Riddle of zebra stripes

Print edition : May 30, 2014

Zebras in Katavi National Park in Tanzania. Photo: Tim Caro/UC Davis

WHY zebras have black and white stripes is a question that has intrigued scientists for centuries. A research team lead by the University of California, Davis, examined this riddle systematically and found that biting flies, including horseflies and tsetse flies, are the evolutionary driver for zebra stripes. The finding was published in a recent issue of the journal Nature Communications.

Experimental work had previously shown that such flies tend to avoid black-and-white striped surfaces, but many hypotheses for zebra stripes have been proposed since Alfred Russel Wallace and Charles Darwin raised the question 120 years ago. In the present work, the researchers 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. Their next step was to compare these animals’ geographic ranges with the geographic distribution of glossinid (tsetse flies) and tabanid (horseflies) biting flies. They then examined where the striped animals and these variables overlapped.

After analysing five hypotheses, the scientists ruled out all but one: avoiding blood-sucking flies. “[T]here was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies,” said the lead author Tim Caro, a UC Davis professor of wildlife biology. Why would zebras evolve to have stripes when other hoofed mammals did not? The study found that, unlike other African hoofed 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.

But why do biting flies avoid striped surfaces? Caro said that now that his study has provided ecological validity to the biting fly hypothesis, the debate can move to why biting flies avoid striped surfaces. A possible reason: In 2012, a researcher from Lund University, Sweden, found that the way light was reflected and polarised from a zebra body was unattractive to the flies.

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