Why Zebras Have Stripes
April 3, 2014
It’s not because zebras want to look flashy to attract mates, or because the stripes help camouflage them from hungry carnivores. It’s not because the stripes help keep zebras cool under the burning African sun, or help zebras tell one another apart. The purpose of the zebra’s handsome markings is to keep away dangerous biting flies, including horse flies and tsetse flies, according to a new study by scientists led by biologist Tim Caro from the University of California, Davis (UC-D). The flies can spread sometimes-fatal diseases and cause blood loss in their victims.
The zebra’s stripes have long puzzled scientists, including the great British naturalist Charles Darwin, who favored the mate selection theory. Scientists have learned that like fingerprints in human beings, no individual zebra’s stripes are identical to those of another zebra. Moreover, the stripes are white, as they mark areas of the fur without black pigment (coloring). Zebras also have black skin beneath their fur. In recent years, the camouflage theory became the most popular.
Zebras live in eastern and southern Africa. Most female zebras have one young, called a foal, every spring. (Fran Allan, Animals Animals)
Caro’s study was the first to examine all five of the main explanations for the purpose of the stripes. The scientists mapped the range of seven species of wild equids in Africa. An animal’s range is the area where it lives naturally. Equids are a family of mammals that also includes horses, asses, and mules. The scientists also mapped the range of biting flies and predators, temperature variations, and wooded areas. The fly theory was a much closer fit than the others. “I was amazed by our results,” said Caro in a press release from UC-D. “Again and again, there was greater stripling on areas of the body in those parts of the world where there was no annoyance from biting flies.” The UC-D scientists found no relationship between the equids and other stripe theories, including camouflage and avoiding predators. The findings supported a 2012 study by Swedish scientists who studied how well boards painted to represent zebra stripes could repel horse flies.
So why don’t all equids have stripes? One theory suggests that the zebras evolved stripes because their relatively short fur makes them particularly susceptable to the flies. And how do stripes repel biting flies? Some scientists have speculated that the stripes might interfere with the flies’ ability to see. But according to Caro, that’s a question for another study.
