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Birds' magnetic sense —— Columbarian Columbuses
鳥類的磁性感知能力：禽類新發現
Birds can navigate by the Earth's magnetic field. How they do it is still a mystery
鳥類能夠利用地球磁場導航。機理尚不明確
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WHERE would people be without magnetic compasses?
人類沒有指南針會怎樣 ?
The short answer is: lost.
很簡單：迷失方向。
By giving human beings a sixth sense — an ability to detect the hitherto invisible magnetic field of the Earth — the compass proved one of the most important inventions ever.
指南針給瞭人類第 6 感，使人能辨別地球無形的磁場，成為最重要的發明之一。
It let sailors navigate without sight of the night sky.
海員不用觀察夜空便可以辨識方向。
And that led to the voyages of discovery, trade and conquest which created the political geography of the modern world.
人們用它進行海上探索，海上交易，攻城掠地，進而開創瞭現代世界的政治版圖。
Imagine, then, what animals which had their own, built-in compasses could achieve.
有些動物有自己內嵌的指南系統。可以想象得出這些動物的能力。
They might spend their summers doing the English Season in Glyndebourne or Henley, and then overwinter in the warmth of Mombasa.
它們可以在戈林德伯恩或亨利鎮消暑，享受自己的英格蘭夏日。然後在溫暖的蒙巴薩島過冬。
They might strike out, like intrepid pioneers, from Angola to Anchorage.
它們可以像無畏的開拓者一樣，從安哥拉獨闖安克雷奇。
They might even, if truly gripped by wanderlust and a hatred of the darkness, live in near-perpetual daylight by migrating from Pole to Pole.
假如它們為旅行所牽絆，為黑暗而煩惱，它們會穿梭於兩極之間，過著永遠有光亮的生活。
And that is just what some birds do.
以上這些隻是鳥類能力的一部分。
Swallows travel between Europe and Africa. Northern wheatears fly from Africa to Alaska, and back.
傢燕在歐洲和非洲之間遷徙。石棲鳥在非洲和阿拉斯加之間遷徙。
Arctic terns each year make the journey from one end of the planet to the other.
每年，北極燕鷗都會從地球的一端飛到另一端。
And they can do it, at least in part, because they do have a magnetic sense denied to humans.
它們能這麼做的原因之一便是鳥類可以感知磁性，而人類不行。
The most familiar avian navigation trick is that pulled off by homing pigeons.
人類最為熟知的鳥類導航技巧就是通過研究信鴿而得到的。
As a consequence pigeons have often found themselves at the sharp end of investigations about how bird navigation in general, and magnetic sense in particular, actually work.
鴿子便處在瞭人類研究的尖端。人們用它研究鳥類整體的導航機能，用它特別研究磁性感應機制。
That pigeons have such a sense was shown more than 40 years ago, by William Keeton of Cornell University, in upstate New York, who attached magnets to pigeons to see if they could still home.
鴿子顯示出此種能力是在 40 年前。當時，紐約州北部康乃爾大學的 William Keeton 把磁體系在鴿子身上，觀察它們是否能夠回傢。
They could not, though birds fitted with non-magnetic dummies managed perfectly well.
結果是它們不能，但是那些帶有仿磁體的鴿子卻回傢。
Since then, experiments on other species have shown magnetic sensitivity is common among birds. What these experiments have not shown, however, is how the birds manage it.
此後的實驗表明，磁性感知能力是鳥類共有的，但並沒有解釋是如何操作的。
See it? Hear it? Smell it?
視覺 ? 聽覺 ? 嗅覺 ?
There are two theories.
理論上的說法有兩種。
One is that the magnetic sensors are grains of magnetite, a form of iron oxide which, as its name suggests, is easily magnetised.
一種是鴿子具有磁感應器，這是一種以氧化鐵形式存在的磁鐵礦粒子。顧名思義，這種物質極易磁化。
The other is that the Earth's magnetic field affects a particular chemical reaction in the retina in a way that reaches into the arcane depths of quantum mechanics.
另一種說法認為，地球磁場能對視網膜裡特定的化學反映產生影響，在某種程序上可以達到神秘量子力學的深度。
The magnetite hypothesis concentrates on birds' beaks.
磁鐵礦假說的焦點是鳥類的喙。
Magnetite grains are common in living things, and are known to be involved in magnetic sensing in bacteria. In birds they are particularly abundant in the beak.
磁鐵礦粒子是生物共有的，廣泛存在於鳥的喙中。
So last year David Keays of the Institute of Molecular Pathology, in Vienna, dissected the beaks of nearly 200 unfortunate pigeons, to find out more.
去年，維也納分子病理學研究所的 David Keays 對將近 200 隻鴿子進行瞭解剖，以期得到更多發現。
What he discovered was not encouraging.
但是，他發現的並不令人鼓舞。
There were, indeed, lots of magnetite grains.
大量鐵磁礦粒子確實存在。
But he had expected they would congregate in some sort of specialised sensory cell akin to the taste buds of the tongue or the hair cells of the ear.
他原以為鐵磁礦粒子會聚集成為專門的感覺細胞，類似於舌頭上的味蕾和內耳毛細胞。
Instead, he found that the beak's magnetite is mostly in macrophages.
但是，他發現，喙部的鐵磁礦主要以巨噬細胞的形式存在，
These are cells whose job is to wander around amoeba-like, chewing up bacteria and debris from other body cells as they go.
這些細胞的職能是以遊離細胞的形式對細胞殘片及病原體進行噬菌。
Not, then, likely candidates as magnetic sensors.
因此，巨噬細胞不可能具有磁感應功能。
Other experiments, though, do suggest the beak is involved.
其它的實驗也包含瞭對喙的研究。
The nerve that connects it to the brain is known as the trigeminal.
聯結喙與腦的神經叫三叉神經。
When Dominik Heyers and Henrik Mouritsen of Oldenburg University, in Germany, cut the trigeminals of reed warblers the birds' ability to detect which way was north remained intact.
德國奧爾登堡大學的 Dominik Heyers 和 Henrik Mouritsena 切斷瞭葦鶯的三叉神經，保留瞭它們辨別北方的能力。
They did, however, lose their sense of magnetic dip.
然而，這些鳥卻失掉瞭磁傾角的感應力。
Dip indicates latitude, another important part of navigation.
磁傾角可以指示緯度，是導航的重要組成部分。
To confuse matters further, some people accept Dr Keays's interpretation of what is going on in the beak,
Keays 對鳥喙解釋使情況更加復雜。但有些人還是接受瞭他的說法。
but think that the relevant magnetite grains are elsewhere — in the hair cells of the ear, which are also rich in iron oxide.
但是這些人認為鳥身體的其它部位也存在磁鐵礦粒子—內耳毛細胞。氧化鐵也富含這種粒子。
If they are right, then from the birds' point of view they are probably hearing the magnetic signal.
假如這些人的假定正確，從鳥的角度來看，它們可能聽得到磁信號。
The main alternative to the nasal-magnetite hypothesis, though, is not that birds hear magnetic fields, but that they see them.
鼻腔內存在磁鐵礦的假說 並不是鳥類可以聽到磁場，而是能看到磁場。
One line of evidence for this is that part of a bird's brain, called cluster N, which gets its input directly from the eyes, seems to be involved in magnetic sensing.
關於此的證明是，鳥大腦中有一部分叫 cluster N，可以直接得到眼部輸送的信息，好像跟磁場感應有聯系。
Experiments Dr Mouritsen's team conducted on robins showed that destroying cluster N destroys a bird's north-detecting sense, and other experiments, on meadow pipits, show that cells in cluster N are far more active when the birds are using their magnetic sense than when they are not.
博士 Mouritsen 研究團隊對知更鳥進行瞭實驗，得出推斷。實驗顯示破壞知更鳥的 cluster N，也就破壞瞭它們識別北方的能力。研究團隊又對草地鷚進行瞭實驗。實驗顯示，鳥類使用磁感應能力的時候，cluster N 細胞異常活躍。
The problem with this idea is that birds' eyes do not have magnetite in them.
此種假說的問題在於鳥類的眼部沒有磁鐵礦。
If they do house magnetism detectors, those detectors must be something else.
假如它們真的起到瞭磁探測器的作用，那麼肯定另有他物。
That something, according to a hypothesis advanced by Klaus Schulten, who works at the University of Illinois at Urbana-Champaign, is a type of retinal protein called a cryptochrome.
在伊利諾斯大學香檳分校工作。據 Schulten，這種他物是一種名為 cryptochrome 的尿視黃醇蛋白。
When hit by light, a cryptochrome produces pairs of molecules called free radicals that are electrically neutral but have unpaired electrons in them.
當受到光照時，就產生名為自由基的分子對。這種自由基呈電中性，其中含有未配對電子。
Electrons are tiny magnets, so they tend to attract each other and pair up in a way that neutralises their joint magnetic fields.
電子就是微小的磁性體。因此，當它們的聯合磁場中合之時，電子就會相互吸引，就會形成組對。
Unpaired electrons, however, remain magnetic, and thus sensitive to the Earth's field.
但是，那些不成對電子仍具磁性，對地球磁場很敏感。
Moreover, because the unpaired electrons in the free radicals were originally paired in the molecule that split to form the radicals, quantum mechanics dictates that these electrons remain entangled.
因為自由基中的那些不成對電子最初存在於分裂成為自由基分子之中，量子力學規定這些電子依然是絞纏的。
This means that however far apart they move, what happens to one affects the other's behaviour.
也就是說，無論雙方離得有多遠，一方的行為會影響另一方。
Calculations suggest the different ways the two radicals feel the Earth's field as they separate is enough to change the way they will react with other chemicals — including ones that trigger nerve impulses, and that, via entanglement, they can transmit this information to each other, and thus affect each other's reactions.
此種假設表明，當兩種自由基分離時，它們感知地球磁場的相反作用足夠能夠改變它們與其它化學物質相互反應的方式――包括那些能產生神經脈沖的化學物質。同時，通過絞纏，它們彼此能互相信息，從而產生相互影響。
This, the calculations indicate, would be enough for a bird's brain to interpret the magnetic field.
此種假設表明，這足可以讓鳥腦識別磁場。
It would probably see a pattern of spots before its eyes, which would remain stationary as it scanned its head from side to side.
鳥眼可能會看到眼前有某種樣式的斑點圖案，當鳥類對其識別之時，眼睛是固定的。
And some birds do, indeed, scan their heads this way when assessing the direction of magnetic north.
其實，當鳥類辨別地磁北極之時，確實能夠用此法掃描頭部。
It is possible, of course, that both hypotheses are right, and that birds have two magnetic senses, with one perhaps concentrated on north detection and the other on detecting dip.
當然，兩種假說都有正確的可能。鳥類也有可能有兩套磁感應能力，一種集中在北方，另一種集中於磁傾角。
But there is something particularly poetic about the idea that even part of this mysterious sixth sense depends on a still-more-mysterious quantum effect — one that Einstein himself described as spooky action at a distance.
這種神秘的第六感覺依賴於更加神秘的量子力學效應。對此還有一種詩意般的解釋，即愛因斯坦自己說的鬼魅般的超距作用。
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