Scientists have long debated the purpose of a zebra’s stripes, and although there have been several theories put forward, one of the most common is that they are used as some sort of camouflage. This month, a study published in PLOS ONE reveals that these stripes don't actually help to conceal this conspicuous animal from dangerous predators.
Zebras have black and white stripes in a predominantly green and sandy-hued landscape. Through human eyes, this color combination seems to make the zebras stand out against their savannah grassland backdrops. However, to several predators, such as the African lion, their own visual systems may mean that this stripe pattern makes zebras harder to spot.
A team of researchers from the University of Calgary and University of California, Davis, decided to investigate this long-standing zoological mystery. Key to this study was how their natural predators actually hunt them down, and which of their senses they primarily use while doing so. Armed with information about these predators’ visual capabilities, the researchers processed a range of images of zebras by passing them through several spatial and color filters.
A zebra as it appears to a human, zebra, lion and spotted hyaena under photopic conditions. Credit: Amanda D. Melin, Donald W. Kline, Chihiro Hiramatsu, Tim Caro
The final images represented, at several different distances, how the predators would see a zebra in a range of environments and at different times of day. The researchers found that beyond 50 meters (164 feet) in daylight, or 30 meters (98 feet) at twilight – peak hunting times – a zebra’s stripes cannot be easily seen by predators, despite being very easy to spot by humans. On the darkest moonless nights, even humans can’t see the stripes beyond 9 meters (29 feet).
Although this may sound like zebras actually have excellent camouflage abilities, there’s a problem: At the point wherein the predators can distinguish the zebra from the background, they will have most likely already smelled or heard their prey. So the stripes are essentially ineffective camouflage.
Zebras are easy to spot for humans, but they’ve evolved to defend themselves against lions and spotted hyenas, not us – and these predators see the world quite differently. bmidgett/Shutterstock
It had previously been suggested that in more woodland areas near to the savannah, the black and white stripes might help them to blend in with the vertical, parallel tree trunks, but this was also shown not to be the case. In the more open savannah environments, where zebras spend most of their days, the researchers noted that lions could see a zebra just as well as any other similar-sized prey with far less eccentric patterns. The waterbuck and the impala both have solid-colored hides, for example.
Ultimately, according to Tim Caro, a UC Davis professor of wildlife biology and co-author on the study, this study is the final nail in the coffin for the camouflage theory. “The results from this new study provide no support at all for the idea that the zebra's stripes provide some type of anti-predator camouflaging effect,” Caro said in a statement. “Instead, we reject this long-standing hypothesis that was debated by Charles Darwin and Alfred Russell Wallace.”
Although previous research by Caro suggests that the stripes may deter parasitic tsetse flies, this study yielded no additional evidence for any of the other theories, including the idea that the unique stripe patterns are used by zebras to identify individuals at a distance.