How did the zebra get its stripes? One theory holds that stripes help confuse predators. But stripes might be primarily to protect zebras from ferocious…insects. That’s according to a study in the Journal of Experimental Biology. [Ádám Egri et al., "Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes"]

Horseflies are abundant in Africa. They deliver painful bites that spread disease and distract animals from grazing. The flies use polarized light hitting water as a guide to places to mate and lay eggs. And they read such light hitting dark mammal hides as a sign of a blood meal.

Zebra embryos start out dark and develop their white stripes before birth. Could the stripes confuse flies?

The researchers went to a farm infested with horseflies, where they set up models of black and white stripes of varying angles and widths, thus changing the direction of the reflected polarized light. They tracked how the patterns affected the flies’ interest.

They then tested models of horses colored black, brown, white or zebra-striped. And again tracked the effect on flies.

Turns out that the black and white stripes on a zebra are optimal for avoiding a horsefly’s attention. Meaning that zebras may have evolved stripes to ward off disease-carrying insects, and to dine in peace.

—Cynthia Graber

[The above text is a transcript of this podcast.]