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Zebras are one of nature’s most captivating animals.

But why zebras have stripes remains a topic of discussion among scientists including Theodore Stankowich, a Cal State Long Beach assistant professor of biological sciences who’s published several research articles with collaborator Tim Caro of UC Davis. They’re interested in the ecology and evolution of animal defense mechanisms and predator-prey behaviors.

In the September 30 online journal Royal Society Open Science, Stankowich and Caro’s finds both agreement and disagreement with another zebra study. The Royal Society is the independent scientific academy of the UK and the Commonwealth, and the world’s oldest scientific organization.

Stankowich and Caro compared their own previous findings to those of a team led by Brenda Larison of UCLA that appeared in a January Royal Society Open Science article, “How the zebra got its stripes: a problem with too many solutions.” Larison’s group studied the plains zebra, one of three zebra species, and how environmental factors influenced variations in striping within this species.

“Our comment focused on the idea that it’s better to look at how different studies and different research methodologies can complement each other and show similar results rather than trying to divide two studies and focus on small differences,” Stankowich said.

“Based on their findings, they suggest that zebra stripes serve as a way to regulate body temperature and to cool off the animal because they found significant associations of variation in the pattern with temperature — more intense striping in environments that have higher temperatures,” Stankowich said. “The also found an association with areas with high precipitation as well, but they focused on temperatures as the most powerful of their findings. Intense striping was associated with high ambient environmental temperatures.”

Stankowich and Caro, along with Amanda Izzo, Hannah Walker (a current 91��ɫ biological sciences master’s student) and Robert C. Reiner Jr. of UC Davis had published their own research, “The function of zebra stripes,” in the April 1, 2014 Nature Communications.

“We found initially in our paper that while there was no significant effect of high temperatures alone on striping across all equids — our paper was on all of the zebra and horse species, not just the one,” he continued. “We combined a factor of high temperature and high humidity for at least six to seven months of the year, which is a good proxy for biting fly activity. We found that this proxy was strongly associated with intense striping. That was clearly the best predictor; it was almost a perfect match for species that had leg stripes and body stripes as well.”

Stankowich and Caro’s comment suggests that Larison and her colleagues’ findings “not only support what we found as well, but also support our hypothesis that biting fly activity is more important as a mechanism for striping evolution than temperature alone is. They found effects of both temperature and precipitation, which is similar to what we found in terms of temperature and humidity.

“Our study was one across all seven equid species and 21 subspecies as well,” he continued. “Their study focused on one single species of zebra. So, their results can’t speak to the original evolution of zebra striping to begin with. It can only speak to explaining variation within the species that they examined, which is really interesting on its own! Whereas, ours, because it looked at striping in zebras and non-striped equids, sheds light on how these factors influenced the original evolution of striping in this entire group.”

Stankowich and Caro also said the Larison group’s suggested mechanism of thermoregulation — that stripes might help cool zebras by creating eddies of air over the animal’s body — isn’t well supported and that field measurements suggest that zebras actually are warmer than other animals.

Larison’s co-authors included Ryan J. Harrigan, Thomas B. Smith, Alec M. Chan-Golston and Elizabeth Li of UCLA; Daniel I. Rubenstein of Princeton University; and Henri A. Thomassen of the University of Tübingen, Germany.

But both teams do agree on several things, Stankowich says. “We both believe that research should be directed toward trying to unlock what the mechanism is behind either biting fly avoidance or cooling. Those appear to be potentially important explanations, and while we believe there’s a clear mechanism for how stripes can deter biting flies and thereby prevent the spread of diseases, we have yet to see a clear mechanism for how striping can influence body temperature.

“We still feel that the most supported hypothesis is the biting fly hypothesis. We both agree that more research is needed.”

Written by Anne Ambrose