー・ウォールにこっそり取り除かれた自分の(作った)隠し場所を掘ったものの,種を回収するのには失敗した。そのことは彼の3番目の仮説,つまり鳥は臭いで餌を見つけ出すという仮説も誤りだということを証明した。

 

[8]  鳥たちが(ひょっとして)土に印をつけているかどうかを見つけ出すために,彼は2番目の実験に進んだ。彼は「オレンジ」と「レッド」が鳥かごの内側でさらに多くの種子を隠し場所に蓄えるようにしてから,(もしあるとしたら)表面の手がかりを覆い隠すために床の半分をならして平らにした。バンダー・ウォールは,もし鳥たちが表面のでこぼこを探しているのなら,ならされていない半分の方でだけ隠し場所を見つけることができるだろうと考えた。しかし,2羽のハイイロホシガラスが鳥かごに戻されたとき,彼らはならされた区域とならされていない区域の両方で種子を掘り出した。そしてそのことは,表面の滑らかさは彼らが場所を覚える能力に影響を与えないことを示していた。今,仮説4が却下されたのである。

 

[9]  ハイイロホシガラスが自分の隠し場所を再び見つけるために,空間的な記憶力を用いているという,ただ1つの可能性だけが残された。それを実験するために,バンダー・ウォールは3番目の実験の計画を立てた。最初の2つの実験と同様に,「オレンジ」と「レッド」がほぐした柔らかい土の中に多くの種子の隠し場所を作り終えるまで,鳥かごの内側にとどめた。それから,前と同様に鳥を(鳥かごから)移動させて,秘かにいくつかの変更を加えた。今回彼は,実験場の半分(の区域)で岩や丸太,その他の目印を正確に20センチ同じ方向に動かした。そして,もう半分の区域は目印をそのままにしておいた。彼は,ハイイロホシガラスはすぐ近くの目印がもとの位置に残された区域でのみ,自分の隠し場所を突きとめるだろうと考えた。

 

[10]  鳥は鳥かごに戻ると,自分が隠した餌を探した。今回,鳥たちは隠し場所の半分しか見つけられなかった。つまり,手をつけていない半分の区域にあるものだけを。鳥かごのもう一方の半分(の区域)では,鳥たちは隠し場所の実際の位置から約20センチ離れた所を掘り起こした。これはその(特定の)場所の目印が動かされた距離と方向にほぼ正確に一致していた。

 

[11]  バンダー・ウォールは,ハイイロホシガラスが空間的記憶力を用いて,隠し場所のそれぞれを自分の縄張り内の目印と結びつけることによって食料を蓄える場所を覚え,冬を乗り切ることを明らかにしたのだった。

解答例

Comprehension

  1. What is the main idea of the text?
  2. Stephen Vander Wall conducted experiments to find the way that Clark’s nutcrackers remember the locations of their caches.
  3. Clark’s nutcrackers cache large quantities of pine seeds in fall to survive in the high mountains during the cold season.
  4. The two Clark’s nutcrackers were able to locate their caches only where the landmarks had been left in place.

 

  1. Complete the following table using the words in the box. Change the form of the words if necessary.

1,2,3 Clark’s nutcrackers (1. store  ) large quantities of pine seeds in many different caches to survive in high mountains during the cold season, and are somehow able to (2.  locate ) most of them.

4,5 Stephen Vander Wall thought of five possibilities to explain how nutcrackers might locate stored seeds. He decided to do his experiments with (3. captive ) birds.

6,7,8 The first experiment using two nutcrackers proved that the birds did not search (4.  randomly ) and that they did not find food by (5. smell  ). The second experiment showed that the birds did not look for surface (6. irregularities ) to find their caches.

9,10 The third experiment showed that the nutcrackers located their caches successfully only where the (7. landmarks ) near the caches had been left in place.

11 Vander Wall showed that nutcrackers use (8. spatial ) memory to remember the locations of their caches.

 

captive  irregularity  landmark  locate  randomly  smell  spatial  store

 

 

  1. Write T for true or F for false for each statement.
  2. Nutcrackers feed mainly on pine seeds because they are easy to find during the cold season. ( F )
  3. In the first experiment, neither of the two nutcrackers found any of Stephen Vander Walls caches of pine seeds. ( T )
  4. In the second experiment, neither of the two nutcrackers found their caches in the raked area. ( F )
  5. In the third experiment, neither of the two nutcrackers found their caches in the area where Vander Wall had moved the landmarks. ( T )
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