经济学人下载:鸟类的磁性感知能力 禽类新发现

Science and technology
科学技术

Birds' magnetic sense
鸟类的磁性感知能力

Columbarian Columbuses
禽类新发现

Birds can navigate by the Earth's magnetic field. How they do it is still a mystery
鸟类能够利用地球磁场导航。机理尚不明确

WHERE would people be without magnetic compasses?
人类没有指南针会怎样？

The short answer is: lost.
很简单：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.
指南针给了人类第6感，使人能辨别地球无形的磁场，成为最重要的发明之一。

It let sailors navigate without sight of the night sky.
海员不用观察夜空便可以辨识方向。

And that led to the voyages of discovery, trade and conquest which created the political geography of the modern world.
人们用它进行海上探索，海上交易，攻城掠地，进而开创了现代世界的政治版图。

Imagine, then, what animals which had their own, built-in compasses could achieve.
有些动物有自己内嵌的指南系统。可以想象得出这些动物的能力。

They might spend their summers doing the English Season in Glyndebourne or Henley, and then overwinter in the warmth of Mombasa.
它们可以在戈林德伯恩或亨利镇消暑，享受自己的英格兰夏日。然后在温暖的蒙巴萨岛过冬。

They might strike out, like intrepid pioneers, from Angola to Anchorage.
它们可以像无畏的开拓者一样，从安哥拉独闯安克雷奇。

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.
假如它们为旅行所牵绊，为黑暗而烦恼，它们会穿梭于两极之间，过着永远有光亮的生活。

And that is just what some birds do.
以上这些只是鸟类能力的一部分。

Swallows travel between Europe and Africa. Northern wheatears fly from Africa to Alaska, and back.
家燕在欧洲和非洲之间迁徙。石栖鸟在非洲和阿拉斯加之间迁徙。

Arctic terns each year make the journey from one end of the planet to the other.
每年，北极燕鸥都会从地球的一端飞到另一端。

And they can do it, at least in part, because they do have a magnetic sense denied to humans.
它们能这么做的原因之一便是鸟类可以感知磁性，而人类不行。

The most familiar avian navigation trick is that pulled off by homing pigeons.
人类最为熟知的鸟类导航技巧就是通过研究信鸽而得到的。

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.
鸽子便处在了人类研究的尖端。人们用它研究鸟类整体的导航机能，用它特别研究磁性感应机制。

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.
鸽子显示出此种能力是在40年前。当时，纽约州北部康乃尔大学的William Keeton把磁体系在鸽子身上，观察它们是否能够回家。

They could not, though birds fitted with non-magnetic dummies managed perfectly well.
结果是它们不能，但是那些带有仿磁体的鸽子却回家。

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.
此后的实验表明，磁性感知能力是鸟类共有的，但并没有解释是如何操作的。

See it? Hear it? Smell it?
视觉？听觉？嗅觉？

There are two theories.
理论上的说法有两种。

One is that the magnetic sensors are grains of magnetite, a form of iron oxide which, as its name suggests, is easily magnetised.
一种是鸽子具有磁感应器，这是一种以氧化铁形式存在的磁铁矿粒子。顾名思义，这种物质极易磁化。

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.
另一种说法认为，地球磁场能对视网膜里特定的化学反映产生影响，在某种程序上可以达到神秘量子力学的深度。

The magnetite hypothesis concentrates on birds' beaks.
磁铁矿假说的焦点是鸟类的喙。

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.
磁铁矿粒子是生物共有的，广泛存在于鸟的喙中。

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，维也纳分子病理学研究所的David Keays对将近200只鸽子进行了解剖，以期得到更多发现。

What he discovered was not encouraging.
但是，他发现的并不令人鼓舞。

There were, indeed, lots of magnetite grains.
大量铁磁矿粒子确实存在。

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.
他原以为铁磁矿粒子会聚集成为专门的感觉细胞，类似于舌头上的味蕾和内耳毛细胞。

Instead, he found that the beak's magnetite is mostly in macrophages.
但是，他发现，喙部的铁磁矿主要以巨噬细胞的形式存在，

These are cells whose job is to wander around amoeba-like, chewing up bacteria and debris from other body cells as they go.
这些细胞的职能是以游离细胞的形式对细胞残片及病原体进行噬菌。

Not, then, likely candidates as magnetic sensors.
因此，巨噬细胞不可能具有磁感应功能。

Other experiments, though, do suggest the beak is involved.
其它的实验也包含了对喙的研究。

The nerve that connects it to the brain is known as the trigeminal.
联结喙与脑的神经叫三叉神经。

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.
德国奥尔登堡大学的Dominik Heyers和Henrik Mouritsena切断了苇莺的三叉神经，保留了它们辨别北方的能力。

They did, however, lose their sense of magnetic dip.
然而，这些鸟却失掉了磁倾角的感应力。

Dip indicates latitude, another important part of navigation.
磁倾角可以指示纬度，是导航的重要组成部分。

To confuse matters further, some people accept Dr Keays's interpretation of what is going on in the beak,
Keays对鸟喙解释使情况更加复杂。但有些人还是接受了他的说法。

but think that the relevant magnetite grains are elsewhere—in the hair cells of the ear, which are also rich in iron oxide.
但是这些人认为鸟身体的其它部位也存在磁铁矿粒子—内耳毛细胞。氧化铁也富含这种粒子。

If they are right, then from the birds' point of view they are probably hearing the magnetic signal.
假如这些人的假定正确，从鸟的角度来看，它们可能听得到磁信号。

The main alternative to the nasal-magnetite hypothesis, though, is not that birds hear magnetic fields, but that they see them.
鼻腔内存在磁铁矿的假说 并不是鸟类可以听到磁场，而是能看到磁场。

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.
关于此的证明是，鸟大脑中有一部分叫cluster N，可以直接得到眼部输送的信息，好像跟磁场感应有联系。

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研究团队对知更鸟进行了实验，得出推断。实验显示破坏知更鸟的cluster N，也就破坏了它们识别北方的能力。研究团队又对草地鹨进行了实验。实验显示，鸟类使用磁感应能力的时候，cluster N细胞异常活跃。

The problem with this idea is that birds' eyes do not have magnetite in them.
此种假说的问题在于鸟类的眼部没有磁铁矿。

If they do house magnetism detectors, those detectors must be something else.
假如它们真的起到了磁探测器的作用，那么肯定另有他物。

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.
在伊利诺斯大学香槟分校工作。据Schulten，这种他物是一种名为cryptochrome的尿视黄醇蛋白。

When hit by light, a cryptochrome produces pairs of molecules called free radicals that are electrically neutral but have unpaired electrons in them.
当受到光照时，就产生名为自由基的分子对。这种自由基呈电中性，其中含有未配对电子。

Electrons are tiny magnets, so they tend to attract each other and pair up in a way that neutralises their joint magnetic fields.
电子就是微小的磁性体。因此，当它们的联合磁场中合之时，电子就会相互吸引，就会形成组对。

Unpaired electrons, however, remain magnetic, and thus sensitive to the Earth's field.
但是，那些不成对电子仍具磁性，对地球磁场很敏感。

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.
因为自由基中的那些不成对电子最初存在于分裂成为自由基分子之中，量子力学规定这些电子依然是绞缠的。

This means that however far apart they move, what happens to one affects the other's behaviour.
也就是说，无论双方离得有多远，一方的行为会影响另一方。

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.
此种假设表明，当两种自由基分离时，它们感知地球磁场的相反作用足够能够改变它们与其它化学物质相互反应的方式──包括那些能产生神经脉冲的化学物质。同时，通过绞缠，它们彼此能互相信息，从而产生相互影响。

This, the calculations indicate, would be enough for a bird's brain to interpret the magnetic field.
此种假设表明，这足可以让鸟脑识别磁场。

It would probably see a pattern of spots before its eyes, which would remain stationary as it scanned its head from side to side.
鸟眼可能会看到眼前有某种样式的斑点图案，当鸟类对其识别之时，眼睛是固定的。

And some birds do, indeed, scan their heads this way when assessing the direction of magnetic north.
其实，当鸟类辨别地磁北极之时，确实能够用此法扫描头部。

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.
当然，两种假说都有正确的可能。鸟类也有可能有两套磁感应能力，一种集中在北方，另一种集中于磁倾角。

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.
这种神秘的第六感觉依赖于更加神秘的量子力学效应。对此还有一种诗意般的解释，即爱因斯坦自己说的鬼魅般的超距作用。