Why do zebras have stripes?

This post is by way of direct quotes from a recent item on the ScienceDaily website together with, at the end, two brief comments on why even this apparently trivial research is significant.

Stripes are not for camouflage

“Looking through the eyes of zebra predators, researchers found no evidence supporting the notion that zebras’ black and white stripes are for protective camouflage or that they provide a social advantage.

“If you’ve always thought of a zebra’s stripes as offering some type of camouflaging protection against predators, it’s time to think again, suggest scientists at the University of Calgary and UC.

” . . . They also measured the stripes’ widths and light contrast, or luminance, in order to estimate the maximum distance from which lions, spotted hyenas and zebras could detect stripes, using information about these animals’ visual capabilities.

“They found that beyond 50 meters (about 164 feet) in daylight or 30 meters (about 98 feet) at twilight, when most predators hunt, stripes can be seen by humans but are hard for zebra predators to distinguish. And on moonless nights, the stripes are particularly difficult for all species to distinguish beyond 9 meters (about 29 feet.) This suggests that the stripes don’t provide camouflage in woodland areas, where it had earlier been theorized that black stripes mimicked tree trunks and white stripes blended in with shafts of light through the trees.

” . . . Stripes also not for social purposes:

“In addition to discrediting the camouflaging hypothesis, the study did not yield evidence suggesting that the striping provides some type of social advantage by allowing other zebras to recognize each other at a distance.

“While zebras can see stripes over somewhat further distances than their predators can, the researchers also noted that other species of animals that are closely related to the zebra are highly social and able to recognize other individuals of their species, despite having no striping to distinguish them.”

The significance of the research is two-fold.  One is that this is the way science works.  We can never assume that what is ‘obvious’ — in this case stripes were thought to be for camouflage purposes — without testing. Science can never prove what is correct, only disprove what is not correct.

The second is that zebras’ stripes may be a good example of ‘genetic drift’.  That is, it results from a mutation which is neutral in evolution — it confers neither a survival advantage nor disadvantage.  Once such a mutation manages to survive for more than a few generations then it gradually breeds into the majority of the species.  Understanding of genetic drift is far from complete and so this zebra research project will add just a little more data into the matter.

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