Zebra stripes help keep horseflies at bay
Have you ever wondered how and why zebras got their stripes? Well, wonder no more: a team of researchers from Hungary and Sweden sheds fresh light on the stripe function and why the stripe pattern has evolved. Presented in the Journal of Experimental Biology, the study was funded in part by the TABANOID ("Trap for the novel control of horseflies on open-air fields") project, which received more than EUR 825 000 in funding under the "Research for the benefit of SMEs (small and medium-sized enterprises)" Theme of the EU's Seventh Framework Programme (FP7).
Horseflies (tabanids) are nasty little insects, biting and carrying disease, and distracting grazing animals from feeding. Dr Gábor Horváth from the Department of Biological Physics, Physical Institute, Eötvös Loránd University in Hungary, along with colleagues, investigated whether these blood-sucking insects are drawn to the black and white striped patterns of the zebra.
Dr Horváth says the bugs are attracted to horizontally polarised light because reflections from water are horizontally polarised, and aquatic insects use this phenomenon to determine the sites of where they can mate and lay eggs.
But female horseflies are also drawn to animals because of the linearly polarised light reflected from their hides. When looking at horses, researchers say horseflies favour dark hides over white ones. The developing zebra embryos start out with a dark skin, but go on to develop white stripes before birth.
The researchers postulate that the zebra's stripy hide could have evolved to alter the appearance of their attractive dark skins and make them less interesting to bloodsuckers like tabanids.
In their study, the researchers used a sample at a horsefly-infected horse farm in Budapest. They varied the width, density and angle of stripes, as well as the direction of polarisation of the light that they reflected. The team used oil and glue to trap the insects. They found that the narrower the stripe, the less attractive it is to the horseflies.
They also tested the attractiveness of white, dark and striped horse models, and found that the striped horse was the least attractive of all three types. They later measured the stripe widths and polarisation patterns of light reflected from real zebra hides, and discovered that the zebra's pattern correlated well with the patterns that were least attractive to horseflies.
"We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies," the authors write. "the selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region [i.e. Africa]."
The TabaNOid project, coordinated by MFKK Invention and Research Center Services Ltd., concluded with the development of a product that will profoundly increase the visibility of participating SMEs in the horse-fly protection business in Europe. As IPR owners, the project partners of the TabaNOid Consortium (LENIS, MEPARK, KŐRÖS-MAROS BIOFARM, JCB, EOHIPPUS and AGROPHARM) will be able to strengthen their competitiveness by producing, distributing and selling an eco-friendly and efficient horse fly control trap.
Eötvös Loránd University
Journal of Experimental Biology