22 November 2021

While thinking of the wild zebras of Maryland, I wondered …

Why do zebras have stripes? There are many theories.

Are zebra stripes for camouflage from predators?

No. A 2016 study by Univ of Calgary and UC Davis examined the visual acuity of the zebras’ top predators, including lions and hyenas, and compared it to what zebras and humans can see.

The study found that beyond 50 meters (about 164 feet) in daylight or 30 meters (about 98 feet) at twilight, when most predators hunt, stripes can be seen by humans but are hard for zebra predators to distinguish. And on moonless nights, the stripes are particularly difficult for all species to distinguish beyond 9 meters (about 29 feet.) This suggests that the stripes don’t provide camouflage.

— UC Davis: Zebra Stripes Not for Camouflage

In fact, to distant lions and hyenas, zebras look as plain as a wild ass.

Are zebra stripes for identifying each other?

Each zebra has stripes as unique as our fingerprints. No two zebras are the same. We humans can visually identify individual zebras by their patterns (see below). However, the same 2016 study found that zebras don’t see their own stripes as well as we do.

Do zebras use the stripes for identity? Not exclusively.

Wikipedia explains that a new zebra mother imprints her own striping pattern, scent and vocalization on her foal so the youngster will recognize and follow her.

Are zebra stripes for cooling?

Maybe. A 2015 study at UCLA posited that the stark black-and-white stripes set up convention waves on the animals’ coats, making them cooler in the African sun. Wider stripes cool better than narrow ones. To support the theory, among the three species of zebras (shown above with wild ass) the widest striped live in the hottest part of Africa while the narrow-striped live in the cooler region.

This theory has not been tested, though.

Are zebra stripes for protection against biting flies?

Yes. This has been fully tested by biting flies, zebras, and horses wearing zebra coats.

In a 2019 study — Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses — zebras and horses wearing zebra stripes were exposed to biting flies. The flies could not land on the stripes.

The zebra stripes seemed to dazzle the flies so much that they couldn’t manage a controlled landing. Flies zoomed in too fast and either veered off just in time — or simply bumped into the zebra and bounced off. The flies didn’t seem to like the zebra coats on horses, either, but their bare heads were fair game.

— New York Times: Why Do Zebras Have Stripes? Scientists Camouflaged Horses to Find Out

Protection from flies is a big advantage for zebras because they have short, bite-able coats and “they are uniquely susceptible to the diseases carried by the flies.”

However, there is no simple answer.

“Really, the striping is kind of extraordinary, so you need something extraordinary to explain it.”

— Why do zebras have stripes? It’s not for camouflage

There are many reasons for having stripes but only the zebras know for sure.

(photos from Wikimedia Commons and PLOS One; click on the captions to see the originals)