Why did the zebra evolve to have stripes? The question has puzzled evolutionary biologists for centuries – and even Charles Darwin couldn’t explain the enigma.
The zebra is completely black as an early embryo, and white stripes only appear in a later embryonic stage, when the production of dark pigmentation is blocked. Each zebra has subtly different stripes, acting like nature’s own barcode.
A popular theory, both in the 19th century and today, is that zebras evolved striped coats as camouflage in tall grass. But, as Darwin noted, the “stripes cannot afford any protection in the open plains of South Africa”.
More recently, biologists have observed that zebras don’t attempt to conceal themselves by freezing in response to predators, but are actually rather mobile and noisy when danger is about.
Darwin suggested that zebras developed their unique stripes to recognise each other, which could be particularly important for male and female courtship. “A female zebra would not admit the addresses of a male ass until he was painted so as to resemble a zebra,” Darwin wrote.
Martin How at the University of Queensland, Australia, agrees that the stripes have an obvious social function. “But it’s possible they appeared for another reason and the social benefits came later.”
How says he has unpublished evidence suggesting that the stripes evolved to confuse predators, giving zebras crucial time to escape. He analysed videos of zebras with a motion detection program that mimics how movement is encoded in the animal brain. Their stripe pattern generated a range of optical illusions which would baffle a predator, he says. This effect was particularly strong when the animals moved together as a herd.
Another suggestion is that the stripes create a visual illusion, which makes the zebra look bigger that it is. Or perhaps the stripes assist with thermoregulation. But there is little evidence to support these claims, so the evolutionary explanation for the zebras’ stripes has remained murky.
Adam Egri at Eötvös University in Budapest, Hungary, has shown that horseflies (tabanids) tend to avoid zebra-like stripes. These biting insects transmit several equine diseases, such as equine infectious anaemia, as well as leaving painful bites.
Heading to a fly-infested farm in Budapest Egri and colleagues painted trays with different black and white patterns, and filled them with salad oil to trap the horseflies. Trays coated with thick horizontal stripes attracted less flies than diagonal lines, or criss-crosses. Thin black stripes mimicking those of the zebra attracted fewer flies than thick lines.
“The stripes are messing with their heads,” says Justin Marshall, a sensory neurobiologist, also at the University of Queensland. “It confuses them and provides an unattractive surface to land on.”
According to experiments carried out by Egri’s team, the stripes could also disrupt polarised light, making zebras less appealing to the pests. Horseflies are attracted to horizontally polarised light because they detect water through horizontal polarisation. At the watering hole, flies drink, mate and lay eggs.
Mike Archer, an evolutionary biologist at the University of New South Wales, Australia, calls this “a delightfully innovative explanation for something that’s long puzzled mammalogists.”
“Having been bitten myself many times by tabanids, which really hurt, this new explanation makes a great deal of sense to me,” he says.
Journal Reference: The Journal of Experimental Biology, DOI: 10.1242/jeb.065540