The surprising reason zebras have stripes - Cella Wright

When an English aristocrat rode through London in a zebra-drawn carriage, he embodied a dream decades in the making, borne from European efforts to dominate the African continent. But 19th century European colonists faced big problems as they tried to control the sprawling African territories they’d claimed, one of which was biting flies. These pests could extract half a liter of horse blood daily while transmitting fatal diseases that devastated domestic horses, leaving colonial powers reliant on thousands of humans to transport goods.

Zebras, however, appeared immune to the same pitfalls as horses. The German Empire deemed them “predestined for the use of military needs,” and undertook the task of domesticating them. Around 1900, former German colonial army officer Fritz Bronsart von Schellendorff placed himself at the mission’s helm.

But he severely underestimated the project. And soon enough, the question of why domesticating zebras was proving so difficult joined another longstanding mystery: namely, why zebras had such conspicuously striped coats— a matter that perplexed prominent scientists— and went on to fuel decades of debate. To understand zebras’ more distinctive qualities, we should start with the big picture.

Zebras belong to the same family as horses and donkeys. After their lineage entered Africa, they evolved into the three zebra species that exist today, living in social herds in eastern and southern Africa, grazing on grasses and evading fierce predators. Scientists have speculated extensively about zebra striping, but not all theories have held up.

For example, the hypothesis that striping has a social function seems unlikely because, while every zebra does have unique patterning, other equids have no trouble identifying individuals in their herd without it. Some have theorized that the pattern helps zebras stay cool in direct sunlight, with the heat differential between their black and white stripes generating cooling air currents. But when scientists tracked the air movements around sunlit zebra hides, they saw no such effect.

Many have also wondered if the patterning works as camouflage or somehow confuses or dazzles predators— perhaps evoking a tangle of tree trunks or creating uncertainty around where the zebra’s body starts and ends. But hyenas and lions probably see zebras as gray until they're in close range, where they can also hear and smell them. Lions can likely also identify their outlines just as easily as they can other, less flamboyantly patterned prey.

And given how frequently lions capture zebras, it doesn't seem like they're all that confused. One hypothesis that does pack a lot of promise concerns those biting flies that horses couldn’t handle. Zebras have shorter hair than other grazers in their regions, possibly making them more vulnerable to the flies’ probing proboscises.

So, perhaps striping somehow acts protectively. Testing this hypothesis, one experiment found that a certain kind of biting fly avoided horses covered in striped and checked rugs, compared to those in solid grey. Another documented biting flies circling horses, zebras, and horses clad in zebra print equally— but landing on zebra-y areas only about a quarter as much.

Biting flies also generally approached zebras at higher speeds and didn't decelerate as usual, causing clumsy overshoots and crash landings. It seems that zebra stripes— and other graphic patterns— interfere with how biting flies process visual information to position themselves when landing, limiting their blood-sucking and disease-transmitting opportunities. But zebras aren't just good at keeping biting flies off their backs.

Around 1900, Bronsart founded an experimental ranch near Mount Kilimanjaro to capture and cross-breed zebras with other equids. Things didn’t go as planned— in part because zebras have a robust set of defensive fight and flight adaptations. Most are capable of running within an hour of birth, and they’re equipped with fierce bites and kicks strong enough to kill a lion.

Bronsart’s operation was also unprepared to meet zebras’ needs. Those Bronsart did gather, which he paid Indigenous people to wrangle, couldn’t produce enough milk for their calves. Within a single year, Bronsart had burned through a five year budget and dozens of the zebras he held in captivity were dead.

Attempts at zebra domestication had failed miserably, leaving those rare instances of zebra taming largely to black-and-white history.