斑馬為什么會有條紋

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."

斑馬為什么會有條紋？這聽起來就像是一個“本該如此”的故事的主題，大作家拉迪亞德·吉卜林永遠不會抽時間去寫。你可能以為目前為止有人已經找到了確切答案，可事實卻是，斑馬為何會有條紋仍舊是生物學上的一個未解之謎。現代斑馬的祖先身上隨機出現了條紋，按照進化法則，這必定擁有某種優勢，使有條紋的斑馬生育的后代比那些沒有條紋的同類生育的更多。隨著一代代的繁衍，有條紋的斑馬會變得越來越常見，最終超過沒有條紋的斑馬。

但是對于那種優勢可能是什么，科學家們多年來一直困惑不已。問題不在于他們對此沒有合理的解釋，而在于合理的解釋太多了。也許條紋使斑馬與植被融為一體，這樣天敵就無法看到它們了。也許當斑馬在開闊的大草原上奔跑時，條紋的存在使天敵難以判斷它們奔跑的速度和距離遠近。（受這種觀點的啟發，一戰期間人們發明了一種用于戰船上的偽裝手段。）也許是因為條紋能讓斑馬免于被攜帶疾病的飛蠅叮咬。也許斑馬用身上的條紋吸引配偶，就像鳥類用色彩艷麗的羽毛吸引異性一樣。也許每只斑馬身上的條紋圖案具備一定的社交功能，讓同一族群的成員之間更容易認出彼此。也許條紋能用某種方法保護斑馬免受酷熱天氣的侵害。

以上這些都是對斑馬條紋的所有看似合理的解釋，科學家們直到最近都難以從中做出抉擇。不過，2014年4月1日出版的《自然通訊》雜志上刊登了一篇論文，該論文對其中一種可能——條紋逼退昆蟲說——提供了迄今為止最有力的證據。“有關蒼蠅的這種假設業已被證明是目前為止最合理的一種說法，”普林斯頓大學的生物學家、斑馬專家丹尼爾·魯本斯坦說（他本人沒有參與這項研究），“而這篇新發表的論文為這種說法提供了新的證據。”讓飛蠅不想叮咬這一功能顯然非常實用，因為這些昆蟲一般都攜帶著致命疾病。此外，盡管吸血蠅類咬上一口只會吸走一小滴血，可要是每天被叮咬成千上萬次，加起來可能就會導致明顯的失血。endprint

在這篇新論文問世之前，已有實驗結果表明叮咬類飛蠅降落時會避開有條紋的表面。“我們知道它們不喜歡條紋表面，盡管還不知道其原因。”加利福尼亞大學戴維斯分校的生物學家、這項新研究的第一作者蒂姆·卡羅這樣告訴我。不過，此前的那些實驗室實驗中所使用的是類似于捕蠅紙那樣的人造表面，而不是活生生的斑馬——這也不難理解，因為讓一只斑馬在實驗室里干站著會很難。如果科學家能讓一只斑馬乖乖配合，他們也許能夠證明斑馬身上的條紋事實上的確可以令飛蠅避而遠之，可即便這樣也無法排除與之相競爭的其他假說。（比如，沒準避免讓獅子發現才是條紋存在的更重要的原因，而要在實驗室驗證這一點可就更難了。）

因此，卡羅和同事們采取了另外一種辦法。他們找來包括斑馬、馬和野驢在內的已知野生馬科動物的所有種及其亞種，共計20種，觀察每個種類身上條紋的多少和出現的位置。然后將這些動物的活動范圍與已有的、可從進化學角度解釋條紋出現可能性的各種不同因素（例如大型天敵的出現、氣候條件或斑馬生活地區的主要植被種類）一一對應。

這些因素中，除了叮咬類飛蠅的大量出現，幾乎沒有一個因素與一個種或亞種是否具有顯著條紋、少量條紋抑或完全沒有條紋關聯那么密切。（事實上，并沒有可用的世界多個地區蒼蠅密集度分布圖，所以科學家就選用了一個代替方案，即蒼蠅所喜愛的炎熱潮濕的環境。這并不是個完美的解決方案，因為從理論上來說，熱度和濕度本身就可能會對條紋的出現有所影響，而與飛蠅完全無關，不過科學家們普遍認為這種可能性幾乎不存在。）盡管此前的研究已經暗示了這一結論，可卡羅和他的同事們還是對實驗結果所展現出的顯著相關性印象深刻。“我感到十分驚訝，”他說，“我們一次次地發現，在那些一年之中往往會有大量叮咬類飛蠅出現的地區，這些動物身上就長著大量條紋或十分密集的條紋。”

這個令人抓狂的簡單問題似乎終于得到了解答，卡羅感到如釋重負。“我們終于進入了新的階段，可以不必再問‘為什么會有條紋？，而開始問‘是什么讓飛蠅不想落到條紋上？”他說。飛蠅叮咬為什么能成為一個如此嚴重的問題，究竟主要是因為會感染疾病呢，還是因為會導致失血？卡羅對這個問題也很感興趣。“這就是做科學研究常出現的情況，”他說，“你解決了一個問題，就會引出六個更多的問題。”

1. Rudyard Kipling： 拉迪亞德·吉卜林（1865～1936），英國小說家、詩人，出生于印度孟買，出版了大量的詩集、長篇小說、短篇小說集和歷史故事集，以及散文、隨筆、游記等。

2. get around to： 抽出時間做

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

4. brethren [?bre?r?n] n. [舊] （某組織或社團的）成員，會員

5. outlast [?a?t?lɑ?st] vt. 比……存在得更長久

6. blend in： 融入；與……融為一體；與……協調

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

8. predator [?pred?t?（r）] n. 天敵

9. savannah [s??v?n?] n. （開闊、平坦的）草原

10. camouflage [?k?m?flɑ??] n. （軍事上的）偽裝，隱蔽

11. plumage [?plu?m?d?] n. （鳥的）全身羽毛

12. plausible [?pl??z?bl] adj. （解釋或說法）似乎真實的，貌似合理的

13. to date： 到目前為止，迄今為止

14. hypothesis [ha??p?θ?s?s] n. 假說；假設。其復數為hypotheses。

15. stand around： 干等著；（什么都不做）干站著

16. deter [d??t??（r）] vt. 阻止；制止

17. competing [k?m?pi?t??] adj. 相互競爭的

18. equid [?ekw?d] n. 馬科動物

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. 使人惱怒地

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