Most of us think zebra stripes are for camouflage, but a University of Calgary study has found that’s not the case, CBC News Calgary reports.
The problem is, we look at zebras through human eyes, said the study’s lead author, Amanda Melin, who’s an assistant professor of anthropology at the University of Calgary.
Truth be told, most mammals don’t see that well — including lions, hyenas, and other predators.
“Knowing that most mammals have pretty poor visual acuity, I thought ‘I bet they can’t even see these stripes very far, especially at night,” Melin said.
It’s more likely that predators have instead heard or smelled their zebra prey before catching a glimpse of them, and that rules out camouflage protection.
Melin and her team conducted a series of calculations that allowed them to estimate how far lions, spotted hyenas, and other zebras can see zebra stripes in daylight, twilight, or during a moonless night, UC Davis reports.
She conducted the study with Tim Caro, a University of California, Davis professor of wildlife biology. Caro and other colleagues provided evidence in previous studies suggesting that the zebra’s stripes give it an evolutionary edge by discouraging biting flies — natural pests of zebras.
The study was published in PLOS one.
How the test was conducted
To test whether stripes camouflage zebras in their natural environment, Melin and her colleagues passed digital images that were taken in Tanzania in the field inside spatial and color filters that simulated how zebras would look to their main predators, especially lions and spotted hyenas, and to other zebras, UC Davis reports.
The scientists also measured the width and light contrast (known as luminance) of the stripes, and that allowed them to estimate how far they could be seen by lions, spotted hyenas, and other zebras, judging from information regarding the visual capabilities of these animals.
What they found:
Zebra predators tend to hunt in woodlands and shrubby areas on moonless nights, TechTimes reports. Under these conditions, a zebra’s stripes can only be distinguished by big cats at about 29 feet. During twilight, the stripes only remain visible within 98 feet. Within short distances, big cats can locate their prey via sense of smell and hearing, the researchers found. When that happens, camouflage isn’t effective.
As for the zebras themselves, they also can’t tell the difference between solid and stripe patterns at long distances, the researchers found. This means that their stripe patterns don’t serve as a social function, either.
In the past, some scientists had suggested that zebra stripes might break up the animal’s outline, making it trickier for predators to spot them, but the researchers found that in grassy, treeless habitats, where zebras usually spend most of their time, lions could see them as easily as they could prey with solid-colored hides, such as topi antelope, waterbuck, and impala.
Caro said that the team’s findings don’t provide evidence or lend support to the theory that zebra stripes provide camouflage against predators.
“Instead, we reject this long-standing hypothesis that was debated by Charles Darwin and Alfred Russell Wallace,” he said.
Darwin and Wallace began the debate over the purpose of zebra stripes 120 years ago, TechTimes notes. Darwin dismissed the idea that the stripes were camouflage and proposed that they were, instead, a tool for sexual selection.
But what about those biting flies?
It turns out that the idea that stripes might repel these bad bugs originated in the 1930s, George Strombopoulos posits. In an earlier study, Caro and his colleagues examined previous attempts to link flies to stripes, and they also studied the striping pattern on the seven species of horses that have stripes.
Then they cross-referenced the patterns with where the animals lived and found there was definitely a tight correlation with areas where biting flies are found. If, for instance, scientists found tsetse flies (which no human or zebra wants to find if they can help it), there were horses with stripes. If tsetse flies weren’t found, neither were stripes. In humans, tsetse flies can cause African trypanosomiasis, and a similar illness in horses.
But what is it about stripes that bug the bugs? Scientists don’t know why the biters avoid them, and they note there’s more research to be done in order to prove this theory.
Stay tuned.
