Are zebras white with black stripes or black with white stripes?

Zebras are iconic for their distinctive coats, but have you ever wondered whether zebras are white with black stripes or black with white stripes?

The stunning black-and-white coloration of the zebra's hide stands out in stark contrast with the dry, brown-and-green, treeless grasslands and savannah woodlands of their home territories of East and southern Africa, according to the African Wildlife Foundation. 

These stripes are unique to each individual. There are three zebra species living today — the plains zebra (Equus quagga), the mountain zebra (E. zebra) and the Grevy's zebra (E. grevyi) — and each of those species has a different striping pattern, too. For some, the darker portions of their hide are black, whereas others have browner coloring, and some have stripes only on their bodies but not on their legs. An extinct subspecies of the plains zebra called a quagga (E. quagga quagga) had minimal striping on its head, mane and neck, according to The Quagga Project. 

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Despite these different patterns and coloring, all zebras have the same skin color: black, said Tim Caro, a behavioral and evolutionary ecologist and conservation biologist at the University of California, Davis. However, this doesn't answer the question of whether their fur is black with white stripes or vice versa. For that, we have to look to the zebra's melanocytes, or the cells that produce pigment for their fur.

Although zebras have black skin, different developmental processes determine their fur color, just like a light-skinned person can have dark hair, Caro said. In fact, zebras actually have more light-colored hair than dark — their bellies are usually light — so it may seem that zebras are white with black stripes. 

But that's not the case. Here's why: Every piece of hair — both light and dark — grows from a follicle filled with melanocyte cells, according to a 2005 review in the Journal of Investigative Dermatology. These cells produce a pigment that determines the color of hair and skin. This pigment is known as melanin; a lot of melanin leads to darker colors, like dark brown or black, while less melanin leads to lighter colors, such as hazel or blond, Live Science previously reported. Zebras' black fur is chock-full of melanin, but melanin is absent from white fur, in essence, because the follicles that make up the stripes of white hair have "turned off" melanocytes, meaning they don't churn out pigment. 

The production of melanin from melanocytes is "prevented during the development of a white hair, but not of a black hair," Caro told Live Science in an email. In other words, for zebras, the animals' default state is to produce black hair, making them black with white stripes, according to Brittanica. 

The exact biological processes behind the zebra's stripes aren't known, but in African striped mice (Rhabdomys pumilio), which have light and dark stripes running along their furry bodies, the gene Alx3 is more active along light stripes than dark stripes, according to a 2016 study in the journal Nature. Alx3 effectively stops a master regulator gene in charge of melanocyte development, leading to light-colored hair, the researchers found.

So, why is the zebra black with white stripes? This unique pattern may keep away biting flies, according to research by Caro and his colleagues. In a study published in 2020 in the journal Proceedings of the Royal Society B, they found that African horseflies landed less frequently on horses wearing striped or checked rugs than they did on horses wearing solid-colored rugs. These biting flies can carry diseases that are fatal to zebras.

"There are very few mammals indeed with contrasting stripes like a zebra," Caro said. "The okapi has similar stripes on the rump, but other than that, no other species has really distinct black and white stripes. My guess is that the fly deterrent function is unique to equids because they are so susceptible to the diseases carried by certain biting flies in Africa."

Originally published on Live Science.