Here’s a question that has puzzled some scientists for more than 100 years: why do zebras have stripes?
Well, the thing is, up until now, no-one has been able to come up with a definitive answer. At least, not
an answer that fits with all the facts. But researchers at the United Kingdom’s University of Bristol think they might finally be on the right track.
When it comes to theories about zebras and their stripes, Professor Tim Caro, from the University of
Bristol’s School of Biological Sciences thinks he’s heard them all – everything from effective camouflage
to ward off potential predators such as lions and other big cats; a sophisticated biological adaption to
help zebras cool off in the blazing African heat, and even as a form of attraction and bonding during
mating and other social interactions among the members of the herd. But, he says, as logical as some of them sound, not one of these theories quite cut the mustard when it comes to examining the scientific data.
But now Professor Tim has come up with a theory of his own. He says he has discovered mounting
evidence that suggests the reason comes from a defensive response to the millions of annoying and
potentially dangerous parasitic flies that inhabit the plains of Africa.
It seems that the zebra’s distinctive black and white patterning utterly confounds the parasitic flies,
confusing them so much that instead of landing and biting the host, they crash into the zebra’s hide and
bounce off before buzzing off to find an easier meal.
In a new paper published today in the journal Proceedings of the Royal Society B (Yes, the Royal Society
of Biological Sciences really has a journal called the Proceedings of the Royal Society B) professor caro
and his fellow Bristol scientists have now provided significant depth to this hypothesis by narrowing
down the possible mechanism at work behind it.
Previously, the same researchers had shown that blood-sucking horseflies would approach horses in
striped rugs as often as plain rugs, but then failed to land or slow down when they got close.
Essentially, stripes dazzled the flies, forcing them to collide with the skin or fly away altogether. In their
new study they explored a potential mechanism explaining how the stripes lead to this outcome: the
Lead author Dr Martin How, also from Bristol’s School of Biological Sciences, said: “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
“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.”
Despite its appeal to visual ecologists, this research found that the aperture effect is not the mechanism
behind fly confusion by comparing fly landings on horses wearing striped versus checked rugs.
Since checked rugs provide visual input free from the aperture effect, one would expect flies to land on
them without difficulty. Yet flies had real difficulties with this pattern – hardly landing on checked or
striped rugs at all. Thus, stripes themselves are not unique to deterring tabanid horseflies, other
patterns can be effective too.
Professor Caro, this 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.”