斑马为什么会有条纹

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。“有关苍蝇的这种假设业已被证明是目前为止最合理的一种说法，”普林斯顿大学的生物学家、斑马专家丹尼尔·鲁本斯坦说（他本人没有参与这项研究），“而这篇新发表的论文为这种说法提供了新的证据。”让飞蝇不想叮咬这一功能显然非常实用，因为这些昆虫一般都携带着致命疾病。此外，尽管吸血蝇类咬上一口只会吸走一小滴血，可要是每天被叮咬成千上万次，加起来可能就会导致明显的失血。endprint

在这篇新论文问世之前，已有实验结果表明叮咬类飞蝇降落时会避开有条纹的表面。“我们知道它们不喜欢条纹表面，尽管还不知道其原因。”加利福尼亚大学戴维斯分校的生物学家、这项新研究的第一作者蒂姆·卡罗这样告诉我。不过，此前的那些实验室实验中所使用的是类似于捕蝇纸那样的人造表面，而不是活生生的斑马——这也不难理解，因为让一只斑马在实验室里干站着会很难。如果科学家能让一只斑马乖乖配合，他们也许能够证明斑马身上的条纹事实上的确可以令飞蝇避而远之，可即便这样也无法排除与之相竞争的其他假说。（比如，没准避免让狮子发现才是条纹存在的更重要的原因，而要在实验室验证这一点可就更难了。）

因此，卡罗和同事们采取了另外一种办法。他们找来包括斑马、马和野驴在内的已知野生马科动物的所有种及其亚种，共计20种，观察每个种类身上条纹的多少和出现的位置。然后将这些动物的活动范围与已有的、可从进化学角度解释条纹出现可能性的各种不同因素（例如大型天敌的出现、气候条件或斑马生活地区的主要植被种类）一一对应。

这些因素中，除了叮咬类飞蝇的大量出现，几乎没有一个因素与一个种或亚种是否具有显著条纹、少量条纹抑或完全没有条纹关联那么密切。（事实上，并没有可用的世界多个地区苍蝇密集度分布图，所以科学家就选用了一个代替方案，即苍蝇所喜爱的炎热潮湿的环境。这并不是个完美的解决方案，因为从理论上来说，热度和湿度本身就可能会对条纹的出现有所影响，而与飞蝇完全无关，不过科学家们普遍认为这种可能性几乎不存在。）尽管此前的研究已经暗示了这一结论，可卡罗和他的同事们还是对实验结果所展现出的显著相关性印象深刻。“我感到十分惊讶，”他说，“我们一次次地发现，在那些一年之中往往会有大量叮咬类飞蝇出现的地区，这些动物身上就长着大量条纹或十分密集的条纹。”

这个令人抓狂的简单问题似乎终于得到了解答，卡罗感到如释重负。“我们终于进入了新的阶段，可以不必再问‘为什么会有条纹？，而开始问‘是什么让飞蝇不想落到条纹上？”他说。飞蝇叮咬为什么能成为一个如此严重的问题，究竟主要是因为会感染疾病呢，还是因为会导致失血？卡罗对这个问题也很感兴趣。“这就是做科学研究常出现的情况，”他说，“你解决了一个问题，就会引出六个更多的问题。”

1. Rudyard Kipling： 拉迪亚德·吉卜林（1865～1936），英国小说家、诗人，出生于印度孟买，出版了大量的诗集、长篇小说、短篇小说集和历史故事集，以及散文、随笔、游记等。

2. get around to： 抽出时间做

3. prolifically [pr??l?f?kli] adv. （人、动物）多育地

4. brethren [?bre?r?n] n. [旧] （某组织或社团的）成员，会员

5. outlast [?a?t?lɑ?st] vt. 比……存在得更长久

6. blend in： 融入；与……融为一体；与……协调

7. vegetation [?ved???te??n] n. 植被；植物；草木

8. predator [?pred?t?（r）] n. 天敌

9. savannah [s??v?n?] n. （开阔、平坦的）草原

10. camouflage [?k?m?flɑ??] n. （军事上的）伪装，隐蔽

11. plumage [?plu?m?d?] n. （鸟的）全身羽毛

12. plausible [?pl??z?bl] adj. （解释或说法）似乎真实的，貌似合理的

13. to date： 到目前为止，迄今为止

14. hypothesis [ha??p?θ?s?s] n. 假说；假设。其复数为hypotheses。

15. stand around： 干等着；（什么都不做）干站着

16. deter [d??t??（r）] vt. 阻止；制止

17. competing [k?m?pi?t??] adj. 相互竞争的

18. equid [?ekw?d] n. 马科动物

19. prevalent [?prev?l?nt] adj. 流行的；盛行的；普遍的

20. proxy [?pr?ksi] n. 代替者；替代物

21. maddeningly [?m?dn??li] adv. 使人恼怒地

22. straightforward [?stre?t?f??w?d] adj. 简单明了的；易懂易做的endprint