Who's The Bigger Pain In The Rear?

Zebras, as we all know, are black or brown with white stripes and each zebra has it’s own customized pattern. The old explanation is that the stripes are camouflage and, as lions are color blind, the fact that the stripes aren’t green and brown is OK. Another explanation is that looking at a herd, the stripes confuse the eye of the predator and make it difficult to pick one out. Yet another suggestion is that they help one zebra recognize another.

The most recent work suggests that the camouflage theory is correct, but it’s not the lions that are the bane of a zebra’s life. It’s a much smaller enemy that they are trying to confuse. Horseflies are a constant pain in the rear and they spread nasty disease, but they have a vulnerability that can be exploited.

They have compound eyes that pick up horizontally polarized light. They use this to pick out and locate water that they need. Black and dark brown horses also reflect horizontally polarized light so the horseflies can zero in nicely. With white horses, any reflection is apparently unpolarized. Egri et al in the Journal of Experimental Biology spotted this difference and tested the effects of a variety of black and white patterns on horseflies (1, 2).

Variously black and white patterned sticky boards were placed in a field and the fly density recorded. The least favored were boards with zebra-like stripes. To go for the clincher, they put four sticky horses out in the field, one that was black, one brown, one zebra-striped and one Dalmatian-dog spotted. Note, these were model horses, so no real horses were bitten. The zebra-striped caught the least number of flies.

Clearly, the camouflage theory is correct and the stripes reduce the chance of zebras being bitten, whether once in a lifetime by a lion or many times during each day by a tabanid (fancy name for horse fly). It is a pity mosquitos are attracted to carbon dioxide and not horizontally polarized light or we could all be more comfortable in the summer with zebra-striped make-up.

  1. http://www.bbc.co.uk/nature/16944753
  2.  http://jeb.biologists.org/content/215/5/736

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