Why does a zebra have black and white stripes? For more than a century, naturalists proposed all kinds of theories. Some believed the alternating patterns broke up the zebra's silhouette in tall savanna grass, helping it hide from stalking predators like lions and hyenas. Others hypothesized that black and white stripes heated and cooled at different rates, generating micro-scale air currents across the skin to cool the animal under the blazing African sun.
However, recent controlled field experiments have provided strong evidence for a completely different target: biting insects.
A deadly savanna nuisance
Across sub-Saharan Africa, wild zebras share their habitat with tabanid flies, commonly known as horse flies and tsetse flies. These insects are not just annoying pests; they inflict painful bites and transmit debilitating, sometimes fatal blood-borne parasites.
Unlike many large African herbivores that have thick, leathery hides, zebras have relatively thin coats, leaving them vulnerable to swarms of blood-feeding insects. Evolutionary pressure to deter biting flies would offer zebras a significant survival advantage.
The 2019 coat experiment
In 2019, evolutionary biologist Tim Caro and a research team from the University of Bristol and the University of California, Davis published a groundbreaking study in the journal PLOS ONE.
To examine how insect vision interacts with zebra stripes, the researchers observed horse flies around captive zebras and domestic horses living on a livery farm in Somerset, England. Using high-speed video cameras, they analyzed each phase of the flies' flight trajectories.
From a distance, horse flies circled and approached zebras and solid-colored horses at virtually identical rates. The dramatic difference occurred only during the final approach.
Crashing instead of landing
When horse flies approached uniformly colored brown, black, or white horses, they smoothly decelerated, spread their landing legs, and touched down gently to feed.
Around zebras, however, the flies failed to slow down. The high-contrast, alternating monochrome stripes appeared to disorient the insects' low-resolution compound eyes. Unable to judge distance properly at close range, the flies either overshot completely and flew right past, or collided headfirst into the zebra's body and bounced off.
As a result, horse flies successfully landed on zebras less than one-quarter as often as they did on solid horses.
To eliminate differences in scent or skin chemistry, the researchers placed three types of fabric blankets on domestic horses: solid black, solid white, and black-and-white striped coats. Horses wearing the striped blankets received far fewer fly landings on their coats than when they wore solid colors.
An evolutionary optical shield
The zebra's iconic coat functions like an optical shield. By disrupting the flight control systems of biting insects, the zebra avoids debilitating bites and life-threatening infections, demonstrating how simple geometric patterns can provide powerful biological protection.
Sources
- University of Bristol News (2019) - Research reveals why the zebra got its stripes
- Axios (2019) - Zebra stripes confuse biting flies, study finds
- PLOS ONE (Caro et al., 2019) - Benefits of zebra stripes: Behaviour of tabanid flies around zebras and domestic horses
This article accompanies episode 7 of our series Weird Animal Bodies.