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Stripes deter tabanid horseflies from landing on zebras and, while several mechanisms have been proposed, these hypotheses have yet to be tested satisfactorily. In new research, scientists at the University of Bristol investigated three possible visual mechanisms that could impede successful horsefly landings (aliasing, contrast and polarization), but additionally explored pattern element size employing video footage of the insects around differently patterned coats placed on domestic horses. They found that horseflies are averse to landing on highly but not on lightly contrasting stripes printed on horse coats.
Over the past decade, evidence for zebra striping being an adaptation to thwart biting fly attack has continued to grow, while suggestions that stripes confuse predators, are a form of camouflage or are a thermoregulatory mechanism all lack empirical support.
In brief, zebra stripes reduce landings of horseflies based on experimental studies with striped artificial targets, horse models, human models, painted cows and comparisons of live plains zebras (Equus burchelli) with domestic horses, and there is a co-occurrence of horsefly annoyance and striping in wild equids (horse family).
In contrast, observational studies of zebras fleeing do not support a confusion effect, stripes can only be resolved by predators at close distances undermining camouflage ideas and experiments with striped objects find no support for a cooling effect.
Nonetheless, the mechanism by which stripes deter biting flies from landing is still poorly understood and lack of knowledge of a mechanism can reduce the credibility of trait function.
“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,” said University of Bristol’s Professor Tim Caro, first author on the study.
“Is it the thinness of the stripes? The contrast of black and white? The polarized signal that can be given off objects?”
“So we set out to explore these issues using different patterned cloths draped over horses and filmed incoming horseflies.”
In the study, Professor Caro and his colleagues found that tabanid horseflies are attracted to large dark objects in their environment but less to dark broken patterns.
All-gray coats were associated with by far the most landings, followed by coats with large black triangles placed in different positions, then small checkerboard patterns in no particular order.
In another experiment, they found contrasting stripes attracted few flies whereas more homogeneous stripes were more attractive.
“This suggests that any hoofed animal that reduces its overall dark outline against the sky will benefit in terms of reduced ectoparasite attack,” Professor Caro said.
The authors found little evidence for other issues that they tested, namely polarization or optical illusions confusing accurate landings such as the so-called ‘wagon-wheel effect’ or ‘the barber-pole effect.’
Now, they want to determine why natural selection has driven striping in equids but not other hoofed animals.
“We know that zebra pelage 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. This needs investigation.”
The study was published in the Journal of Experimental Biology.
Tim Caro et al. 2023. Why don’t horseflies land on zebras? J Exp Biol 226 (4): jeb244778; doi: 10.1242/jeb.244778