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?“) Zebras watch a photographer in Tanzania’s Serengeti National Park. Photograph by Brian Hilsmeyer, National Geographic Your Shot
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.”
Studying Stripes
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.
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.”)
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. Follow Christine Dell’Amore on Twitter and Google+.
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