You’ve heard how the zebra evolved its stripes to camouflage itself in the long grass, right? You may well have wondered why on earth such a glaring pattern would work. Maybe you wondered why it is, then, that most camouflage just means looking like the backdrop, period. If you were fool enough to ask, your teacher probably informed you that lions are colourblind, so the zebra need only be striped. You probably didn’t pursue the question long enough to discover that lions are not necessarily colourblind.
Well now, from “How the Zebra Got Its Stripes” (ScienceDaily, Feb. 9, 2012), we learn,
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.
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.
Actually, flies could well be a bigger problem for zebras than lions are, because flies affect all zebras, not just the unluckier ones. All zebras have an investment in a fly deterrence factor. So it makes more sense that the bizarre pattern could be an adaptation against flies.
Down with Darwinian just-so stories, and up with research!
Zebras in the snow (no horseflies!):
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