Monday, February 27, 2012

How the zebra got its stripes

Why does the zebra have stripes? Perhaps they provide camouflage in the African savannah; perhaps they break up the animal's outline making it more difficult for predators such as lions.

However, research published recently in the Journal of Experimental Biology suggests that the stripes may confer protection against a different sort of predator.

An international team of scientists, led by Gábor Horváth of the Eötvös University, Hungary, found that the zebra's stripes help ward off biting flies.

Such flies are widespread in Africa, and, as well as spreading disease, they cause annoyance and disrupt grazing.

According to Horváth, these insects are guided to a wet habitat suitable for breeding by homing in on horizontally polarized light, which results when light is reflected off water. However, blood-sucking female flies also locate their victims by following polarized light reflected from their skin.

The research, carried out in Hungary, showed that a horse model with zebra stripes attracted far fewer horseflies (tabanids) than did homogeneous black, brown, grey or white equivalents.

The research team found that as the width of the stripes decreased the horseflies were less attracted. They also found that the width of the stripes on zebras coats fell within the range that was most disruptive to these biting flies.

Tabanids have been shown to respond strongly to linearly polarized light,” they explain “and we demonstrate here that the light and dark stripes of a zebra’s coat reflect very different polarizations of light in a way that disrupts the attractiveness to tabanids.”

It appears to be the polarisation of the reflected light, rather than the colour , that is significant. The research team found that even on homogeneous grey surfaces the attractiveness to tabanids was reduced if the surface had bands of alternating polarisation.

We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies”, says the team. They add, “The selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region [Africa]”.

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