Here’s why zebras have stripes

By James Gamble via SWNS

Scientists believe they have finally discovered why zebras have unique and sharply outlined stripes: to protect them from attacks by horseflies.

A new study on the animals' fur has revealed that stark black-and-white stripes and small dark patches are particularly effective in warding off the disease-spreading insects.

Researchers from the University of Bristol deduced that thin stripes on the zebras' backs serve to minimalize the size of monochrome dark patches the flies find appealing.

The team now hope to conduct further studies which may provide answers to why natural selection has led to stripes in the horse family but not in other hooved animals.

Horseflies are often large and agile in flight, and only the female horseflies bite animals - including humans - to obtain blood.

Though their bites are generally considered unharmful to humans, their bites can transmit equine infectious anemia - also known as swamp fever - to horses, which can prove deadly.

Though it was already known that horseflies are less likely to land on striped objects, until now it was not known what about the stripes repelled them.

The study, led by Professor Tim Caro and Dr. Martin How, both of the University of Bristol’s School of Biological Sciences, attempted to find out which aspect of stripes horseflies were averse to.

The team conducted experiments by throwing blankets comprising different black-and-white patterns over the backs of horses, and filming the landing horseflies.

Caro explained: "We knew that horseflies are averse to landing on striped objects - a number of studies have now shown this.

"But it is not clear which aspects of stripes they find aversive.

“Is it the thinness of the stripes? The contrast of black and white? The polarized signal that can be given off objects?

"We set out to explore these issues using different patterned cloths draped over horses and filmed incoming horseflies."

The experiments showed that, though tabanid horseflies are attracted to large dark objects in their environment, they are less attracted to broken dark patterns.

All-grey coats were associated with by far the most horsefly landings, followed by coats with large black triangles placed in randomized positions and a black-and-white small checkerboard pattern.

The researchers also found that coats of contrasting stripes attracted few flies, whereas more homogeneous or similar stripes attracted more.

Following all experiments, Caro and his team concluded that any hoofed animal which reduces its overall dark outline will reduce the chance of a parasitical attack on its coat.

The team found little extra evidence of other tested issues; such as polarization or optical illusions affecting horsefly landings.

Caro added: “This suggests that any hoofed animal that reduces its overall dark outline against the sky will benefit in terms of reduced ectoparasite attack.”

However, Caro's team now want to explore why natural selection has led to striping among equids - the horse family - but not other hoofed animals such as camels, giraffes, deer, sheep, goats and cattle.

Caro added: "We know that zebra pelage – or fur - is short, enabling horsefly mouthparts to reach the skin and blood capillaries below, which may make them particularly susceptible to fly annoyance.

"But more important, perhaps, is that the diseases that they carry are fatal to the horse family but less so to ungulates, a family consisting mainly of large mammals with hooves.

"This needs investigation.”

Caro's study was published in the Journal of Experimental Biology.