WHY DO ZEBRAS HAVE STRIPES?

by Whit Gibbons

September 11, 2016

Q: In a column about animal color patterns, you noted that researchers have given several different explanations for why zebras have stripes. I went to the internet to check out scientific articles about zebra striping but found them very confusing so eventually gave up. What are the generally accepted explanations? And out of curiosity, do any of them refer to Rudyard Kipling's "Just So Stories" and his brief but charming explanation for why a zebra has stripes?

A: One reason you might have difficulty interpreting most scientific explanations for why zebras have stripes is that virtually all of the studies are inconclusive in explaining the adaptive significance of the stripes. No conclusion is categorically better than any of the others.

The earliest and most common scientific explanations were related in some way to camouflage. More than a century and a half ago, the biologist Alfred Russell Wallace proposed that the striping made zebras less conspicuous at night when they went to waterholes. The contrasting pattern would also be effective in some forest situations or in tall grass. Charles Darwin challenged Wallace's explanation by noting that zebras standing out on an open, short grass plain would be obvious to lions and other predators. Darwin's criticism seems a bit simplistic, but as he was Charles Darwin ...

The camouflage idea has persisted and one suggestion is that zebras evolved in partially shaded forested areas where the stripes served as camouflage. However, many zebras now inhabit the open plains but have not yet lost their stripes. The stripes in this case are considered holdovers from an earlier time. One recent proposal is that the stripes have neither a value nor a cost to a zebra in the open so no strong natural selection exists to alter the current genetics.

The dazzle effect, which can be considered a form of camouflage, has been offered as an intriguing idea for how a herd of striped zebras might escape from a predator. In this situation the striped pattern creates moving vertical lines difficult for a lion or a leopard to focus on. Thus the entire herd can sometimes escape before an individual has been singled out for capture by a confused cat.

One scientific proposal that has not gotten much traction among zebra biologists has been the thermoregulation hypothesis. Black stripes and white stripes absorb the sun's rays at different rates, with the black stripes getting hotter than the adjacent white ones. This creates a subtle but persistent airflow across the body of a zebra, resulting in a cooling effect over the skin. Another idea that has been tested experimentally is that the black-and-white pattern serves in some way to deter tsetse flies and other biting insects. And of course with any species, individual recognition is critical in a social group. Perhaps zebras recognize striping patterns the way we recognize faces.

One position taken in several summary articles about zebra striping is patently incorrect. In referring to Kipling's "Just So Stories," some authors assert that Kipling addressed how the leopard got his spots and the camel his hump, but he never explained the zebra's stripes. Wrong! "How the Leopard Got His Spots" explains why the zebra became striped - camouflage in the forest, so the leopard couldn't find it. As Mr. Kipling put it, "And after another long time, what with standing half in the shade and half out of it, and what with the slippery-slidy shadows of the trees falling on them, the Giraffe grew blotchy, and the Zebra grew stripy ..."

My own answer for why zebras have stripes is that each of the scientific hypotheses has merit, some more than others but collectively they confer an adaptive advantage and survival value to a zebra. This doesn't advance much beyond what Kipling, Wallace or Darwin offered but seems as suitable an explanation as any based on the facts as we know them today.

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