Why’d the zebra evolve its stripes? Perhaps because stripes seem to keep off horseflies, a new study suggests. There’s good evolutionary reason to escape the ravages of horseflies, at least for horses and their relatives; though flies are just annoying pests from the human perspective, horsefly-bitten horses can grow skinny and have trouble producing milk for their young. And as soon as baby-making is affected by something in the environment, adaptation isn’t far behind.
Other research has shown that horseflies prefer to land on black horses instead of white, which got Gabor Horvath, author of the recent study, thinking about how they’d react to black-and-white specimens, such as zebras. Of course, actual zebras can be hard to experiment on, as The Economist notes in an article on the research:
[Real zebras] insist on moving around and swishing their tails. The team therefore conducted their study using inanimate objects. Some were painted uniformly dark or uniformly light, and some had stripes of various widths. Some were plastic trays filled with salad oil (to trap any insect that landed). Some were glue-covered boards. And some were actual models of zebra. They put these objects in a field infested with horseflies and counted the number of insects they trapped.
The first thing they found was that just as light patches attracted fewer flies than dark, striped patches attracted even fewer than uniform colors. And the striped pattern that had the fewest flies had stripes the width of zebra stripes. This suggests that zebra stripes may be especially good at keeping the flies off. But why is that? The Economist again:
Dr Horvath thinks it might be related to a horsefly’s ability to see polarised light, which imposes a sense of horizontal and vertical on an image. Horseflies are known to prefer horizontal polarised light. Possibly, the mostly vertical stripes on a zebra confuse the fly’s tiny brain and thus stop it seeing the animal.
So for some creatures, horseflies included, orienting stripes a certain way can make things invisible. Makes you wonder, what out there is invisible to us, because of our own sight limitations? We already know that many organisms can see ultraviolet light, inhabiting a world with more colors than we do and lots more unusual bird and butterfly patterns. Additionally, as optical illusions prove, we see all kinds of stuff that isn’t really there.
Though it’s hard to feature missing a giant mammal in your path, science has shown that humans as well as flies can do it (though for different, more complicated reasons). Try it for yourself here.
Image courtesy of chaouki / flickr