Scientists are providing new evidence to answer the longstanding question about why zebras have stripes. It appears stripes make terrible landing strips, bamboozling the fierce blood-sucking flies that try to feast on zebras and carry deadly diseases.
Researchers on Feb. 20 described experiments demonstrating that horse flies have a difficult time landing on zebras while easily landing on uniformly colored horses. In one experiment, the researchers put cloth coats bearing striped patterns on horses and observed that fewer flies landed on them than when the same horses wore single-color coats.
“We showed that horse flies approach zebras and uniformly colored horses at similar rates but that they fail to land on zebras, or striped horse coats, because they fail to decelerate properly, and so fly past them or literally bump into them and bounce off,” said behavioral ecologist Tim Caro of the University of California-Davis, lead author of the research published in the journal PLOS ONE.
Photo: REUTERS
照片：路透
Close cousins to horses and donkeys, the world’s three zebra species, known for their black-and-white striped bodies, roam Africa’s savannas eating a variety of grasses. Their stripe patterns vary among individuals, with no two alike.
There had been four main hypotheses about the advantages zebras accrued by evolving stripes: camouflage to avoid large predators; a social function like individual recognition; thermoregulation, with stripes setting up convection currents along the animal’s back; and thwarting biting fly attacks. “Only the last stands up to scrutiny,” Caro said. “Most biologists involved with research on mammal coloration accept that this is the reason that zebras have stripes.” African horse flies carry diseases such as trypanosomiasis and African horse sickness that cause wasting and can be fatal.
The researchers videoed horse flies as they tried to prey on captive zebras and domestic horses at a livery in North Somerset, England. Stripes did not deter flies from a distance, as they circled horses and zebras at similar rates. But the flies managed to land on zebras less than a quarter as often. University of Bristol biologist and study co-author Martin How said stripes may dazzle flies somehow once the insects venture close enough to see them with their low-resolution eyes.
Photo: AP
照片：美聯社
“In addition to stripes that prevent controlled landings by horse flies, zebras are constantly swishing their tail and may run off if horse flies do land successfully, so they are also using behavioral means to prevent flies probing for blood,” Caro said.
(Reuters)
科學家近日提出新證據，為存在已久的問題──「為何斑馬身上有條紋」──提出解答。原因可能在於：條紋會形成糟糕透頂的降落跑道，以迷惑那些想要在斑馬身上飽餐一頓，身上又帶有多種致命疾病的兇猛吸血蒼蠅。
研究人員於二月二十日描述他們進行的實驗，顯示馬蠅降落在斑馬身上時困難重重，卻能輕易降落在毛色均勻的馬身上。在其中一項實驗中，研究人員為馬披上條紋圖案的外套，結果觀察到降落在牠們身上的蒼蠅數量，比同一批馬穿上單色外套時還少。
這份研究發表於期刊《公共科學圖書館—綜合》，主要作者為加州大學戴維斯分校的行為生態學家提姆‧卡洛。他指出：「我們證明馬蠅飛近斑馬和單色馬的比例相近，但卻無法降落在斑馬，或是帶條紋的馬外套上，因為馬蠅未能適當減速，以致於它們要不是飛過馬，就是直接撞上馬身，然後彈飛出去。」
斑馬是馬和驢的近親，世界上共有三個種類，牠們以身上黑白相間的條紋著稱，漫步於非洲的大草原上，以各式草類植物為食。身上的條紋圖案依不同個體而有差異，每隻都不一樣。
關於斑馬藉由演化出條紋而獲得的優勢，歷來有四種主要假設，分別是形成保護色以躲避大型掠食動物、類似個體識別的社交功能、藉由條紋沿著背產生對流氣流調節體溫、以及阻撓蒼蠅的叮咬攻擊。卡洛指出：「只有最後一項假設經得起檢驗。」他表示：「大多數研究哺乳類身上色彩的生物學家都接受這個推論，認為這是斑馬身上有條紋的原因。」非洲馬蠅帶原的疾病包括錐蟲病以及非洲馬疫，會引起消耗性症侯群，嚴重時可能致命。
研究人員在英國北索美塞特的一間代養馬場進行攝影，記錄馬蠅試圖襲擊被圈養的斑馬和家馬。這類昆蟲繞著家馬和斑馬飛行的比例相近，顯示在一段距離以外，條紋並未對馬蠅構成威嚇。不過，馬蠅成功降落在斑馬身上的次數竟少於停在家馬身上的四分之一。英國布里斯托大學的生物學家馬汀‧郝伊是這份研究的共同作者，他表示，一旦蒼蠅探索到夠近的距離，足以用低解析度的眼睛看到斑馬時，條紋某種程度上會讓它們感到目眩眼花。
卡洛進一步指出：「除了利用條紋妨礙馬蠅進行控制型著陸，斑馬也會不斷揮動尾巴，如果馬蠅成功著陸，斑馬也可能會逃走，所以牠們也會利用行為方式來阻止這些求血若渴的蒼蠅。」
（台北時報章厚明譯）
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