Zebras in Tanzania. Gil Eliahu

Why Do Zebras Have Stripes? One Theory Shot Down, 17 to Go

Despite challenging lack of sun and zebras, Swedish scientists debunk hypothesis that the interchange of black, white stripes created mini-climate on animal's fur

A new study published over the weekend in Nature Scientific Reports has debunked a fondly held if outré theory that zebras evolved stripes in order to stay cool. Zebra stripes are definitely cool, but it isn't because evolution created a mini-climate on their fur, it turns out.

And how were stripes supposed to create this mini-climate?

Black absorbs sunlight better than white. The black stripes would get hotter than the white stripes during the African day. And thus, went the theory, as the hot air above the black stripes collided with the cooler air above the white stripes, small vortexes would form.

These mini-vortices would behave like tiny little fans and cool the savannah animal as it peacefully grazed under the sun.

This theory has been debunked in a place with no zebras nor much sun: Sweden, where Susanne Åkesson of Lund University and colleagues elaborate what does not explain why the zebra has stripes.

Åkesson et al performed thermographic experiments using simulated zebras made of water-filled metal barrels covered with animal pelts colored black, white, grey and striped patterns.

"The barrels were installed in the open air for four months while their core temperature was measured continuously," write Åkesson and the team.

\ BAZ RATNER/ REUTERS

The long and short of it is that the sunlit zebra-striped barrels reproduced well the surface temperature characteristics of sunlit zebras. Well and good. However, they found no significant differences in core temperature between striped barrels and grey barrels, even on many hot days, independent of the air temperature and wind speed.

"The stripes didn't lower the temperature. It turns out stripes don't actually cool zebras," Åkesson remarked.

It is true that a barrel pretending to be a white cow had lower core temperature than a barrel disguised as a black cow. But clearly zebra-striped coats do not keep the animal inside cooler than a gray coat might.

So now there are only 17 theories for why the zebras has stripes, down from 18, the scientists say. And that's not counting Rudyard Kipling's hypothesis that the stripes were created when the animal was standing in the forest beneath "the slippery-slidy shadows of the trees." 

The 17 surviving theories include camouflage: keep in mind that color vision is not great in many of the predators. What looks to a six-foot-tall human like an obvious white and black quadruped in the brownish savannah grass may look less obvious to a three-foot-tall hyena.

Another notin is that like the lionfish, the zebra is using stripes as a warning. In the fish's case, its saying "I am poisonous." In the case of the zebra, it is saying, "I have a poisonous personality."

There is also a theory, from 2012, postulating that stripes help the zebra against the misery of tsetse flies and other insects. That theory was proposed by none other than Åkesson herself, based on the observation that under lab conditions, tsetses prefer simulations of dark animals to simulations of white ones. That theory has yet to be proven with actual zebras, which is a challenge, because they don’t live in Sweden.

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