The mystery of why zebras have their stripes has baffled scientists – now a dazzling answer

The mystery of why zebras have their characteristic stripes has puzzled researchers for over a century.

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

Stripes acting to confuse predators are another common explanation, but they’re also wrong when looking at the science. Instead, a growing body of evidence suggests that it’s the parasitic flies that are confused by the zebra’s distinctive patterns.

In a new article published today in the journal Proceedings of the Royal Society B, 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 approach horses in striped rugs as often as single rugs, but fail to land or slow down as they approach.

The mystery of why zebras have their characteristic stripes has puzzled researchers for over a century. Credit: Amelia Gillard, University of Bristol

Essentially, the stripes dazzled the flies, forcing them to either collide with the skin or fly away completely. In their new study, they explored a potential mechanism explaining how scratches lead to this result: the openness effect.

Lead author Dr Martin How, also from Bristol School of Biological Sciences, said: “The aperture effect is a well-known optical illusion which, in human vision, is also known as barber effect name. Movable bands, such as those on rotating barber panels outside barber shops, appear to move at right angles to the band, rather than in their true direction, so the post appears to be moving toward up, rather than around its axis.

“We looked to see if this illusion also takes place in the eyes of biting flies that land on striped hosts.

“When a fly approaches a landing surface, it will adjust its speed based on how quickly the surface expands in its vision, allowing for a slow, controlled landing.

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

Three zebras drinking. Credit: Amelia Gillard, University of Bristol

Despite its appeal to visual environmentalists, this research found that the openness effect is not the mechanism behind fly confusion when comparing fly landings on horses wearing striped rugs and checkered rugs.

Since verified rugs provide visual entry with no opening effect, one would expect flies to land on them without difficulty. Still, flies had real difficulty with this pattern – barely landing on checkered or striped rugs. So the stripes themselves are not unique in deterring tabanid horseflies, other patterns can be effective as well.

Professor Caro, co-lead author of this article, added: “These exciting studies not only bring us closer to understanding one of the world’s most iconic and photogenic species, but they will be of great interest to them. farmers who are trying to reduce fly damage. bites and even horse clothing businesses.

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Reference: August 19, 2020, Proceedings of the Royal Society B.

This study was funded by the Institute for Advanced Studies at the University of Bristol and the Royal Society.