Michael+Lemonick

How did the zebra get its stripes？ It sounds like the theme for a "Just So" story that Rudyard Kipling1） never got around to2） writing. You would think that someone would have come up with the definitive answer by now， but， in fact， the reason zebras have stripes remains a biological mystery. The laws of evolution suggest that the random emergence of stripes on the ancestors of modern zebras must have had some sort of advantage that allowed them to reproduce more prolifically3） than their unstriped brethren4）. The striped animals would have become more common with each generation， ultimately outlasting5） the ones without stripes.

But scientists have puzzled for years over what that advantage might have been. The problem isn't that they have no good ideas—it's that they have too many good ideas. Maybe stripes let zebras blend in6） with vegetation7）， so that predators8） can't see them. Maybe they make it hard for predators to judge a zebra's speed and distance when it's running on the open savannah9）. （This idea inspired a form of camouflage10） used by ships during the First World War.） Maybe stripes discourage disease-carrying flies from biting. Maybe they attract mates， much as elaborate plumage11） does for birds. Maybe individual stripe patterns serve a social function， allowing herd members to recognize each other more easily. Maybe stripes somehow protect against excessive heat.

These are all plausible12） explanations for a zebra's stripes， and until recently it's been hard to choose between them. But now a paper， published in the April 1st issue of Nature Communications， offers the best evidence to date13） in favor of one thesis： that the stripes repel insects. "The fly hypothesis14） has proven to be the best one so far，" Daniel Rubenstein， a Princeton biologist and zebra expert， who wasn't involved in the research， said. "And this new paper adds new support to the idea." Discouraging bites from flies is obviously useful， since the insects often carry fatal diseases. Also， while a single bite from blood-eating flies extracts just a tiny droplet of blood， thousands of bites per day can add up to significant blood loss.

Before the recent paper， there was already experimental evidence that biting flies avoid landing on striped surfaces. "We know they don't like it， although we don't know why，" Tim Caro， a biologist at the University of California Davis， and the lead author of the new study， told me. But previous laboratory experiments used artificial surfaces like flypaper， not living zebras—understandable， since getting a zebra to stand around15） in a lab would be tough. If scientists could get a zebra to cooperate， they might show that their stripes do， in fact， deter16） flies， but this would do nothing to eliminate the competing17） hypotheses. （Maybe avoiding lions is an even more important reason to have stripes， for example， and testing that in a lab would be even tougher.）endprint

So Caro and his colleagues tried a different approach. They took all twenty known species and subspecies of wild equids18）， including zebras， horses， and wild asses， and looked at how much striping each group has and where on the body it appears. Then they matched the range of the animals to the various factors that have been suggested as evolutionary reasons stripes might have appeared—the presence of large predators， for example， climate， or the kind of vegetation that is prevalent19） where zebras live.

Almost none of these factors correlated strongly with whether a species or subspecies was boldly striped， subtly striped， or stripeless—except for the prevalence of biting flies. （Actually， there aren't good maps of fly concentrations in many parts of the world， so the scientists used a proxy20）： hot and humid conditions， which flies love. It's not a perfect solution， since heat and humidity could， in theory， have some effect on striping that has nothing to do with flies， but that's widely considered to be unlikely.） Despite hints from the earlier research， Caro and his colleagues were struck by how clear the correlation turned out to be. "I was rather surprised，" he said. "We found again and again that many stripes or intense striping is associated with areas that tend to have many biting flies over the course of the year."

Caro is relieved that this maddeningly21） straightforward22） question appears to have been answered at last. "We've finally gotten to the stage where we can stop asking the question 'Why stripes？' and start asking 'What prevents flies from landing on stripes？' " he said. Caro is also interested in the question of whether it's primarily disease or blood loss that makes fly bites such a problem. "That's what happens in science，" he said. "You answer one question and it leads to six more."

斑马为什么会有条纹？这听起来就像是一个“本该如此”的故事的主题，大作家拉迪亚德·吉卜林永远不会抽时间去写。你可能以为目前为止有人已经找到了确切答案，可事实却是，斑马为何会有条纹仍旧是生物学上的一个未解之谜。现代斑马的祖先身上随机出现了条纹，按照进化法则，这必定拥有某种优势，使有条纹的斑马生育的后代比那些没有条纹的同类生育的更多。随着一代代的繁衍，有条纹的斑马会变得越来越常见，最终超过没有条纹的斑马。

但是对于那种优势可能是什么，科学家们多年来一直困惑不已。问题不在于他们对此没有合理的解释，而在于合理的解释太多了。也许条纹使斑马与植被融为一体，这样天敌就无法看到它们了。也许当斑马在开阔的大草原上奔跑时，条纹的存在使天敌难以判断它们奔跑的速度和距离远近。（受这种观点的启发，一战期间人们发明了一种用于战船上的伪装手段。）也许是因为条纹能让斑马免于被携带疾病的飞蝇叮咬。也许斑马用身上的条纹吸引配偶，就像鸟类用色彩艳丽的羽毛吸引异性一样。也许每只斑马身上的条纹图案具备一定的社交功能，让同一族群的成员之间更容易认出彼此。也许条纹能用某种方法保护斑马免受酷热天气的侵害。

以上这些都是对斑马条纹的所有看似合理的解释，科学家们直到最近都难以从中做出抉择。不过，2014年4月1日出版的《自然通讯》杂志上刊登了一篇论文，该论文对其中一种可能——条纹逼退昆虫说——提供了迄今为止最有力的证据。……