Scientists solve the riddle of zebras' stripes

April 1, 2014
UC Davis scientists have learned why zebras, like these plains zebras in Katavi National Park, Tanzania, have stripes. Credit: Tim Caro/UC Davis

Why zebras have black and white stripes is a question that has intrigued scientists and spectators for centuries. A research team led by the University of California, Davis, has now examined this riddle systematically. Their answer is published April 1 in the online journal Nature Communications.

The scientists found that biting flies, including horseflies and tsetse flies, are the evolutionary driver for zebra's stripes. Experimental work had previously shown that such flies tend to avoid black-and-white striped surfaces, but many other hypotheses for have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago. These include:

  1. A form of camouflage
  2. Disrupting predatory attack by visually confusing carnivores
  3. A mechanism of heat management
  4. Having a social function
  5. Avoiding ectoparasite attack, such as from biting flies

The team mapped the geographic distributions of the seven different species of , horses and asses, and of their subspecies, noting the thickness, locations, and intensity of their stripes on several parts of their bodies. Their next step was to compare these animals' geographic ranges with different variables, including woodland areas, ranges of large predators, temperature, and the geographic distribution of glossinid (tsetse flies) and tabanid (horseflies) biting flies. They then examined where the striped animals and these variables overlapped.

After analyzing the five hypotheses, the scientists ruled out all but one: avoiding blood-sucking flies.

"I was amazed by our results," said lead author Tim Caro, a UC Davis professor of wildlife biology. "Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies."

While the distribution of in Africa is well known, the researchers did not have maps of tabanids (horseflies, deer flies). Instead, they mapped locations of the best breeding conditions for tabanids, creating an environmental proxy for their distributions. They found that striping is highly associated with several consecutive months of ideal conditions for tabanid reproduction.

Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies.

"No one knew why zebras have such striking coloration," Caro said. "But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it."

Yet in science, one solved riddle begets another: Why do biting flies avoid striped surfaces? Caro said that now that his study has provided ecological validity to the biting fly hypothesis, the evolutionary debate can move from why zebras have stripes to what prevents biting flies from seeing striped surfaces as potential prey, and why zebras are so susceptible to biting fly annoyance.

Explore further: How the zebra got its stripes

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4 / 5 (3) Apr 01, 2014
Previously reported (March 2012) by Kathryn Knight in "HOW THE ZEBRA GOT ITS STRIPES", J Exp Biol 2012 215:iii. ; doi:10.1242/jeb.070680
1.2 / 5 (9) Apr 01, 2014
Not proper way to do research.
Just make the skin uniformly colored by Autotransplantation.
After complete healing, check the no. of flies biting the striped side & Non-striped side.

The comparison should validate or invalidate....
Actually ANY RESEARCH on Zebra Stripes is nothing buttB.S!
1 / 5 (3) Apr 01, 2014
Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies.{/q]

A heroic attempt of explanation. Skeptics pounce on the inheritable (hereditary) traits of hair length.

Test and report the research results.
Breed longer-haired zebras without stripes.
Do they die out or survive along aside their successfully striped cohorts?
Which of the two prevails?
A long term study.
1 / 5 (2) Apr 01, 2014
"Just make the skin uniformly colored by Autotransplantation."

"Breed longer-haired zebras without stripes."

When you "breed out" the stripes what color will the Zebra be? White or Black? Grey?

1 / 5 (2) Apr 01, 2014
Do longer-haired zebras share the same advantage of not being bitten as striped zebras?
'Advantage' here means survival rate.

What colors do all other long-haired mammals exhibit that exist in the same environment?
Those are the colors that contributed to these mammals survival without the additional hazard of bites.
1 / 5 (2) Apr 02, 2014
Published April 1st?
5 / 5 (5) Apr 02, 2014
Nice to see that it could be adjudicated and that the partly swedish team had the correct hypothesis.

@tadchem: That was the original work that won out, yes. The interesting thing here is that they looked at a couple of main proposed hypotheses and could compete them successfully. (E.g. one winner.)

@betterexists: Whenever I see someone go "not [a] proper way to do research" I can be fairly certain that they have never done any. Or considered that they could compete their suggested method against the one used. Or that their method would work in practice. (Here it would be unethical, not to say costly and slow even if done.)

Here it was a successful method, so one proper way.
1 / 5 (6) Apr 02, 2014
Funny. Then, Why do we have lines on our Palms?
For the Palmists to tell us about our Future?
Stop Funding Dumb Research activities hereafter.
Rather, do research at the molecular, genomic levels for stripe production. Use that knowledge in Textile Industry, Whatever!
These FOOLISH scientists were born during Jesus time & now roaming amidst us.
1 / 5 (6) Apr 02, 2014
These Idiots have awful lot of research pathways ahead of them if money is added to their bank accounts.
So many birds with so many colors & streaks& stripes.
Peacock People! Minute sized. Brain size matters indeed.
5 / 5 (1) Apr 05, 2014
Some-how, I don't think a zebra-striped jacket would deter Scottish midges...
2 / 5 (2) Apr 06, 2014
Actually, Zebras know that stripes are cooler than spots. DOH!
Apr 06, 2014
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