斑馬條紋：防蚊蟲(chóng)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. “Thats 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 dont 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(dòng)物中比較招人喜愛(ài)的一個(gè)成員，事實(shí)上它也是最古老的成員之一。盡管斑馬看起來(lái)比普通的馬和驢要更具異域風(fēng)情，但其實(shí)它的外形更接近早期的馬科動(dòng)物祖先。

但是，斑馬為什么會(huì)進(jìn)化出其標(biāo)志性的黑白相間條紋，這是科學(xué)家們近幾十年一直在爭(zhēng)論的話(huà)題。傳統(tǒng)觀點(diǎn)認(rèn)為斑馬的黑白條紋可以使他們隱藏在高的草叢中不被發(fā)現(xiàn)——這也是躲避獅子好方法，因?yàn)楠{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(huì)覺(jué)得是一道刺眼的光線。這種光線最吸引蚊蟲(chóng)是因?yàn)槠浜芟袷菑乃嬲凵涑龅墓饩€，而蚊蟲(chóng)通常會(huì)在水面上產(chǎn)卵。

進(jìn)化生物學(xué)家蘇珊娜·阿肯斯在先前的研究中發(fā)現(xiàn)：黑色的馬比棕色或白色馬的毛發(fā)更能折射出類(lèi)似偏振光的光。

阿肯斯和她的團(tuán)隊(duì)對(duì)此進(jìn)行了實(shí)驗(yàn)，測(cè)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(shù)量，他們發(fā)現(xiàn)馬蠅最喜歡的是黑馬，而斑馬條紋則是很好的驅(qū)蟲(chóng)劑，而且條紋越窄越好。

這一結(jié)果或許有助于解釋為什么斑馬身上最細(xì)的條紋都長(zhǎng)在臉上和腿上?！耙?yàn)檫@些地方的皮膚最薄，”阿肯斯說(shuō)道。

但是為什么黑白相間的條紋比白色更能有效地驅(qū)趕馬蠅呢？白色是偏振光反射率最低的顏色。阿肯斯解釋道，黑白相間的條紋最后被證實(shí)是具有“最理想的干擾偏振光反射信號(hào)的功能”。

黑白相間的條紋能夠不規(guī)則地反射出偏振光和非偏振光，這使得別的動(dòng)物“很難從周?chē)沫h(huán)境中將斑馬識(shí)別出來(lái)”。因此，黑白相間的條紋起到了保護(hù)色的作用，幫助斑馬躲避蚊蟲(chóng)和大型貓科動(dòng)物。

那么我們很自然地聯(lián)想到一個(gè)問(wèn)題：如果條紋真是馬蠅的克星的話(huà)，那么馬——作為斑馬的近親——為什么不也進(jìn)化成條紋狀的皮膚呢？

答案或許是：非洲的馬蠅數(shù)量以及種類(lèi)比溫帶的更多，所以斑馬需要進(jìn)化出抗敵能力的壓力更大。

另外一個(gè)很明顯的問(wèn)題是：為什么其他物種沒(méi)有進(jìn)化出這種驅(qū)蟲(chóng)功能，而且如果他們進(jìn)化出了這種功能，又會(huì)有什么結(jié)果？例如，如果人類(lèi)的皮膚都是黑白相間的條紋圖案，那么人類(lèi)曾經(jīng)歷的不同社區(qū)之間的暴力行為也許就不會(huì)那么惡劣了?；蛟S，這才是我們所有人都應(yīng)該思考的問(wèn)題。