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. )
Follow UD News at Twitter!