Zebra stripes evolved to keep pesky insects at bay, according to the most thorough study to date on the subject.
All three species of zebra have bold stripes in comparison to other African grazers like buffalo and antelope. This so-called stripe riddle has puzzled scientists—including Darwin—for over a century, leading to five main hypotheses: that the stripes repel insects, provide camouflage, confuse predators, reduce body temperature, or help the animals interact socially. (See “Zebra Stripes Evolved to Repel Bloodsuckers?“)
For the first time, scientists played all of these theories against each other in a statistical model—and the result was pretty much, well, black and white.
“We found again and again and again [that] the only factor which is highly associated with striping is to ban biting flies,” said study leader Tim Caro, a biologist at the University of California, Davis.
“I was delighted to see the results were so strong in one direction.”
For the study, Caro and colleagues collected data from a vast range of sources, including museum collections and historical maps.
First, the team looked at variations in striping patterns across the seven living species of the equid group—which includes horses, asses, and zebras—and their 20 subspecies. Most have some sort of striping somewhere on their bodies.
They also noted where the stripes occurred on the body—for instance, the face, belly, or rump. (See pictures of zebras in National Geographic magazine.)
The team then mapped where current and extinct equid species live, where biting flies are found, the ranges of predators like lions and hyenas, distribution of forests, and other environmental factors that could influence the evolution of stripes. The data was then entered into a statistical model to find out which variable best explains striping. (Download zebra-stripes desktop wallpaper.)
The results showed that the range of striped species overlaps with where biting flies are most active—regardless of species and where the stripes occur on the body, according to the study, published April 1 in the journal Nature Communications.
Not the Last Word?
Brenda Larison, a biologist at the University of California, Los Angeles, who studies stripes in plains zebras, said the new study’s approach is “broad brush,” and that more specific research may be needed. (Related: “Resolving the Riddle of Why the Zebra Has Stripes.”)
That’s why “the story is likely to be much more complex, and this is unlikely to be the last word on the subject,” said Larison, who has received funding from National Geographic Society’s Committee for Research and Exploration.
Though Larison agrees that deterring flies is the “best supported hypothesis to date, most of the other hypotheses aren’t well studied, and there is still a lack of direct evidence,” she said.
“We really need to know what happens with live zebra in the field before we can be sure.”
Scientists haven’t actually observed zebras in the wild to see if biting flies avoid alighting on them, in part because it’s difficult to get that close to the animals. It’s also not known why biting flies steer clear of stripes.
However, study author Caro said he’s confident that biting flies swarming around a mixed group of herbivores would avoid zebras. (Watch a video of competing zebras.)
We’ve “moved the debate to the next stage—we can discount all [the other] hypotheses pretty conclusively,” he said.