Science Figures Interpreted and Analyzed by RealClearScience
It's dangerous being a zebra. These ungulates, which graze on the plains (as well as mountains and grasslands) of Africa, face many existential threats. For instance, the very act of eating grass can be lethal, as anthrax spores reside in the soil. Ingesting these spores can lead to a deadly gastrointestinal infection. Zebras must also avoid human hunters. And they need lots of water, so they must never wander very far away from waterholes. However, the waterhole is deceptively tranquil; simply taking a quick drink can result in being eaten by a lion.
Fortunately for the zebras, they have evolved white and black stripes as camouflage to confuse lions. That's the conventional wisdom, anyway. But, as is so often the case, the conventional wisdom appears to be wrong.
New research in Nature Communications concludes that zebras' stripes are actually for preventing bites from nasty insects, such as tabanids (horse flies). For some reason, these flies avoid landing on black and white striped surfaces.
For their study, the authors collected stripe pattern data on seven wild equids: plains zebra, Grévy's zebra, mountain zebra, African wild ass, Przewalski's horse, kiang, and Asiatic wild ass. They then overlaid a map of these equids' ranges with information on predators' ranges, temperature, biomes, tsetse fly ranges, and tabanid ranges (which were determined by proxy using measurements on temperature and humidity). They found that the presence of equid stripes most closely correlated with the ranges of biting flies, such as tabanids and tsetse flies. In other words, equids are more likely to have stripes if they live in an area that also has biting flies.
Simultaneously, the authors' map mostly eliminated the other possible explanations that exist for zebra stripes: camouflage, predator confusion, body temperature control, and social identification. However, the presence of rump and leg stripes correlated with the range of hyenas, so it is possible that the stripes play a role in keeping hyenas at bay. The authors argue, though, that adult zebras are generally too big and powerful for hyenas to hunt, so it is unlikely that striping patterns play an important role here. Also, Asian equids that were once hunted by wolves and tigers do not have stripes.
Thus, the evidence from this and previous studies indicates that zebras have stripes to avoid flies, not lions. Now, they need to find a way to avoid humans.
Source: Tim Caro, Amanda Izzo, Robert C. Reiner Jr, Hannah Walker & Theodore Stankowich. "The function of zebra stripes." Nature Communications 5:3535. (2014). DOI: 10.1038/ncomms4535
(Image: Zebras via Paulmaz/Wikimedia Commons)