Why do zebras have stripes? It was a question we thought we had the answer to – because they serve as camouflage to protect the equine species from predators – but new research published in Friday’s edition of the journal PLOS One suggests that this may not be the case after all.
In the new study, lead author Amanda Melin, an assistant professor of biological anthropology at the University of Calgary, and colleagues from the University of California-Davis explained that the camouflage hypothesis is flawed because it looks at the issue “through human eyes.”
Melin’s team reframed the issue by performing a series of calculations through which they were able to estimate the distances at which lions, spotted hyenas or other zebras could spot the stripes during the daytimes, at twilight or during a moonless night. They found that the stripes could not help the creatures hide, because they would have already been detected in other ways.
“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,” study co-author Tim Caro, a professor of wildlife biology at UC-Davis, explained in a statement. “Instead, we reject this long-standing hypothesis that was debated by Charles Darwin and Alfred Russell Wallace.”
Visual capacity tests reveal that stripes don’t help zebras hide
In previous research, Caro and other colleagues found evidence that the stripes benefit zebras by discouraging biting flies. Now, the new study found that stripes could not be involved in helping the creatures blend into the background of their environment or distorting their outlines, because by the time predators see them, they would have already heard or smelled them.
The study authors reached this conclusion after taking digital field images obtained in Tanzania and running them through spatial and color filters meant to simulate how zebras would appear to lions and spotted hyenas, as well as to their fellow zebras. They measured stripe length and the luminance, or light contrast, in order to determine the maximum distance from which all of these other creatures could detect stripes, based on each individual animal’s visual capacity.
Melin, Caro and their colleagues found that beyond 164 feet (50 meters) in daylight and 98 feet (30 meters) in twilight, stripes can be easily spotted by humans but are far more difficult for the predators that hunt zebras during these times to detect. It becomes even harder for lions, spotted hyenas or other zebras to see stripes during moonless nights, as they are unable to distinguish the spots at distances greater than 29 feet (9 meters), according to the authors.
As such, this indicates that the stripes do not provide zebras with camouflage in wooded areas, and that the stripes do not protect them from becoming prey. Furthermore, in treeless habitats, the researchers found that lions had no more difficult of a time seeing striped zebras than other, smaller prey with solid-colored hides, indicating that stripes offered no additional protection in these environments either.
The researchers also concluded that there was no evidence to support the notion that stripes had a social benefit, allowing zebras to recognize one another from a distance. While they did find that zebras could see stripes over slightly greater distances than lions or spotted hyenas, they said that close relatives of zebras could also easily recognize other members of their species, despite their lack of stripes.
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Feature Image: A zebra grazing on the grassy plains gazes at the researchers’ chart used for color-calibrating images. (Tim Caro/UC Davis)
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