The question of whether zebras are white with black stripes or black with white stripes might seem like the set-up for a classic joke like this one:
Q: What’s black and white and red all over?
A: A zebra with a sunburn!
But this question is no joke, because it actually does have an answer: zebras are black with white stripes.
At first glance, it may appear the opposite is true—after all, the black stripes of many zebras end on the belly and towards the inside of the legs, revealing the rest as white. But looks are deceiving in the zebra’s case.
All of a zebra’s fur, both black and white, grows from follicles that contain melanocyte cells. These cells are present in all animals, and they’re primarily responsible for generating the pigment that gives colour to skin and hair. In both cases, melanocyte cells produce melanin—the pigment—that is outwardly visible. In zebras, chemical messengers determine which melanocytes deliver pigment to which section of fur, thus creating the zebra’s black and white pattern. What’s important about zebras is that their white fur represents an absence of melanin; white is not its own pigment. Since white stripes only exist because pigment is denied, black is understood to be the “default” colour of a zebra.
Beneath all that fur, zebras have black skin, too. A shaved zebra, without any stripes, could be almost unrecognizable as an all-black animal.
Question answered! However: researchers still aren’t sure why zebras have stripes at all. In the history of the study of zebras, researchers have proposed at least 18 different theories on why zebras have stripes, with explanations ranging from camouflage to protection against predators to marks of uniqueness like a human fingerprint. “People have been talking about zebra stripes for over a hundred years, but it's just a matter of really doing experiments and thinking clearly about the issue to understand it better,” ecologist Tim Caro told BBC Future in 2019. Caro was commenting on a team who, while studying a herd of zebras at Hill Livery in the United Kingdom, dressed horses in black-and-white striped outfits and let them loose among zebras and horses without fake stripes to hopefully gain insight on the purpose of stripes.
Recent studies have focused on testing a few of the possible theories for zebras’ stripes, the most popular being protection from biting flies and thermoregulation. In a 2014 study by Caro and others, they found that striping on animals is more common in areas rife with biting flies, potentially meaning that biting flies struggle to see a striped or black-and-white surface as a safe place to land. At Hill Livery, too, fewer flies landed on zebras and horses with striped coats than on horses without striped coats.
Though this is one of the most-tested theories on the stripe issue, not all scientists are convinced. Retired animal lab technician Alison Cobb told BBC Future that she didn’t think avoiding biting flies was important enough to spark an evolutionary feature like stripes. She favours the theory of thermoregulation, which posits that black stripes absorb heat to warm zebras in the morning chill and white stripes reflect light to cool zebras in the afternoon heat. Cobb said: “Every zebra must avoid getting hot, and biting flies will come at certain places, and certain times of the year, but they are by no means as definite or frequent a threat as overheating.”
More research is necessary to determine which theory, if either, is correct. If we have to wait on an answer on why zebras have stripes, at least we know which colour they truly are.