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

The question of whether zebras are white with black stripes or black with white stripes seems like the context for a classic joke:

Question: What is it that is all black, white and red?

A: Sunburnt zebra!

But this is no joke, because it does have an answer: Zebras are black with white stripes.

At first glance, the opposite may be true—after all, many zebras have black stripes that terminate on the belly and inside of the legs, revealing the rest to be white. But zebra looks can be deceiving.

All of a zebra's fur, whether black or white, grows from hair follicles that contain melanocytes. These cells are found in all animals and are primarily responsible for producing the pigments that give skin and hair their color. In both cases, melanocytes produce melanin (pigment) that is visible on the surface. In zebras, chemical messengers determine which melanocytes deliver pigment to which part of the fur, creating the zebra's black-and-white pattern. Importantly for zebras, their white fur represents a lack of melanin. White is not its own pigment. Since the white stripes only exist because the pigment is rejected, black is considered the zebra's "default" color.

Underneath all this fur, the zebra's skin is also black. A shaved zebra, without any stripes, is almost unrecognizable as an all-black animal.

Question answered! However: Researchers are still not sure why zebras have stripes. Over the history of zebra research, researchers have proposed at least 18 different theories as to why zebras have stripes, with explanations ranging from camouflage to defense against predators to unique markings like human fingerprints. "People have been talking about zebra stripes for over a hundred years, but it was just a matter of really doing experiments and thinking clearly about the problem to understand it better," ecologist Tim Caro told BBC Future in 2019. Carlo commented on a team that, while studying a herd of zebras at Hill Livery in the UK, dressed the horses in black and white stripes and let them roam freely among zebras and horses without false stripes, hoping to gain insight into their purposeful stripes.

Recent research has focused on testing a number of possible theories for zebra stripes, the most popular of which are protection from fly bites and temperature regulation. In a 2014 study by Carlo et al., they found that stripes on animals were more common in areas where biting flies are prevalent, which may mean that biting flies have a harder time seeing striped or black-and-white surfaces as safe landing spots. At Hill Livery, fewer flies landed on zebras and horses with striped coats than on horses without striped coats.

Although this is one of the most tested theories about the streak problem, not all scientists are convinced. Retired animal laboratory technician Alison Cobb told BBC Future she didn't think avoiding biting flies was enough to trigger evolutionary traits like stripes. She subscribes to the thermoregulatory theory, which suggests that in the morning cold the black stripes absorb heat, making the zebra feel warm, while in the afternoon heat the white stripes reflect light, making the zebra feel cooler. "Every zebra must avoid heat, and biting flies will be present in certain places and times of year, but they are by no means as clear or frequent a threat as overheating," Cobb said.

More research is needed to determine which theory, if any, is correct. If we have to wait for answers about why zebras have stripes, at least we know what color they actually are.

animal tags： zebra