June 26, 2007 > TechKnow Talk
Why are Zebras Striped?
By Todd Griffin
This simple question turns out to have no simple answer. There are many theories as to why zebras have developed stripes, but no one knows for sure. As with many genetic adaptations, there may be a combination of reasons, all providing some benefit to the survival of the species. Thus, several theories may be correct, though some seem more plausible than others.
Members of the horse family, zebras live in southern and eastern Africa, mostly in grasslands where their primary predator is the lion. Zebras are herbivorous animals subsisting largely on grasses and can live for 40 years. They are capable of running at speeds up to 40 mph. There are at least three distinct species of zebra all similarly marked with alternating black and white or brown and white stripes. In addition to lions, they may also fall victim to cheetahs, leopards, or hyenas.
Some have suggested zebra stripes appear similar to the long grasses in which the herds often graze, helping them blend into their surroundings. Though the big cats are not totally color blind, they do not see the full palette of colors familiar to humans. So perhaps they are unable to distinguish the green or brown grasses from the dark and light stripes, lending this theory some credence. However, zebras rarely stand motionless. Their reaction to imminent danger is to flee, which would certainly differentiate a zebra from the grass.
Another theory has been advanced that something about the nature of the stripes renders zebras essentially invisible to lions at night. The great cats and hyenas all hunt primarily at night or early morning, and have excellent night vision. There have been reports that human observers using night vision goggles find zebras nearly invisible. Are night vision goggles an accurate simulation of how a lion sees at night? It's difficult to say but unlikely these large, noisy, and smelly animals can escape the notice of lions, also equipped with an excellent sense of smell and hearing. An impaired ability to see the prey would make the chase and kill more challenging, but lions will hunt in full daylight if hungry enough, so the night invisibility theory is not a fully satisfying answer.
There is no doubt that the stripes do act as camouflage in some way. Cats hunt by charging the herd, isolating a single, vulnerable individual, and running it down. As a zebra herd stampedes away from the predator, the array of moving stripes may be confusing, appearing as a mass of undulating black and white, and making it difficult to perceive and target a single animal. Young and weak animals would tend to blend into the overall swirling sea of stripes, from which the devastating kicks of large hooves could suddenly emerge.
This last theory is supported by the fact that zebras are "hard-wired" to congregate. Though they live in small family units, families typically group together to form huge herds, sometimes numbering in the thousands. In fact, researchers have discovered that if a striped pattern is painted on a wall, zebras will be attracted to it and stand next to it.
Another type of "predator" in the African grasslands is the fly. Many types of biting flies torment large animals. One of these, the bloodsucking tsetse fly, carries potentially fatal diseases such as nagana, to which the zebra is vulnerable. There is evidence that flies are attracted to large, dark objects. The stripe pattern may render the zebra less appealing to flies.
As the stripes confuse predators, they may at the same time help zebras identify each other. The striping pattern of each animal is subtly unique, due to growth patterns, body shape, and injuries, and seems to be instantly recognizable to other zebras. Among a herd that may reach thousands of individuals, the ability to recognize family members by their markings must come in quite handy.
Again, several of these theories are probably part of the answer. The "mass of moving stripes" theory has considerable merit in the opinion of the TechKnow Guy. It's easy to imagine the confusion and frustration of a predator with poor color vision faced with a vast, kaleidoscopic display of black and white shapes whirling among the grasses. The stripes also likely offer significant defense against attack by flies. Their utility as a means of identification is perhaps a fortunate side effect of these other survival-promoting qualities.
The zebra's crafty camouflage is not effective, however, in their struggle against human encroachment on their habitat. Though they are not endangered, their range is being gradually reduced, as they are forced to compete for water with domestic livestock in many areas. Poaching for meat and skins has also reduced their numbers.
By the way, most zoologists believe the zebra is essentially a black animal with white stripes. The skin is entirely black; stripes are a manifestation of hair color. Hair is pigmented where black stripes are seen, and lacks pigmentation, or melanin, in white areas. All hair follicles are capable of producing melanin, but those in white regions are genetically inhibited from doing so.
This differentiation of white and black areas occurs at a relatively early stage of the zebra's fetal development (3-5 weeks) when some cells are suppressed from passing melanin into the hair. Stripes are thought to be of equal spacing (about 0.4 mm apart) when they first appear in the embryo. Some species have more stripes than others, varying from 26 to 80 stripes. This may result from equally spaced stripes expressed at slightly different stages of fetal development (smaller or larger embryos) in different species. The ultimate pattern is dependent on subsequent growth of the animal, which distorts the initially uniform and parallel spacing, especially around the hindquarters.