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.
