Zebra Stripes

Zebras, horse like animals native to the grasslands of Africa, are known for their distinctive black and white stripes. Historically many scientists thought zebras' stripes served to camouflage (hide) them from predators, such as lions and hyenas. This assumption was based on the observation that other animals, such as tigers, have similar stripes that make them less visible. However, in recent years scientists have noted that zebras' environment and behavior are not well suited to camouflage by stripes. Tigers often inhabit heavily forested areas in which there, vertical stripes help them blend in with the surrounding trees, but zebras typically inhabit open grasslands. Unlike tigers, who use stealth to stalk their prey, zebras are herbivores that rarely hold still when threatened by predators; instead, they rely on their good eyesight to spot predators at a distance and flee at any signal of danger.

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A zebra's stripes may help to protect it in other ways. Stripes may help zebras blend in with each other, rather than blend in with their environment. Zebras live in large herds, and they flee as a group when threatened. The dense pattern of moving zebra stripes may appear as a mass of confusing images, making it difficult for predators to target individual zebras. Catching a fleeing zebra requires a precisely timed final leap, and a zebra's stripes may interfere with predators' perception of distance. Because zebras have some ability to defend themselves by using their hind legs to kick at pursuing predators, at times powerfully enough to cause serious injury, a zebra's distinctive stripes may also serve as a warning that encourages predators to seek less dangerous prey. However, none of these explanations is strongly supported by observation. Zebras are killed by lions about as frequently as other unstriped prey animals.

More recently, scientists have suggested that the primary purpose of stripes is to protect zebras from biting flies. The grasslands of Africa are home to a number of species of flies that feed on the blood of large mammals and can cause considerable damage through blood loss and the transmission of diseases. Laboratory tests have shown that biting flies are less likely to land on objects covered in black and white stripes than on solid black on white surfaces. It is not yet known why biting flies would avoid striped objects, but scientists have suggested that the explanation lies in the mechanisms of flies' vision, which is simple and apparently confused by stripes. The stripes may make it difficult for flies to detect the outline of a zebra's body or cause the flies to confuse a zebra for a collection of thin, vertical objects that do not resemble potential victims. Scientists have analyzed the stomach contents of wild biting flies and have found relatively little zebra blood.

If stripes are an effective means of protection against biting flies. then why have other animals not evolved stripes as well? One theory is that zebras are particularly vulnerable to flies because of their unusually short hair and developed stripes as an alternative protective measure. Horses, which are closely related to zebras, are indeed extremely susceptible to biting flies when imported to Africa. They are frequently infected by fly-borne parasitic infections which are often fatal even when veterinary treatment is provided. Horses do not develop immunity to the parasites and so can be repeatedly re-infected.

Other puzzles remain. For instance, biting flies seem to be most discouraged by horizontal stripes, but the stripes on zebras are mostly vertical. More mysteriously, not all zebras are covered in stripes: the quagga, an extinct subspecies of zebra, had fainter stripes that were present only on the front half of its body. If stripes protect from flies, why would the quagga lack protection on half of its body? This cannot be explained by environmental factors because quaggas' range overlapped with that of fully striped zebras. One possible explanation is that if they only have stripes on half of their body, quaggas can more easily distinguish between their own and other species of zebra, making it easier for them to follow the solid-colored hindquarters when fleeing from predators. This behavioral hypothesis is unfortunately impossible to test, because scientists can only study the quagga through preserved museum specimens and a handful of nineteenth-century photographs.