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Old 10th February 2012, 03:14 PM
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Default How the Zebra Got Its Stripes

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How the Zebra Got Its Stripes


Zebra. If there was a 'Just So' story for how the zebra got its stripes, I'm sure that Rudyard Kipling would have come up with an amusing and entertaining camouflage explanation. But would he have come up with the explanation that Gábor Horváth and colleagues from Hungary and Sweden have: that zebra's stripes stave off blood-sucking insects? (Credit: © davy liger / Fotolia)

ScienceDaily (Feb. 9, 2012) — If there was a 'Just So' story for how the zebra got its stripes, I'm sure that Rudyard Kipling would have come up with an amusing and entertaining camouflage explanation. But would he have come up with the explanation that Gábor Horváth and colleagues from Hungary and Sweden have: that zebra's stripes stave off blood-sucking insects?
The team publishes their discovery that zebra stripes is the least attractive hide pattern for voracious horsefiles in the Journal of Experimental Biology at http://jeb.biologists.org/.
Horseflies (tabanids) deliver nasty bites, carry disease and distract grazing animals from feeding. According to Horváth, these insects are attracted to horizontally polarized light because reflections from water are horizontally polarized and aquatic insects use this phenomenon to identify stretches of water where they can mate and lay eggs. However, blood-sucking female tabanids are also guided to victims by linearly polarized light reflected from their hides. Explaining that horseflies are more attracted to dark horses than to white horses, the team also points out that developing zebra embryos start out with a dark skin, but go on to develop white stripes before birth. The team wondered whether the zebra's stripy hide might have evolved to disrupt their attractive dark skins and make them less appealing to voracious bloodsuckers, such as tabanids.
Travelling to a horsefly-infested horse farm near Budapest, the team tested how attractive these blood-sucking insects found black and white striped patterns 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 team found that the patterns attracted fewer flies as the stripes became narrower, with the narrowest stripes attracting the fewest tabanids.
The team then tested the attractiveness of white, dark and striped horse models. Suspecting that the striped horse would attract an intermediate number of flies between the white and dark models, the team was surprised to find that the striped model was the least attractive of all.
Finally, when the team measured the stripe widths and polarization patterns of light reflected from real zebra hides, they found that the zebra's pattern correlated well with the patterns that were least attractive to horseflies.
"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 and they add, "The selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region [Africa]."
http://www.sciencedaily.com/releases...0209101730.htm


ABSTRACT ONLY :-

Egri, Á., M. Blahó, et al. (2012). "Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes." The Journal of Experimental Biology 215(5): 736-745.
Quote:
The characteristic striped appearance of zebras has provoked much speculation about its function and why the pattern has evolved, but experimental evidence is scarce. Here, we demonstrate that a zebra-striped horse model attracts far fewer horseflies (tabanids) than either homogeneous black, brown, grey or white equivalents. Such biting flies are prevalent across Africa and have considerable fitness impact on potential mammalian hosts. Besides brightness, one of the likely mechanisms underlying this protection is the polarization of reflected light from the host animal. We show that the attractiveness of striped patterns to tabanids is also reduced if only polarization modulations (parallel stripes with alternating orthogonal directions of polarization) occur in horizontal or vertical homogeneous grey surfaces. 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. We show that the attractiveness to tabanids decreases with decreasing stripe width, and that stripes below a certain size are effective in not attracting tabanids. Further, we demonstrate that the stripe widths of zebra coats fall in a range where the striped pattern is most disruptive to tabanids. The striped coat patterns of several other large mammals may also function in reducing exposure to tabanids by similar mechanisms of differential brightness and polarization of reflected light. This work provides an experimentally supported explanation for the underlying mechanism leading to the selective advantage of a black-and-white striped coat pattern.
http://jeb.biologists.org/content/215/5/736.abstract
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