Why do zebras have stripes?
An alternate theory has emerged to explain the zebra's distinctive colouring. Yi-Di Ng reports.
Many people assume that a zebra's stripes help it hide from predators. But against the golden brown of the savannah, nothing stands out more clearly than a brilliant black and white striped coat. Biologist Tim Caro from the University of California believes the stripes do indeed afford protection – not from lions or leopards but blood-sucking flies.
Arguments over the reason for the zebra’s stripes go back a long way. Charles Darwin did not see how the stripes could offer protection on the open plains but Alfred Russel Wallace disagreed: “They may be indeed protective when the animal is at rest among herbage, the only time when it would need protective colouring,” he said.
Others have proposed that stripes help keep zebras cool, or are a wearable bar code to provide an instant visual ID, or that they repel insects. Without setting a foot in the wild, Caro and his colleagues decided to pit these hypotheses against each other using a simple approach – correlation.
They mapped the distribution of zebras and their plainer cousins – wild horses and asses – across Africa and Asia. They overlaid these maps with temperatures and the distribution of large predators and biting flies.
The depredations of flies could be a strong enough selective
pressure to drive the evolution of stripes.
Only one factor strongly correlated with the distribution of stripes: swarms of horseflies and tsetse flies. “This really does seem to give a lot more weight to the idea that these stripes have evolved to protect against insects,” says University of Bristol biologist Martin How.
So do flies care about stripes? Yes. In 2012, Ádám Egri from Eötvös University in Budapest reported a study on horseflies’ preferences for trays of salad oil. Judging by the number of flies in the oil, they preferred their trays painted solid rather than striped, and the thinner the stripes, the greater their aversion. Flies generally annoy zebras most around their legs and face. “I think it is no coincidence that the thinnest stripes are found on a zebra’s legs and face,” says Caro.
He believes the depredations of flies could be a strong enough selective pressure to drive the evolution of stripes. He points out that instead of delivering needle-like stings like mosquitoes, horseflies slice through skin leaving painful slow-healing wounds prone to infection. They also have ravenous appetites and are capable of draining up to 500 mL of blood a day from a single animal. Both tsetse and horseflies also carry diseases like trypanosomiasis, fatal to zebras (and humans). Zebras are particularly vulnerable to these vicious bites because they have much thinner coats than their savannah companions, the wildebeests and antelopes. Yet the rate of fly-borne diseases in zebras is much lower than in their unstriped companions. Analysis of blood meals in tsetse fly stomachs also shows that they dine less frequently on zebras.
“It is a correlative study rather than a proof,” warns Martin How, pointing to the next obvious question: why don’t flies like stripes? “Now we need to find out exactly how this mechanism works.”