The Compass Within 


By Davide Castelvecchi D. カステルベッキ
English 日本語 日本語
For what must have felt like an interminable six months back in 2007, Sabine Begall spent her evenings at her computer, staring at photographs of grazing cattle. She would download a satellite image of a cattle range from Google Earth, tag the cows one by one, then pull up the next image. With the help of her collaborators, Begall, a zoologist at the University of Duisburg-Essen in Germany, ultimately found that the unassuming ruminants were on to something. On average, they appeared to align their bodies with a slight preference toward the north-south axis. But they were not pointing to true north, which they could have located using the sun as reference. Instead they somehow knew how to orient themselves toward the magnetic north pole, which is hundreds of kilometers south of the geographic pole, in northern Canada.  2007年の6カ月間,それは動物学者ビーガル(Sabine Begall)にとってうんざりするほど長く感じられたことだろう。ドイツのデュースブルク・エッセン大学に所属する彼女は,毎晩コンピューターの画面に向かい,放牧された牛の群れの写真に目を凝らして過ごした。放牧地を撮影した衛星写真をグーグルアースからダウンロードしては,1頭ずつ目印を付ける作業をひたすら繰り返したのだった。
A follow-up study found more evidence that animals as large as cows can react to the earth’s magnetic field: the aligning behavior vanished in the vicinity of high-voltage power lines that drowned out the relatively subtle signals coming from the planet.  その後の調査で,牛のような大型動物が地磁気に反応することを示すさらなる証拠が得られた。比較的弱い地磁気の信号をかき消してしまうような高圧送電線の近くでは,こうした整列行動が見られないのだ。
Until a few decades ago studies such as Begall’s would have been met with derision. Everyone knew that organic matter does not respond to weak magnetic fields such as the earth’s and that animals do not come equipped with bar magnets to use as compasses. Franz Anton Mesmer’s 18th-century belief in “animal magnetism”—the notion that breathing creatures harbor magnetic fluids in their bodies—had long been relegated to the annals of charlatanism.  数十年前なら,ビーガルの研究は物笑いの種となっただろう。有機物が地磁気のような弱い磁場には反応しないこと,コンパスの役割を果たす棒磁石を動物が“内蔵”していないことは常識だった。18世紀にメスメル(Franz Anton Mesmer)が唱えた動物磁気(生物の体内に磁気を帯びた液体が存在しているという考えで,「メスメリズム」とも呼ばれる)は,長く偽科学の好例と見なされてきた。
Today the scientific community accepts that certain animals do read and respond to magnetic fields and that, for many of them, being able to do so should be helpful to survival—although why cattle would want to align magnetically is still mysterious. A magnetic sense has, in fact, been well documented in dozens of species—from seasonal migrants such as robins and monarch butterflies to expert navigators such as homing pigeons and sea turtles; from invertebrates such as lobsters, honeybees and ants to mammals such as mole rats and elephant seals; and from tiny bacteria to humongous whales.  しかし今日の科学界は,ある種の動物は磁場を感知して反応するという考え方を受け入れており,またそうした動物の多くにとって磁気感知能力が生存競争を生き抜く一助になったはずだと考えている。ただし,牛がなぜ磁場の向きに沿って並びたがるかは依然として謎のままだ。
What no one knows for sure yet is exactly how creatures other than bacteria do it. Magnetism is “the one sense that we know the least about,” notes Steven M. Reppert, a neurobiologist at the University of Massachusetts Medical School in Worcester.  しかし,これらの動物がどのように磁場を感知しているかについては,細菌を除くと不明だ。「磁気感覚は最もわかっていない感覚だ」と,マサチューセッツ大学医学部の神経生物学者レパート(Steven M. Reppert)は言う。
In the past decade or so, however, collaborations of biologists, earth scientists and physicists have begun to propose plausible mechanisms and to pinpoint candidate anatomical structures where those mechanisms may be at play. None of these ideas has yet gained the acceptance of the full scientific community, but the experimental evidence found so far is truly mesmerizing. Some animals may even harbor more than one type of magnetic organ. And whereas certain biological magnetic field sensors seem to behave much like ordinary bar magnet compasses, others may well be rooted in subtle quantum effects.  だが過去10数年で,生物学者と地球科学者,物理学者の共同研究によって,妥当と考えられるメカニズムが提案され,それらが機能していそうな解剖学的構造が特定されるようになってきた。そうした仮説のうち科学界全体に受け入れられるに至ったものはまだないが,これまでに見つかった実験的証拠は実に驚くべきものだ。一部の動物に至っては,複数タイプの磁気感覚器を備えている可能性がある。また,方位磁石とほぼ同様に機能する生物学的な磁場センサーもあれば,微妙な量子効果に基づくセンサーもあるようだ。
The subject continues to have its share of controversy. But increased interest in magnetic reception and rapid improvements in experimental techniques could lead investigators to solve the mystery of this unusual sense in the next few years.  この研究テーマは依然として議論を呼び続けている。しかし,磁気感受に対する関心の高まりと,実験技術の急速な進歩を受け,この不思議な磁気感覚の謎は今後数年のうちに解明されるかもしれない。