Image by kow loon from Pixabay
Researchers now know that the zebra’s stripes deter parasitic flies. Yet so do other patterns. Could the stripes have other uses too?
Image by kow loon from Pixabay
The puzzle of why nature gave zebras have their characteristic stripes has perplexed researchers for over a century.
In the past decade, Professor Tim Caro at the University of Bristol’s School of Biological Sciences in the UK 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 playing a role in the animal’s social interactions.
Stripes helping to confuse predators is another common explanation. But it too is flawed as it is not supported by the scientific research data.
Instead, mounting evidence suggests that it is parasitic flies that are confused by the zebra’s distinctive patterning.
In a new paper published recently in the academic journal Proceedings of the Royal Society B, Bristol University scientists have added credibility to this hypothesis by narrowing down the possible mechanism.
Their latest research follows on from a previous study, where they found that blood-sucking horseflies would approach horses draped in striped rugs as often as horses draped in plain rugs. But they had enormous difficulty landing on the striped rugs, yet no difficulty with the plain ones.
Essentially, say the researchers, stripes dazzled the flies and cause them to collide with the skin or fly away.
In their new study, they explored a possible way to explain how the stripes lead to this outcome: the so-called ‘aperture effect’.
The aperture effect is a well-known optical illusion also known as the barber-pole effect. Moving stripes, such as those on the rotating signs outside barbershops, appear to move at right angles to the stripe and so the pole seems to move upwards.
“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 [such as zebras],” explained Dr Martin How, lead author of the study.
“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.”
He continued: “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 to land successfully.”
But is turns out that, while the flies certainly had enormous difficulty landing on stripes, the stripes did not appear to cause the flies to experience an ‘aperture effect’.
Given that the researchers now know that flies struggle to land on both striped and checked rugs (but not plain ones), it is clear that patterns – including perhaps other types of patterns not yet tested – do deter flies and that zebras clearly derive benefit from this because of their unique patterned skin.
“Not only do these exciting studies bring us closer to understanding one of the world’s most iconic and photogenic [animal] species, they will be of great interest to farmers attempting to reduce the damage caused by fly bites, and even to horse-wear companies,” said Professor Caro.