Zebra Stripes May Serve A Totally Different Purpose Than Once Thought

Zebra stripes are striking and beautiful, but what purpose do they serve?

A new study suggests the stripes help the animals fight off biting flies, which can transmit disease.

This contradicts a popular theory in recent years that the stripes serve as camouflage against large predators. As recently as last year, lab studies supported this theory.

Looking at the bold black-and-white patterns, most people would think a zebra's stripes would be a problem for the equids (horse-like animals) - literally making them a bull's-eye for predators on the dusty open African plains.

How could the stripes possibly be used as camouflage?

Bold Stripes as 'Razzle Dazzle'

A recent theory posited that the dazzling stripes serve as a motion camouflage. Papers published in the journal PLoS ONE in 2011 and 2013 suggested that the stripes help conceal moving objects, such as the zebra, and confuse predators about how fast the animal is moving, making it harder to discern the animal's outline, especially in a herd.

This dazzle camouflage was used on warships in World War I. The bold pattern on the boat shown in the picture on the right disguised the bow.

These razzle-dazzle-camouflage studies suggest that the strategy works, but they were carried out in a lab using computers and moving squares and human eyes, not animals. We can't tell whether animal eyes react to the stripes the same way.

Even if the stripes do camouflage the animals, we can't say that they developed for this purpose. Indeed, another question is why other animals don't have black and white stripes? Perhaps they serve a different purpose altogether.

Fighting Flies With Stripes

Another set of new studies provides evidence for a completely different reason behind the striped coats: fending off flies.

The idea was first put forth by researchers in the Journal Of Experimental Biology last year.

The researchers found that a zebra statue with a striped coat attracted fewer horseflies than a zebra statue with a brown, white, grey, or black coat. The narrower the stripes, the fewer flies.

The flies are attracted to light reflecting off water, which is twisted in a certain way known as "linear polarized." The researchers proposed that the black and white stripes of the zebra created an opposite polarizing effect in the flies' eyes by confusing them.

There are seven species of equids that the researchers analyzed. Three have prominent black and white stripes, like the zebra, one has stripes only on its lower legs, and the other three have gray or brown coats with no stripes. Within these species, some subspecies have more or less striping.

The new study, published in the journal Nature Communications on Apr. 1, analyzed the striping and geographic ranges of these species and subspecies, comparing them to the geographic ranges of biting flies known as tabanids, which can carry disease. They found that black and white stripes were more prevalent in zebras and related species that live in areas that were home to biting flies for more than seven months a year.

You can see in the graphic below how the striped pattern of the animals' coats (shown by the white to black dots on the inside circle) coincide with the biting-fly activity in their range (the outer circle of dots going from blue to red/orange).

They also compared the geographic ranges of these animals to ranges of predators who might be influenced by a "razzle dazzle"-camouflage technique. They didn't find any evidence that the coat serves as camouflage.