No, zebra stripes not for camouflage

Contrary to the century-old notion that stripes seen on zebras offer certain kind of camouflaging protection against predators, a study has found that they are quite unlikely to do so. The study was published on January 22 in the journal PLOS ONE.

To be effective as a camouflage, the stripes should be able to blend in with the background of the environment or in breaking up the outline of the zebra.

The reason why the stripe pattern’s camouflaging capacity has gained ground is because it has been looked at from our ability to see the stripes. It has never been tested from the point of view of the actual predators like lion and hyena.

So in order to check if the stripes served its primary objective, scientists used lion- and hyena-specific spatial and colour filters to mimic the visual system of the predators. The maximum distance from which lions and hyenas could resolve the stripes was measured to see if the stripes served to conceal the animal (zebra).

What Amanda D. Melin, the first author of the paper from Washington University, found was that the visual system of the predators was able to identify zebras but unable to resolve the stripes beyond 50 metres in daylight and 30 metres in twilight. It was as low as nine metres on a moonless night.

Put simply, if the predators are unable to resolve the stripes from such short distances, the ability of the stripes to blend with the environment and help in concealing the zebra from its predators beyond these distances is very low if not nil. As a result, zebras are clearly discriminable by lions and hyenas both under daylight and twilight conditions.

Also, zebras spend most of their time in open treeless habitats, and the stripes do not help in disrupting the outline of the animal in these settings; a lion will be able spot it from a long distance.

The study thus found that under daylight conditions, humans can resolve zebra stripes at considerably greater distances than can large carnivores or zebras. However, the predators and humans performed equally under moonless night condition.

“Stripe visibility decreases dramatically as light falls. At dusk, when hunting by carnivores normally begins, humans can resolve stripes from greater distances than other mammals: three times those of lions and five times further than spotted hyaenas,” they write.

So during twilight conditions, lions can discern stripes at 25-56 metres and hyenas at 15-34 metres. At distances greater than these, zebras appear grey to both the predators; the stripes serve no purpose whatsoever.

Under moonless night condition, lions can see the stripes only if they are as close as 6-14 metres from zebras and it is 4-9 metres for hyenas.

“These findings suggest strongly that stripes themselves are unlikely to be a form of crypsis [camouflage] at far distances because predators would not be able to discern the black stripes against a treed background, or see white stripes blending in with bright shafts of light between trees,” the write.

However, at very short distances, the stripes help zebras to blend with the background. But at such short distances, the predators can “smell or hear zebras” moving or breathing.

“In sum, our results do not lend support to stripes being a form of anti-predator crypsis, leading us to reject this longstanding hypothesis on the adaptive significance of zebra striping,” they conclude.

Published in The Hindu on January 24, 2016