California State University, Long Beach (CSULB) biology faculty member Theodore Stankowich knows the answer to a question even evolutionary genius Charles Darwin couldn’t solve—Why do zebras have stripes?
A research team that included Stankowich, examined the stripe mystery systematically in an article that appeared April 1 in the online journal Nature Communications.
Stankowich and his research colleagues, led by UC Davis’ Tim Caro, found that biting flies, including horseflies and tsetse flies, are the evolutionary driver for zebra stripes, not camouflage, not heat management, not predator defense and not social interaction. Co-authors include UC Davis’ Amanda Izzo, Hannah Walker and Robert C. Reiner Jr. An illustration of the Equid family tree was created by 2013 CSULB graduate Rikesh Patel.
“There have been experiments that have shown that biting flies don’t detect animals with stripes,” said Stankowich, an assistant professor who also has studied such topics as why skunks stink. “The stripes disrupt the fly’s ability to detect the animal in nature.”
Stankowich and crew also looked at the geographic distribution of zebras, horses and asses, and noted differences in zebra stripe patterns. They then overlapped the data with variables such as temperature, terrain, predator range and biting fly distribution.
“What we found was that if a zebra species overlapped an area with at least six month’s high humidity, where tabanid flies like to live, those species are very likely to have stripes,” Stankowich explained. “We looked at the range of each species and overlapped them with how many predators such as the spotted hyena or lion overlapped those same areas.”
The team mapped the geographic distributions of the seven different species of zebras, horses and asses, and of their sub-species, noting the thickness, locations and intensity of their stripes. Their next step was to compare these animals’ geographic ranges with different variables, including woodland areas, ranges of large predators, temperature and the geographic distribution of tsetse flies and tabanid flies (horseflies).
The team treated each of 21 sub-species as a different entity. For instance, in equus burchelli, there are five sub-species including the extinct Quagga which was brown but with faint zebra stripes on its neck. “What we found in almost every analysis that popped out was that the presence of tabanid flies for at least six to seven months out of the year heavily influenced the evolution of striping,” he said. “That is especially so for legs. Flies prefer to land on horses or zebras at about leg height. Tabanid fly activity was, by far, the best predictor of striping. It was almost a perfect fit.”
There is a hypothesis that suggests stripes serve as a warning to predators or camouflage. “Zebras can kick and defend themselves quite well against predators but were stripes found in areas of high hyena presence?” Stankowich asked. “If a zebra’s range has a lot of woodland overlap, that might be an indication that woodland encourages the evolution of stripes as a form of camouflage, but that didn’t pan out.”
The study looked at hair length and hide thickness. “The species of zebras that have stripes have shorter hair and thinner hides than other equids that don’t have stripes,” he said. “They live in hotter areas so the combination might have to do with heat regulation. Maybe thinner hides and shorter hair make a cooler zebra. But to protect against the flies, the stripes are helpful.”
There are differences between the stripes. “Belly stripes are different from neck stripes,” Stankowich noted. “Every zebra has its own striping pattern. Different sub-species vary widely in the types of stripes they have. Some have thin stripes all over their bodies while others have very thick stripes on their rumps. One sub species has ‘shadow stripes’ where, in between the regular stripes, there are light brown stripes. There is a lot more variation than you might imagine.”
They even looked at the presence or absence of stripes as well as the degree of intensity of striping. “We found some correlation between leg striping and belly striping in connection with the presence of hyenas,” he said. “Zebras can defend themselves effectively against hyenas. If you are a zebra and you can deter a hyena by telling the hyena you can hurt him, you want stripes on your rump or legs where they are the most likely to be seen.”
A Long Beach resident who was born in Whittier, Stankowich earned his bachelor of arts in biological sciences from Cornell University in 2000 before receiving his master’s and Ph.D. in animal behavior from UC Davis, the latter in 2006.
The CSULB faculty member’s main interest is the evolution of coloration with special attention paid to defenses. He believes there is a natural curiosity about why zebras have stripes.
“Up to now, it has been speculative hypotheses over many years going back to Darwin’s time,” Stankowich said. “Now that we can perform comparative evolutionary analyses, we can begin to answer questions like what factors influence the evolution of traits like coloration. With these tools in our toolbox, we are able to ask other questions as well.”