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？

斑马是马科动物中比较招人喜爱的一个成员，事实上它也是最古老的成员之一。尽管斑马看起来比普通的马和驴要更具异域风情，但其实它的外形更接近早期的马科动物祖先。

但是，斑马为什么会进化出其标志性的黑白相间条纹，这是科学家们近几十年一直在争论的话题。传统观点认为斑马的黑白条纹可以使他们隐藏在高的草丛中不被发现——这也是躲避狮子好方法，因为狮子是色盲。但是有最新研究表明，这种黑白相间的条纹也扰乱了另外一种体型更微小的叮咬者——吸血马蝇——的视线。

雌性马蝇以吸食血液为生，容易被偏振光所吸引——这是一种沿着某一特定方向传播的光波，我们看上去会觉得是一道刺眼的光线。这种光线最吸引蚊虫是因为其很像是从水面折射出的光线，而蚊虫通常会在水面上产卵。

进化生物学家苏珊娜·阿肯斯在先前的研究中发现：黑色的马比棕色或白色马的毛发更能折射出类似偏振光的光。

阿肯斯和她的团队对此进行了实验，测量了被黏在涂上了胶水的黑白条纹板子或马模型上的马蝇数量，他们发现马蝇最喜欢的是黑马，而斑马条纹则是很好的驱虫剂，而且条纹越窄越好。

这一结果或许有助于解释为什么斑马身上最细的条纹都长在脸上和腿上。“因为这些地方的皮肤最薄，”阿肯斯说道。

但是为什么黑白相间的条纹比白色更能有效地驱赶马蝇呢？白色是偏振光反射率最低的颜色。阿肯斯解释道，黑白相间的条纹最后被证实是具有“最理想的干扰偏振光反射信号的功能”。

黑白相间的条纹能够不规则地反射出偏振光和非偏振光，这使得别的动物“很难从周围的环境中将斑马识别出来”。因此，黑白相间的条纹起到了保护色的作用，帮助斑马躲避蚊虫和大型猫科动物。

那么我们很自然地联想到一个问题：如果条纹真是马蝇的克星的话，那么马——作为斑马的近亲——为什么不也进化成条纹状的皮肤呢？

答案或许是：非洲的马蝇数量以及种类比温带的更多，所以斑马需要进化出抗敌能力的压力更大。

另外一个很明显的问题是：为什么其他物种没有进化出这种驱虫功能，而且如果他们进化出了这种功能，又会有什么结果？例如，如果人类的皮肤都是黑白相间的条纹图案，那么人类曾经历的不同社区之间的暴力行为也许就不会那么恶劣了。或许，这才是我们所有人都应该思考的问题。