【高考风向预测 · 外刊精读 33】Not for Camouflage?|斑马为什么非要穿一身黑白条纹?

Zebra stripes have inspired explanations ranging from camouflage to cooling, but comparative and experimental evidence increasingly points toward protection from biting flies. 

For more than a century, scientists have debated why zebras evolved such dramatic black-and-white coats. One obvious explanation is camouflage. Perhaps the stripes help zebras blend into grass or confuse predators such as lions when a herd begins to run. Other ideas have suggested that stripes help zebras recognize one another, regulate body temperature or avoid biting insects.

The difficulty is that all of these explanations sound reasonable. Science therefore needs more than a good story.

In 2014, researchers compared the major hypotheses systematically. They examined how strongly different zebra and equid populations were striped and compared those patterns with environmental factors. If predators were the main evolutionary pressure, stronger striping might be expected where predator pressure was greater. If cooling were the key, striping should correspond closely with temperature. Instead, one of the strongest and most consistent relationships involved biting flies. Regions associated with greater activity of tabanid horseflies and tsetse flies tended to be associated with stronger striping.

This was useful evidence, but it still did not explain how stripes protected an animal. Researchers therefore moved from geographic comparison to direct experiments.

In a 2019 study, scientists observed horseflies around captive zebras and domestic horses living in similar conditions. The flies approached and circled both types of animals at similar rates. The difference appeared at the final stage. Horseflies landed much less successfully on zebras. High-speed video showed that flies approaching ordinary horses usually slowed down before landing. Near zebras, however, many continued flying too fast, passed over the animal or collided briefly with the coat instead of making a controlled landing.

The researchers then performed a clever control experiment. Domestic horses wore black, white and zebra-striped cloth coats on different occasions. Because the same horse could wear all three patterns, smell, body shape and much of the animal's behavior remained constant. Again, far fewer flies successfully landed on the striped coat than on the plain black or white coats. Interestingly, the horse's uncovered head did not gain this protection. That result helped show that the pattern itself — rather than simply some difference between horses and zebras — influenced fly behavior.

But one puzzle remained. Why exactly do stripes interfere with landing? Researchers have proposed several mechanisms. Stripes might create misleading motion signals, disrupt the visual outline of a large dark object, or interact with the limited resolution of a fly's visual system.

A 2023 experiment tested horses wearing many different patterns. Large dark regions attracted more landings, while sharply contrasting patterns broken into smaller dark areas attracted fewer. This supported the idea that stripes interfere with the visual information flies use during close approach.

Even so, the exact mechanism continues to be refined. That is an important part of the story. Scientists can have strong evidence that a trait provides a benefit before they completely understand the sensory mechanism producing that benefit.

The zebra's stripes may no longer be quite the mystery they once were. But "we know what they help with" is not always the same as "we know exactly how they work."

Camouflage 为什么流传这么久？因为它 sounds plausible——黑白条纹、一群斑马、高速奔跑、狮子眼花，故事非常完整。但科学需要问：What prediction does this explanation make?然后拿真实数据去比。2014 年的跨物种、跨地区比较没有发现 predator avoidance 和 striping 之间同样一致的关系，却发现了 biting-fly 变量。

它主要做的是 comparative analysis：比较不同物种、亚种、地理范围、环境变量。它发现 striping 和 biting fly exposure 关系最稳定，但这首先属于 association evidence——不是把一群斑马随机分成"有条纹"和"没条纹"。

为什么这个实验强很多？假如只比较 zebra 和 horse，你会遇到大量 confounding variables：它们可能气味不同、行为不同、毛发不同、体型也有差异。所以研究人员干脆用 the same horse 换不同外套。

如果斑马条纹的功能是"让苍蝇远远看见就不来"，那么研究应该发现 zebra 周围 circling flies 明显少。但是 2019 实验发现：苍蝇围绕 zebra 和 horse 接近、盘旋的频率没有显著差异。真正差别是 landing。所以条纹更可能影响 final approach，而不是 long-distance attraction。

2019 年的 video analysis 发现：horseflies 接近普通马时会 decelerate（减速），像飞机准备 landing；但接近 zebra 时，它们更容易继续高速靠近，结果 overshoot 或者 briefly touch 然后离开。所以研究人员推测：条纹影响了苍蝇在最后几厘米或较近距离里的 visual control of landing。

因为知道 stripes work 还不等于知道 what property of stripes works——可能是条纹很细？黑白反差？polarized light？视觉 motion illusion？所以研究人员让马穿上灰色、大黑色三角、checkerboard、不同 contrast 的 stripe 等各种外套。结果：large dark patches 更容易吸引 landing，而被高对比、小块图案打碎的大面积轮廓更少被落上。

现在证据可以比较有力地支持 function：条纹有助于减少 biting-fly landings。但是 mechanism——苍蝇的视觉系统究竟在哪一步、为什么、如何被干扰——还在继续研究。2023 实验更支持大面积 dark object 被高对比图案打散的解释，并没有支持当时测试的 polarization 以及若干经典视觉错觉机制；2025 年的建模研究又提出，条纹可能通过视觉采样中的 aliasing 干扰苍蝇对运动的分析，说明机制层面的解释仍在继续发展。

也不要说太过。2014 研究的准确结论是：在他们的系统比较中，没有发现 consistent support for camouflage / predator avoidance。这和"条纹在任何时候绝对没有任何视觉防御作用"仍然不是同一个强度。科研语言特别喜欢 no consistent support，而不是 impossible。

因为 biting flies 并不只是"烦"。雌性 tabanid horseflies 需要 blood meal 帮助产卵，它们会造成 blood loss 和 irritation；更重要的是，在非洲环境中，一些吸血昆虫还可以传播对 equids 严重的疾病。2014 研究因此讨论了 zebra 较短的 coat hair、parasite attack、disease transmission 以及 blood loss 可能产生的选择压力。