Zebra stripes have attracted the attention of scientists (and nature lovers, in general) for decades. One of the hypotheses put forward for its existence was that the unequal combination of white lines on a black background in the body of these equines (of which there are three different species) was an evolutionary result that offered them a slight defense against large predators; a partial camouflage against the attack of lions, for example.
For a few years, however, this possibility seems to have been ruled out, as indicated by the results of a study published in the journal PLOS One in January 2016 by researchers from the United States, Canada, and Japan. Evidence has not been found either that the stripes that give their name to a well-known traffic sign help these African animals in the face of heat waves, as indicated by another hypothesis.
On the other hand, more recent scientific work, such as the one published in 2019 by experts from the University of Budapest (Hungary) in the Royal Society Open Science journal, point to the role of stripes as a repellent for biting insects such as horseflies as more likely.
Now, a curious study led by researchers from the University of Bristol (United Kingdom) shows a possible explanation for the antiparasitic effect of zebra stripes. Specifically, after various tests with models and simulations (it cannot be denied that the images are curious), experts believe they have discovered that the key is that the zebras' fur has fine stripes and a sharp outline, a visual characteristic that repels (or at least, it does not attract) these relatives of flies that are characterized by causing strong bites even on the most resistant skin.
The results of this new research have been published in the Journal of Experimental Biology (January 17), revealing that sharp distinctions between black and white and small dark spots are particularly effective in thwarting horsefly attacks. These features specifically eliminate the outline of large dark monochromatic spots that attract horseflies at close range.
The research was led by Professors Tim Caro and Martin How, both from the University of Bristol's School of Biological Sciences. "We knew that horseflies are reluctant to land on striped objects; several previous studies have shown this, but it was not clear what aspects of stripes are aversive to them," Tim Caro details in a note released by his university.
"Is it the thickness of the stripes? The contrast between black and white? The polarized signal that objects can emit?" were some of the questions posed by these experts before starting to investigate using different patterned fabrics with which that horses were covered, to see how the horseflies responded.
The team found that these insects are attracted to large, dark animal-like objects and are less likely to approach uneven or contrasting color patterns.
In trials with costumed horses, the most attractive cloaks to horseflies were all-grey, followed by those featuring large black triangles placed in different positions, and finally, cloaks with small patterns resembling jumbled chessboards. In another experiment, the researchers found that high-contrast stripes were less attractive to horseflies, while smoother, smoother stripes were more attractive.
The team led by Caro and How believe that zebra stripes reduce insect attacks by blurring their overall body outline, especially when the white lines contrast sharply with the black ones.
On the other hand, they did not observe any relationship with the possible effect of light polarization or optical illusions such as those known as the wheel cart effect or the barber effect (from the old moving signs of barbershops).
The authors indicate that, for the moment (they would need more studies, as they say in these cases), they cannot propose any hypothesis to explain the fact that zebras are one of the few equids that have stripes, almost all of them being destined for Prefers blood-sucking Diptera.
