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How the zebra got its stripes, this time really

plains zebras/Tim Caro, UC Davis

The last we heard (last December), the answer was:

Computer models that tracked the appearance of the patterns revealed that they worked as optical illusions to provide “misleading information” for both predators and pests:


But now from ScienceDaily:

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 zebra stripes have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago.

These include:

A form of camouflage

Disrupting predatory attack by visually confusing carnivores

A mechanism of heat management

Having a social function

Avoiding ectoparasite attack, such as from biting flies

The team mapped the geographic distributions of the seven different species of zebras, 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.”

See also:

Abstract Despite over a century of interest, the function of zebra stripes has never been examined systematically. Here we match variation in striping of equid species and subspecies to geographic range overlap of environmental variables in multifactor models controlling for phylogeny to simultaneously test the five major explanations for this infamous colouration. For subspecies, there are significant associations between our proxy for tabanid biting fly annoyance and most striping measures (facial and neck stripe number, flank and rump striping, leg stripe intensity and shadow striping), and between belly stripe number and tsetse fly distribution, several of which are replicated at the species level. Conversely, there is no consistent support for camouflage, predator avoidance, heat management or social interaction hypotheses. Susceptibility to ectoparasite attack is discussed in relation to short coat hair, disease transmission and blood loss. A solution to the riddle of zebra stripes, discussed by Wallace and Darwin, is at hand.–Tim Caro, Amanda Izzo, Robert C. Reiner, Hannah Walker, Theodore Stankowich. The function of zebra stripes. Nature Communications, 2014; 5 DOI: 10.1038/ncomms4535

The authors sound superconfident. Here are three questions a friend wondered about:

1. Why do Bengal tigers have stripes (but lions don’t). Nor do most life forms that cope with parasites.
2. Wouldn’t evolving some simpler pest control strategy be easier and more likely? Fewer mutations would be needed.
3. Why wouldn’t the flies just mutate so as to ignore the stripes?

Note: One might think that parasites are more numerous in hot locations, but the parasites of the far northern summer are notorious. Is striping often used there as a defense? (The real message here is how difficult it is to determine an actual cause in nature. )

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Perhaps the bugs adapted too quick and now think the stripes are vertical blinds and they are just avoiding windows... It's easy to create these stories. Heartlander
Heartlander Thats funny and a little right,. Indeed with all those bugs so desperate for meals surely they could evolve eyes or something to deal with the stripes. The zebras would lose in that arms race and not be here. So either they didn't lose because the bugs never mattered or they are not really here. Perhaps the stripes are giving US the illusion they are here and it really is just clouds of bugs! Evolution can do anything they say. Robert Byers
Of course, they willingly overlook the most likely reason of them all: zebras got their stripes because the designers felt that they looked cool. As a designer, I, too, think that they look quite cool indeed. Asking how zebras got their stripes is like asking how did a Louis Vuitton bag get to look like a Louis Vuitton bag and insisting that the answer must not make mention of Monsieur Vuitton and his work as a designer. Mapou
Considering the life cycle of the fly as compared to the zebra as well as the mutations required, it seems a fly would easily evolve the ability to overcome a striped zebra defense. Heartlander
What absolute utter poppycock. Not surprising though, seems the Darwin faithful are grasping at straws and their theories are becoming more ridiculous the more desperate they become. humbled
I think the stripes represent a old society where there were armies of predators seeking the zebras. nOt just a few cats like today. I think the stripes are just to make predators fail, on the run, to segregate the zebras from each other and so stop a leap. The cats etc don't leap and hope to hit home. they need a good landing target. I don't think its from bugs. tHats asking for too much selection speciality. natural selection never makes a dent in real nature. Robert Byers

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