Saturday, February 18, 2012

Weekend Post - Why do zebras have stripes?

Why do zebras have stripes? New research suggests it might be more complicated than you think.

While it would be nice to think that zebras have stripes just so our country can have a cool national symbol I’m afraid we can be that self-centered. The zebra came first.

Let’s begin with an age-old question. Is a zebra a white horse with black stripes, or a black horse with white stripes? We now know the answer. It turns out that when they’re growing in their mother’s uterus zebras begin dark-skinned but develop white stripes before they’re born. You have to wonder why this would happen. What advantages does it offer the baby zebra to invest all that energy into growing such a complicated pattern instead of growing stronger bones, a bigger brain or stronger muscles.

Image c/o Wikipedia here:
Camouflage is an obvious answer. It might seem surprising to those of us who live in a country with lots of zebras but most people around the world find it shocking to think that a zebra’s stripes might offer it camouflage. How on earth, they wonder, can an animal with such a vivid pattern of such contrasting colors hide in a grey, green and brown environment? That’s until you see them in the wild and you realize that black and white merges at a distance to grey, providing excellent cover in grass, trees and bush.

Clearly that’s something that gives an individual zebra a chance at avoiding lions. Experts estimate that around 4-5 million years ago the first zebras began to appear. Presumably the first horse-like creatures with a hint of stripes must have had a slight advantage over their non-striped cousins. Critically, it must have given them a greater chance of surviving long enough to pass on their genes to their offspring. The unfortunate ones without the stripes would have been more likely to be visible in the bush and provide lions with lunch. Nature, in effect, selected the striped ones to pass on their improved genes. That’s why this type of evolution is called “natural selection”. It’s not a breeder, supernatural or not, who does the selecting, nature does it.

However, it might be a bit more complicated than just being able to hide from lions in long grass. Scientists from Sweden and Hungary recently came up with another reason why stripes might be useful. Based on some simple experiments they found evidence that stripes protect zebras from blood-sucking insects like horseflies.

They started with the knowledge that horseflies are more attracted to darker horses than to paler ones. They wondered whether stripes might somehow disrupt the attraction the dark skins held for the blood-sucking flies. That’s exactly what they found.

The researchers placed various boards of different colors and patterns in the fields surrounding a horse farm in Hungary. These boards were all covered in glue that trapped any flies that landed on them. At the end of the day the researchers just had to count the trapped flies to see which pattern attracted the most. They expected to see most flies on the darker boards, least on the palest and the stripy boards somewhere in between.

In fact they did confirm that darker boards seem to attract more flies but the surprising thing was that stripes were even less attractive to the flies than a white board. When they tried to work out which pattern of stripes were least attractive they discovered that it was exactly the pattern you find on a zebra.

So now it’s a bit more complicated. Stripes provide camouflage and enable the zebra effectively to hide from predators but they also might provide protection against bites. For this new factor to be plausible the researchers need to suggest a way in which this offers the zebras a greater chance of having babies than another that gets bitten more. The spread of disease is an obvious possibility. Fewer fly bites probably means a lower chance of fatal disease.

But there’s more than just this. Other scientists have suggested that stripes enable a herd of zebras to more effectively confuse predators and enhance their chance of escape. It’s even been suggested that the stripes are a complex form of heat regulation.

This is a great example of how evolutionary pressure is complex and often involves a wide variety of influences. It’s one of the things that makes science so wonderfully interesting. Easy, simple answers are often just too easy and too simple. Science is like the rest of life and truth. Deliciously complex.


You can see the BBC coverage of the story see here. A bit more detail can be seen at The Journal of Experimental Biology here. For a bit of background on the evolution of the horse see the Wikipedia page here. For more detail on the zebra see here.

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