Why Do Zebras Have Stripes? Science Has Finally Told Us

APTOPIX North Korea Daily Life — A pair of zebras brush against each other at a zoo in Pyongyang, North Korea, Thursday, April 26, 2012.

Joe Mathieu is the anchor and executive editor of WGBH's Morning Edition. Before joining WGBH Radio, Joe worked for six years as morning anchor on WBZ NewsRadio in Boston, where he was part of the team that received a Peabody Award for coverage of the Boston Marathon bombings. Joe also received the Edward R. Murrow Award for best newscast in a major market.

Heather Goldstone is the science editor at WCAI, the Cape and Islands NPR Station. She holds a Ph.D. in ocean science from M.I.T. and Woods Hole Oceanographic Institution, and spent a decade as an active researcher before leaving the lab to become a writer.

February 27, 2019

Updated August 01, 2023

This story might sound like something out of a Monty Python skit, but our science correspondent Heather Goldstone assures us it is real science and it answers an age-old question: Why do zebras have stripes? WGBH News' Heather Goldstone spoke with Morning Edition host Joe Mathieu. The following transcript has been slightly edited for clarity.

Joe Mathieu: This is a pressing research topic, right?

Heather Goldstone: I don't know about pressing, but it's definitely an interesting evolutionary question, right? Zebras are one of multiple species of horses, essentially, and they're the only ones that are striped. So, why? There have been multiple ideas out there, but it turns out they kind of haven't stood up to testing. There were ideas like it might help a zebra stay cool. So one researcher actually covered barrels of water in striped and patched and different colored cloth to test this idea and in fact, the stripes were no cooler. It's been thought it might confuse predators or might be part of social structures in zebras. But again, none of that really held up to testing. And so at the moment, the prevailing idea has been that stripes actually help avoid fly bites.

Mathieu: Flies? Like bug bites? Why would they do that?

Goldstone: Well, that's the thing. They don't really know. And we can come back to that. But the stripes correlate with biting fly pressure. So zebras live in Africa, where there are more flies with potentially deadly diseases that they tend to carry. So the idea was, maybe there's some evolutionary pressure there. And so what they decided to do was actually test that. And how they did that was to compare horses and zebras — and also horses wearing zebra-striped horse blankets — on a farm in the U.K., both with video up close of flies flying and trying to bite them and also with visual observations.

And what they found was that the flies are equally interested in the horses and the zebras — or the zebra-colored horses — but they don't seem to be able to land on the stripes. Lead researcher Tim Caro of the University of California, Davis, said: "When they're coming into the horse, they slow down, they're starting to decelerate from about half a second away before they make a controlled landing. Whereas when they're coming into a striped pattern, they continue to fly fast. They don't decelerate."

Mathieu: So that's it?

Goldstone: Yeah, I mean, Caro said he was watching them and they would literally bump into or bounce off of the zebra or zebra-striped blanket, and they think maybe it's affecting their visual system. Caro really thinks that this is the answer to why zebras have stripes. You can hear him in his own words.

"There's no question in my mind that the reason that equids have stripes is to stop biting fly attack. I think that the reasons for that still need to be investigated," Caro said.

All right. So this comes back to the question of, How would stripes affect the visual system of flies? Especially when you think about the size difference between the size of the stripes on the side of a zebra and a little tiny fly. But apparently that's what Caro is going to be up to next, trying to to figure out how it is that these stripes would confuse flies.

Mathieu: It's already been asked in the studio this morning. Dare I ask you: Black with white stripes, or white with black stripes?

Goldstone: You know, and I had to ask Caro while I had him on the line. And he hedged a little bit. He said, you know, it's complicated, but they start off black in the womb and then they develop stripes. So Caro may not be willing to to nail it down, but to me, that sounds like black with white stripes.

Mathieu: I love this. Heather Goldstone, definitive — our science correspondent and host of Living Lab Radio, where you hear great stories like this. This is like the best story of the day. Catch the show this Sunday at noon right here on 89.7 WGBH Radio. Weekly conversations with our science correspondent Heather Goldstone right here on WGBH's Morning Edition.