How The Zebra Got His Stripes
Brett Smith for redOrbit.com – Your Universe Online
Zebras are known for their signature black-and-white stripes and a new study published in the journal Nature Communications has found more evidence that biting flies are actually the reason behind the evolution of this unique coloration.
The origin of the stripes was actually debated by Alfred Russel Wallace and Charles Darwin 120 years ago. Theories surrounding their value include a form of camouflage, a distracting effect on predators, heat dissipation and social value. However, the team was able to conclude that the stripes protect against fly bites, as these insects tend to avoid black-and-white striped surfaces.
To reach their conclusion, the study team first mapped the regional distributions of the seven different types of zebras, horses and asses, as well as their subspecies. The team also noted the solidity, locations, and intensity of their stripes for a number of areas on their bodies. Next, the team compared these animals’ topographical ranges with various factors, including wooded areas, the territories of large predators, temperature factors – and the geographic distribution of two biting flies, tsetse flies and horseflies.
Finally, the team evaluated where the striped animals and these factors overlapped. They were able to rule out all possibilities except one: protecting against the two biting flies.
“I was amazed by our results,” said study author Tim Caro, a University of California-Davis professor of wildlife biology. “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.”
While the distribution of tsetse flies has been well-documented, the researchers used knowledge of the best breeding conditions for horseflies, creating an environmental substitute for their distributions. They found that striping is strongly connected with several consecutive months of ideal conditions for horsefly breeding.
Because zebra hair is shorter than the mouthpart length of biting flies, they are uniquely susceptible to biting flies and therefore developed the unique coloration, the study team said.
“No one knew why zebras have such striking coloration,” Caro said. “But solving evolutionary conundrums increases our knowledge of the natural world and may spark greater commitment to conserving it.”
The results of the study do beg the question: Why do biting flies avoid black-and-white surfaces? That fact was discovered in 2012 on a horsefly infested farm in Budapest.
To see what would attract the flies the most, the researchers varied the patterns on several models. Some models were solid black or white, or black and white striped squares, as well as black, brown, white, or striped life-sized plastic horses. They also used gray squares with varying amounts and widths of stripes to see how patterns affected the flies’ preference. The flies were caught in vegetable oil as they landed on the various experimental squares. The researchers also coated the plastic models with clear, odorless glue to grab the flies as they landed.
The researchers found that the stripes were the least attractive pattern to the horseflies more so than the solid white surface. The striped pattern reflects light in multiple directions, potentially disrupting the polarized light beams biting flies use to find water and mud where they mate and lay eggs.