There have been many hypotheses why zebra have stripes, including predator confusion, crypsis or thermoregulation, but the correct answer was unknown – until now.
Plains zebra Equus quagga boehmi (Joachim Huber)
An international team of researchers, led by Dr. Gábor Horváth of the Eötvös University, Hungary, has found that zebra’s stripes stave off bloodsucking insects.
The researchers have successfully demonstrated that a zebra-striped horse model attracts far fewer horseflies than either homogeneous black, brown, gray or white equivalents.
A paper describing the discovery appears today online in the Journal of Experimental Biology.
In Africa, horseflies (tabanids) deliver nasty bites, carry disease and distract grazing animals from feeding. These insects are attracted to horizontally polarized light because reflections from water are horizontally polarized and they use this phenomenon to identify stretches of water where they can mate and lay eggs.
Equus burchelli zebra model, used in the experiment (Gábor Horváth et al)
The team tested how attractive these blood-sucking insects found black and white stripes by varying the width, density and angle of the stripes and the direction of polarization of the light that they reflected.
Trapping attracted insects with oil and glue, the researchers found that the striped patterns attracted fewer flies as the stripes became narrower, with the narrowest stripes attracting the fewest horseflies.
The team also tested the attractiveness of white, dark and striped horse models, and found that the striped model was the least attractive of all.
“Tabanids have been shown to respond strongly to linearly polarized light, 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”, the team wrote in the paper.
“We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies. The selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region (Africa)”.
