Zebras, horses, and donkeys are all closely related, yet zebras are the only one within the species to have stripes; the only African mammal in the region with stripes, too. These species inhabit a wide variety of different niches such as, grasslands, savannas, woodlands, thorny scrublands, mountains, and costal hills. Until recently, limited information was available on the subject, but many hypothesis have been deduced over the years. These theories include, the stripes evolving as a form of camouflage, disrupting predatory attack by visually confusing carnivores, a mechanism of heat management, a social function, and evolving as a defense against ectoparasite attacks, such as biting flies.
Published on April 1st 2014 2014, in the online journal, Nature Communications, by the University of California- Davis, a new study finds that biting flies, like horseflies (tabanid) and tsetse (glossinid) flies, are a significant influence in the evolution of stripes. A team of researchers mapped and compared geographic distributions for seven different species of horses, zebras, donkeys, and other subspecies measuring different variables such as, the thickness, locations, intensity of their stripes on several parts of their bodies, ranges of large predators, temperature, and the geographic distribution of the biting flies. The researchers found that in areas where zebras had greater stripe density, they also suffered from a greater annoyance of bug biting. Lead author Tim Caro, a UC Davis professor of wildlife biology said, "Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies." Previous experimentation determined that these species of flies avoid black-and-white stripe patterns. Together, both sets of data suggest that overlapping areas with higher bug density influenced zebras to have greater striping patterns, as a defense. Zebras have shorter hair than most mammals in their region, making them more susceptible to the biting bugs. This study provides logical insight into the evolution of stripes on zebra as a defense against biting bugs.
Evolution is influenced by a magnitude of factors and changing even one can result in major phenotypic or genotypic changes. This is a beautiful example of species interacting, showing how a battle between two species can result in an observable pattern we can study. It cannot be taken as definitive proof as to why stripes evolved, but it suggest that in terms of their geographical distribution a coevolutionary arms race between biting flies and zebra striping has a strong correlation. I believe these other theories hold great value and its possible they all played a major role in the evolution of stripes. However, some factors have a greater influence then others, but as to which is the greater force, is still unknown.
Maxwell Liner (B)