Why Not Polka Dots? The Reasons Behind Zebra Stripes
Photo: Thomson Safaris guest, Cheryl Harvey
When we think of zebras, “stripes” is probably the first thought that comes to mind. It’s the species’ most recognizable trait, not to mention a perennial fashion favorite.
But why such a distinctive look? Is there any reason (more persuasive than a story from Aesop) for zebras’ immediately identifiable coats?
The prevailing theory for some time has been that stripes act as zebra camouflage. According to this argument, the striped patterns make it difficult for a predator to pick out any one creature from a herd, delaying its ability to chase after just one animal.
The problem is, there’s just not much evidence for this explanation. It’s possible that it works this way, but that’s just a best guess; no experiments have tested the theory rigorously.
There is evidence, however, that stripes deter a very different kind of hunter: bloodsucking horseflies.
Scientists studying horses found that the darker an animal’s coat, the more horseflies it attracted. In Africa, these pests are not only nuisances, they carry germs that could be dangerous to a herd.
The scientists already knew that zebra embryos start with dark skin, developing the white stripes over the course of gestation; they posited that these stripes might be an adaptation to keep away as many flies as possible.
So they did what any good researcher would: they headed to a fly-infested horse farm near Budapest to test their theory out.
They assumed that fully-white surfaces would attract the least flies, and that a variegated black-and-white pattern would attract fewer flies than a black coat, but more than white.
They were wrong: the striped surfaces attracted the fewest flies of all.
The researchers then took it one step further, testing stripe patterns of varying widths. The takeaway? The pattern that most closely resembled those on actual zebra hides was least attractive to flies overall.
Though the researchers can’t be certain why the stripes have this effect, they posited that it might have to do with the orientation of light waves that reflect off a zebra’s skin vs. those which tend to attract horseflies (vertical vs. horizontal, respectively).
That signature striping may do more for the zebras than just keep the flies away, but the scientists who performed the research (published in 2012 in the Journal of Experimental Biology) believe the primary evolutionary reason behind zebra stripes is pest repelling.
No one has yet tested whether some other pattern (polka dots? argyle?) is even more effective against flies.
Who knows; that might REALLY be a horse of a different color…
