Smart Stripes

One question that I had since I was little is "Why do zebras have stripes?" Some theories say that when a zeal of zebras are running away from a predator, such as a lion, the stripes blend into each other which prevents the lion from pinpointing its target. There are also theories that say the stripes aid in camouflage or that they help zebras recognize one another. However, one theory, proposed by scientists in Sweden and Hungary, says that the stripes were adapted to ward off the irritating horseflies in Africa. Horseflies are known for their painful, blood-sucking bites, and unfortunately I can vouch for this statement. A few summers ago, while relaxing on the beach, I felt sharp, stinging sensations on my leg. When I looked down there were 3 huge horseflies flying around my knees. Soon after, the place where the flies bit me turned very red and we had to go home. The pain was worse than the time I was stung by a bee. Since the bites are so uncomfortable, and even have the potenial to transmit disease, it is no wonder why the zebras adapted a way to repel the irksome horseflies. But how can their stripes have that ability? Well, it all has to do with the way light is reflected.

Horseflies are attracted to water because that is where they are able to reproduce and lay their eggs. Since water gives off horizontally polarized reflections, horseflies are drawn to horizontally polarized light as well because they associate it with their breeding ground. Remember that light travels in waves. So, the horseflies are really attracted to the horizontally polarized light waves.

The researchers in Sweden and Hungary did an experiment where they placed and observed life-size model horses in a field in Budapest.(There was one black model, one brown model, one white model, and one striped model to resemble a zebra) Their purpose was to test which color has a greater horsefly attraction. They found that the darker colored horses gave horizontally polarized light and the white horse gave unpolarised light. In order to see which model the Horseflies were attracted to more, they put special glue on the models and recorded the data and horsefly-count every other day. The model with the most Horseflies on it would be the most attractive and the model with the least would be the least attractive. After collecting data, the researchers were amazed to find that the striped model was the least attractive. (They thought it would be the white model) One reason for this may be because the stripes confuse the horseflies.

Researchers also studied the widths of zebra stripes by placing a black board, a white board, and a couple of striped boards with different stripe lengths into the field. Using glue and capturing the horseflies that landed on each, it was found that the striped boards attracted the lowest number of horseflies. The striped boards with the narrowest stripes, which have a strong resemblance to the stripes of a real zebra, were the least attractive of all.

Since this is just another theory, the other theories have not been proven wrong and are still possible. Experiments for this theory have not yet been conducted in Africa, so nothing is final. Please comment on which theory you think is true. Do you think this theory of repelling horseflies is a valid argument? Also consider, why didn't natural selection favor stripes in horses as well?

Sources:

"How the Zebra Got Its Stripes" ScienceDaily February 9, 2012

Victoria Gill "Zebra stripes evolved to keep biting flies at bay" British Broadcasting Corporation February 9, 2012

Horsefly Picture Credit: Vicky Austin via Flickr

Zebra Picture Credit: Safari Partners via Flickr