The University of Bristol researchers have uncovered the reason behind zebra fur being thinly striped and clearly defined.
A photograph of a zebra
Their findings, published in the Journal of Experimental Biology, reveal that the strong black-white contrast and small dark patches in zebra fur are particularly effective in deterring horsefly attacks. These specific characteristics eliminate the outline of large monochrome dark areas that are attractive to horseflies at close range.
A photograph of a zebraThe team theorizes that the thin back stripes serve to minimize the size of local features on a zebra that are appealing to the biting flies.
The research was led by Professor Tim Caro and Dr. Martin How both from the University of Bristol’s School of Biological Sciences.
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| Horse with a patterned blanket. Credit: Martin How |
Prof 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? So we set out to explore these issues using different patterned cloths draped over horses and filmed incoming horseflies.”
The team found that tabanid horseflies are attracted to large dark objects in their environment but less to dark broken patterns. All-grey 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.
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| A researcher observes a horse with a patterned blanket. Credit: Martin How |
Professor 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.”
The team 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 the team wants to determine why natural selection has driven striping in equids – the horse family -but not other hoofed animals.
Professor Caro added: “We know that zebra pelage – 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. This needs investigation.”
Reference: “Why don’t horseflies land on zebras?” by Tim Caro, Eva Fogg, Tamasin Stephens-Collins, Matteo Santon and Martin J. How, 17 February 2023, Journal of Experimental Biology.