斑馬條紋：防蚊蟲的絕妙法寶

Horseflies， the females of which feed on blood， are attracted to polarized light—light waves that are oriented in a particular direction and that we experience as glare. This glare lures the bugs most likely because it resembles light reflected off water， where they lay their eggs.

On horses， black fur reflects polarized light better than brown or white， as evolutionary ecologist and colleagues Susanne Akesson found in a previous study.

After experiments in which her team measured the number of horseflies that became trapped on gluey， striped boards or models of horses， the team found that the flies’ favorite is the black coat and zebra stripes are the best fly repellent—the narrower the stripes the better.

The result may help explain why zebras’ skinniest stripes are on their faces and legs. “That’s also the place where you have the thinnest skin，” said Akesson. But why would striped skin be more effective than white， which has the lowest reflectivity of polarized light？ The black-and-white pattern， Akesson said， turns out to be“ideal in its function of disrupting this signal of reflected polarized light.”

Because the coat reflects light in alternately polarized and nonpolarized patterns， the zebra “is more difficult to single out relative to the surroundings.” It is， in effect， camouflaged to flies as well as to big cats.

Then a question naturally comes to us. If stripes truly are Kryptonite to horseflies， why don’t horses—close evolutionary relatives to zebras—sport the pattern too？

The answer may be in the fact that there are more horseflies， and more horsefly species， in Africa compared to more temperate regions. Zebras would have been under more pressure to evolve a deterrent.

Another obvious question， though， is why other species have not evolved this elegant form of fly repellent， and what the consequences would have been if they had. If humans， for example， were black-and-white striped then the history of intercommunal violence the species has suffered might not have been quite as bad. One for all of us to ponder， perhaps？

斑馬是馬科動物中比較招人喜愛的一個成員，事實上它也是最古老的成員之一。盡管斑馬看起來比普通的馬和驢要更具異域風情，但其實它的外形更接近早期的馬科動物祖先。

但是，斑馬為什么會進化出其標志性的黑白相間條紋，這是科學家們近幾十年一直在爭論的話題。傳統觀點認為斑馬的黑白條紋可以使他們隱藏在高的草叢中不被發現——這也是躲避獅子好方法，因為獅子是色盲。但是有最新研究表明，這種黑白相間的條紋也擾亂了另外一種體型更微小的叮咬者——吸血馬蠅——的視線。

雌性馬蠅以吸食血液為生，容易被偏振光所吸引——這是一種沿著某一特定方向傳播的光波，我們看上去會覺得是一道刺眼的光線。這種光線最吸引蚊蟲是因為其很像是從水面折射出的光線，而蚊蟲通常會在水面上產卵。

進化生物學家蘇珊娜·阿肯斯在先前的研究中發現：黑色的馬比棕色或白色馬的毛發更能折射出類似偏振光的光。

阿肯斯和她的團隊對此進行了實驗，測量了被黏在涂上了膠水的黑白條紋板子或馬模型上的馬蠅數量，他們發現馬蠅最喜歡的是黑馬，而斑馬條紋則是很好的驅蟲劑，而且條紋越窄越好。

這一結果或許有助于解釋為什么斑馬身上最細的條紋都長在臉上和腿上。“因為這些地方的皮膚最薄，”阿肯斯說道。

但是為什么黑白相間的條紋比白色更能有效地驅趕馬蠅呢？白色是偏振光反射率最低的顏色。……