Zebra stripes and the aperture effect

Researchers have long pondered over the reason for zebra’s stripes.

Over the past decade, Professor Tim Caro at the University of Bristol’s School of Biological Sciences has examined and discredited many popular theories such as their use as camouflage from predators, a cooling mechanism through the formation of convection currents and a role in social interactions.

Stripes acting to confuse predators is another common explanation, but it too is flawed when looking at the scientific data. Instead, mounting evidence suggests that it is parasitic flies that are confounded by the zebra’s distinctive patterning.

In a new paper published in the journal Proceedings of the Royal Society B, the Bristol scientists have now provided significant depth to this hypothesis by narrowing down the possible mechanism.

Previously, the same researchers had shown that blood-sucking horseflies would approach horses in striped rugs as often as plain rugs, but then fail to land or slow down when they got close.

It seems the stripes dazzled the flies, forcing them to collide with the skin or fly away altogether. In their new study, the research team explored a potential mechanism explaining how stripes could lead to this outcome: the “aperture effect.”

Lead author Dr Martin How, also from Bristol’s School of Biological Sciences, explains: “The aperture effect is a well-known optical illusion that, in human vision, is also known as the barber-pole effect. Moving stripes, such as those on the rotating barber-pole signs outside barbershops, appear to move at right angles to the stripe, rather than in their true direction, so the pole appears to move upwards, rather than around its axle.

“We set out to see if this illusion also takes place in the eyes of biting flies as they come to land on striped hosts.

“As any fly approaches a landing surface, it will adjust its speed according to how quickly the surface expands across its vision, enabling a slowed and controlled landing.

“Stripes however could disrupt this ‘optic flow’ through the aperture effect, leading the fly to believe the landing surface is further away than reality. Thus, the fly fails to slow down or land successfully.”

This latest study assessed flies landing on horses wearing striped, checked, or plain grey rugs. The researchers found that flies hardly landed on checked or striped rugs at all.

The researchers explain that, since checked rugs provide visual input free from the aperture effect, one would have expected flies to land on them without difficulty. But they didn’t.

So, it is not only stripes that deter tabanid horseflies, other patterns can be effective too. As the “aperture effect” would not apply to the checked pattern, the researchers could discount it as the mechanism behind fly confusion.

Professor Caro, the paper’s senior co-author, added: “Not only do these exciting studies bring us closer to understanding one of the world’s most iconic and photogenic species, they will be of great interest to farmers attempting to reduce the damage caused by fly bites and even general horse-wear companies.”

For more details, see:

Zebra stripes, tabanid biting flies and the aperture effect

Martin J. How, Dunia Gonzales, Alison Irwin and Tim Caro

Proceedings of the Royal Society B Volume 287 Issue 1933

https://doi.org/10.1098/rspb.2020.1521