The purpose of zebra stripes has perplexed science at least since Charles Darwin debated the matter with fellow Victorian naturalist Alfred Russel Wallace. Today, according to Caro, no fewer than four groups of researchers study the question.
They do not lack for hypotheses: In Caro’s new book, “Zebra Stripes,” he outlines nearly 20 of them. Among the most commonly tossed around — and accepted by the general public, not to mention safari guides — are that the stripes serve as camouflage or confuse predators. Biologists have alternatively surmised that the markings help zebras identify one another, ward off biting flies, regulate a zebra’s temperature or warn attackers in the way a skunk’s stripes signal that it’s got stinky spray and is willing to use it.
But none of these has been proved. So Caro decided to test each of these ideas one by one, in the field, over a decade of summers. This was the scientific method on repeat: Trial and error after trial and error. And because studying large wild mammals is no easy task, Caro had to get creative. Coming up with experiments was, he said, “the thing that taxed me most.”
Photos provide the best illustration of Caro’s systematic, sometimes wacky quest, which he refers to as a “personal discovery.”
To determine how well zebra stripes served as camouflage, Caro cut life-size horse shapes out of plywood, painted them with various markings and had colleagues observe them at dusk and dawn, when lions and other animals that like to dine on zebras are most active. But light would reflect differently off paint than animal hair, so Caro draped zebra, wildebeest and impala skins over chairs and clotheslines and had colleagues observe them in different lights, including in the middle of the night. “This experiment cost me some social capital in that my colleagues could see little point in being waked to observe animal pelts,” Caro writes. It also indicated that zebras are pretty easy to see.
But how humans see zebras might be different from how lions do. So Caro simulated a lion’s-eye view. No big revelations there.
Previous studies have found that biting flies, including tsetse flies, don’t like to land on striped surfaces. But Caro knew that because flies typically land on moving animals, stripes in motion would need to be tested. So Caro had a tailor in the Tanzanian capital of Dar-es-Salaam make him some black-and-white striped pajama-like suits, wore them while walking slowly through zebra territory and counted how many flies landed on him.
Then he did the same while draped in the pelts of zebras and their more drab peers.
This was nerve-racking, Caro said. “Walking through national parks that have high lion populations draped in a zebra skin while a long way from the car is not the most sensible thing to do,” he deadpanned.
But it, like the other experiments, did not lead to a silver bullet.
“Every summer, I’d go out with a new piece of equipment and set of ideas, and inevitably I’d come back in October depressed because they hadn’t worked,” Caro said. “After a while, you really think, “Gosh, I don’t really think I know why zebras are striped.’”
In the end, Caro found his answer in the library at the University of California at Davis, where he is a wildlife biology professor. There, he and colleagues used all of the hypotheses to make maps of where zebras and other horse-like species with varying degrees of stripes live, overlaid those with maps showing where lions and biting flies are found, and accounted for other factors like temperature and climate.
“And the one that came out loud and clear from that comparative analysis was that stripes are associated with biting fly abundance,” he said.
That’s right: Zebras have stripes to deter flies.
Caro said this did not totally shock him. Flies are irritating, but they can transmit lethal diseases to zebras and drain them of lots of blood. But reaching a conclusion — one he says he is “absolutely convinced” is the right one — was a relief after all that effort, as well as an illustration of the imperfect way science can work, he said.
“Just imagine you’ve got 10 soldiers. I’m knocking them all over except one,” he said. “Dispelling the idea that [stripes have] to do with temperature or confusion is as important in a sense as confirming that it’s biting flies.”
As for those other zebra-stripe researchers? Caro said he is pretty sure his work has convinced them, too, though he’s open to argument. For now, Caro is turning his sights to those flies and why they dislike stripes so much.
If that sounds less interesting than mammal markings, stay tuned for another forthcoming paper from Caro.
Its title: “Why is the panda black and white?”