People often think that zebras have stripes for crypsis, or camouflage, reasons. For color-blind predators, the stripes could make them hard to see in tall grasses. Or, that their striking black-and-white stripes are a form of disruptive coloration, working to obscure the body contour or confuse predators and parasites through optical illusions or a distracting effect called motion dazzle when they run as a herd. (Check out some illusions here, here and here: where does one end and the other begin?) Still others have suggested that it has nothing to do with predator avoidance, that the stripes help manage heat by reducing thermal loads. Well those are all great hypotheses, but no one’s ever tested all of them together.
So, pitting a bunch of ideas against each other, Tim Caro from the University of California, Davis, and colleagues examined the geographic distribution of current and extinct equid species (which include zebras, horses and asses). Among the seven species, and some subspecies, that were examined, there were both animals with stripes and without. Then they compared the geographic ranges with environmental variables, such as large predators, ectoparasite breeding conditions, and temperature.
They found that the ranges of the most distinctively striped species (Equus burchelli, E. zebra, and E. grevyi) overlap remarkably with the areas where disease-carrying blood-suckers, like horseflies (tabanids) and tsetse flies (glossinids), are active and particularly annoying. This was consistent across different types of striping -- facial, neck, flank, rump, belly, or leg stripes of varying intensity and thickness, as well as shadow striping -- and different equid species and subspecies.
"Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies,” Caro says in a press release. After all, certain flies are known to avoid black and white surfaces, preferring uniformly colored ones (though why that’s the case is another mystery for another day).
In contrast, they didn’t find consistent support across species for hypotheses about camouflage, predator avoidance, heat management, or some aspect of social interaction. But why zebras? According to the study, unlike other African hooved mammals living in the same areas, zebras have hair that’s shorter than the mouthpart length of biting flies, making them particularly susceptible to annoyance.
The findings were published in Nature Communications this week.
Inconspicuous stripes.
The hodgepodge of reflected light patterns from a zebra's coat render
the animal unappealing to biting insects such as horseflies.
If you're planning a trip to
Africa, pack that zebra-print shirt that's been hiding in the back of
your closet. A new study finds that zebra stripes disrupt light patterns
that tsetse flies and horseflies use to find food and water.
The discovery, experts say, is an exciting step forward in solving
the riddle of why zebras sport such unique patterns. "It's the first
really convincing evidence to come in the 120 years since people started
to debate this issue," says Tim Caro, a behavioral ecologist and
conservation biologist at the University of California, Davis, who was
not involved in the study.
Researchers have speculated about zebra stripes for years, says
Graeme Ruxton, a visual communication researcher at the University of
Glasgow in the United Kingdom, who was not involved in the current work.
Reasons for the jailhouse pattern include confusing predators, being
camouflaged in grasslands, and allowing the animals to pick each other
out in large herds.
But no one had tested these hypotheses experimentally, says Susanne
Åkesson, an evolutionary ecologist at Lund University in Sweden and one
of the authors of the new study. She was curious about whether zebra
stripes were attractive to tabanids, a family of insects that includes
tsetse flies and horseflies—notorious pests that can transmit illnesses
such as sleeping sickness and Chagas disease. Earlier research testing
tsetse fly preferences for black, white, or striped landing surfaces
found that the flies preferred black squares, but the testing didn't
explain why.
Åkesson and her colleagues started with something they knew about
horseflies. Their previous work found that the insects were most
attracted to dark-colored horses, compared with white horses, because
darker coats reflected light waves oriented in the same direction. This
polarized light was the same as that reflected from pools of water,
where the flies lay their eggs. White coats didn't reflect this type of
polarized light.
To see whether a zebra's white stripes would make it less attractive
to tabanids, the team set up a series of experiments on a Hungarian
horse farm. They tested the attractiveness of solid black or white, or
black and white striped squares to tabanids, as well as black, brown,
white, or striped life-sized plastic horses. They also tested gray
squares with varying numbers and widths of stripes to see how patterns
affected horsefly preference. Researchers used vegetable oil to trap
tabanids that alighted on their experimental squares and coated the
plastic horses with a clear, odorless glue to nab flies when they
landed.
It turns out that stripes are even less attractive to tabanids than solid white-colored surfaces, the team reports online today in TheJournal of Experimental Biology.
That's because stripes reflect multiple light patterns, whereas solid
colors reflect uniform patterns, which these flies prefer, the authors
say. So even a solid white square is preferable to stripes.
The team notes that these results need to be confirmed with zebras in
their natural habitat. Odors can make colors more attractive to tsetse
flies, they point out, and zebras in the wild emit very strong smells.
It's possible that eau de zebra may overwhelm off-putting visual cues
for tabanids looking for a meal
Zebras don't really encounter tsetse flies out on the open plain,
Caro says. But Åkesson points out that zebras and flies both hang out
near water.
Despite these caveats, this paper is a very exciting piece of work,
Ruxton says. It's far and away the most rigorous experimental study on a
reason for zebra stripes by an order of magnitude, he says.
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