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Apr 03 2014

Why do zebras have stripes?

The stripes on a zebra are quite striking and cry out for an explanation. The popular one is that it helps camouflage them against predators by mimicking long grass or trees. But a new study suggests that stripes came about because they help to repel flies. Yes, really.

Researchers from the University of California at Davis knew that certain flies avoid black and white surfaces, so they wondered: Could zebra stripes have evolved to keep the animals free from suffering the bites of those very same flies, which can carry fatal diseases? To tackle that question, the researchers examined the distribution of zebras and the locations of the best breeding grounds for the stripe-averse flies. Sure enough, they found that they overlap. The same was true for other animals in the horse family that had stripes on various parts of their bodies.

I am always a little wary of evolutionary explanations for characteristics that seem a little too glib, because of the danger of falling for ‘just so stories’ of superficial plausibility and ignoring the problem of confounding variables. So even if stripes repel certain kinds of flies, would this provide sufficient selection pressure to drive the process? Could there be other factors that have a greater effect?

As we like to say in science, more research is needed.

10 comments

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  1. 1
    busterggi

    Sure they can claim it was to deter biting flies but they really did it because stripes make them look thin.

  2. 2
    Menyambal

    I thought it was because when a lioness chases a herd, there’s so much flickering vertical that she starts wondering, “Are they black with white stripes, or white with black stripes?”

  3. 3
    Dan

    This kind of hypothesis might be (at least partially) testable by comparison with okapis. The Okapi is a giraffe relative that lives in central Africa. Their bodies are mostly brown, but with black & white stripes, similar to zebras, on their legs. If flies bite Okapis at different rates on their bellies vs. legs, that would lend support to this hypothesis.

  4. 4
    Pierce R. Butler

    Why do zebras have stripes?

    To protect them from submarines, of course.

  5. 5
    Randy Lee

    a herd of Zebras were arguing whether their stripes were white on black or black on white. After many months of fighting and arguing one Zebra decided he would go up to heaven and ask God what the truth was. When he arrived he was greeted at the gate by St. Peter who asked the nature of his business. St. Peter told him God was very busy and he would be limited to one question..
    Thinking he better just be direct to the point, the Zebra asked, “God are Zebra stripes white on black, or black on white? God said, “You are what you are.”
    Not understanding what he meant, but being too intimidated to ask another question, the zebra returns to earth all sad. The other zebras excitingly asked, What did he say, what did he say, ..!” The Zebra answers and says in a sad tone, “You are what you are”. ‘But I don’t know what he means’. Then from back in the back a little zabra speaks up and says, “Well, that’s easy. That means we are black on white. Cuz if we was white on black he would have said “You is what you is”.

  6. 6
    Mano Singham

    @Pierce,

    Interesting!

  7. 7
    Trebuchet

    But what about zebrafish? Perhaps PZ should research that.

    @4: That was the first thing I thought of as well.

  8. 8
    Mobius

    What I suspect (and I have heard this proposed by biologist) is that the stripes work like the dazzle camouflage used by the British in WWI and WWII. It doesn’t hide the target, but it confuses the eye as to range and direction of motion. And in a herd, it would work even better since the predator sees a wall of moving stripes with no distinct target.

    Dazzle camouflage was extremely effective when subs had to set their torpedoes manually and send them on a straight course. Similar conditions would apply to a predator trying to decide which way to run to intercept the prey. Not an exact analog (since the predator can make corrections), but still somewhat apt.

  9. 9
    Ritesh

    I always think that most of the preys that run from their predator, run in a big group. More striking or weird or abnormal or interesting a pattern is smaller the group size is. In other words more camouflage you have at an individual level lesser is the need to move in larger groups.

    On a larger scale it will create even more confusion for a predator to actually estimate and managing his/her speed if you just have a simple black and white pattern, but you manage to form a big enough group. You know Strength in Unity!

    Was group running evolved first or stripes or both simultaneously complimenting each other to avoid the predator? I hope this makes sense.

  10. 10
    Don Cates

    Larry Moran at Sandwalk has a post up on this. Why assume it is adaptive? The default should be that it was due to drift. You need evidence that whatever effect you claim (or show) for stripes also provides an increase in reproductive success.
    Why don’t all African herd animals have stripes if they are so adaptive?

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