Scientists solve riddle of zebras’ stripes

Scientists at the University of California, Davis, have learned why zebras, like these plains zebra in Katavi National Park, Tanzania, have stripes. Photo: Tim Caro/UC Davis
Scientists at the University of California, Davis, have learned why zebras, like these plains zebra in Katavi National Park, Tanzania, have stripes. Photo: Tim Caro/UC Davis

The spectacular black and white stripes that adorn zebras are designed to discourage biting flies, according to scientists.

The purpose of the stripes has intrigued researchers for centuries.

Now, a team led by the University of California, Davis, has examined the riddle systematically.

Their findings, published today in the online journal, Nature Communications, reveals that biting flies, including horseflies and tsetse flies, are the evolutionary driver for the stripes.

Experimental work had previously shown that such flies tended to avoid black-and-white striped surfaces, but many theories to explain the purpose of zebra stripes have been proposed since Alfred Russel Wallace and Charles Darwin debated the problem 120 years ago.

These include:

  • A form of camouflage.
  • Disrupting predatory attacks by visually confusing carnivores.
  • A mechanism of heat management.
  • Having a social function.
  • Avoiding ectoparasite attack, such as from biting flies.

The team mapped the geographic distributions of the seven different species of zebras, horses and asses, and of their subspecies, noting the thickness, locations, and intensity of their stripes on several parts of their bodies.

The next step involved comparing these animals’ geographic ranges with different variables, including woodland areas, ranges of large predators, temperature, and the geographic distribution of tsetse flies and horseflies (biting flies).

They then examined where the striped animals and these variables overlapped.

After analyzing the five hypotheses, the scientists ruled out all but one: avoiding blood-sucking flies.

“I was amazed by our results,” said lead author Tim Caro, a professor of wildlife biology at the university.

“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.”

While the distribution of tsetse flies in Africa is well known, the researchers did not have maps of horsefly and deer fly distribution. Instead, they mapped locations of the best breeding conditions for tabanids, creating an environmental proxy for their distributions.

They found that striping is highly associated with several consecutive months of ideal conditions for horsefly and deer fly reproduction.

Why would zebras evolve to have stripes whereas other hooved mammals did not?

The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies.

“No-one knew why zebras have such striking coloration,” Caro said. “But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it.”

Yet in science, one solved riddle begets another: Why do biting flies avoid striped surfaces?

Caro said that now that his study had provided ecological validity to the biting fly hypothesis, the evolutionary debate can move from why zebras have stripes to what prevents biting flies from seeing striped surfaces as potential prey, and why zebras are so susceptible to biting-fly annoyance.

Co-authors on the study included Amanda Izzo and Hannah Walker with the university’s Department of Wildlife, Fish and Conservation Biology; Robert Reiner Jr, of the Department of Entomology and the Fogarty International Center, National Institutes of Health; and Theodore Stankowich with the Department of Biological Sciences at California State University, Long Beach.

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