Tim Caro: 'To determine which characteristics of stripes deter horseflies, we explored different patterned cloths draped over horses and filmed the horseflies' response'

Researchers at the University of Bristol have shed light on the mystery surrounding the unique striped pattern found on zebra fur. In a groundbreaking study published earlier this year in the Journal of Experimental Biology, Professor Tim Caro and Dr. Martin How from the University of Bristol's School of Biological Sciences explain why zebras sport thin stripes and sharp outlines, revealing that these specific characteristics effectively deter horsefly attacks by eliminating the attractiveness of large monochrome dark patches to the insects. The findings suggest that the thin stripes minimize appealing local features on zebras that horseflies are drawn to, offering valuable insights into the evolutionary adaptation of the horse family.

“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," Caro said in a February press release from the University of Bristol. "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."

For years, scientists have speculated on the purpose of zebra stripes, with various theories proposed. However, the team at Bristol believes they have finally unraveled the mystery by focusing on the behavior of horseflies. After conducting a series of experiments using patterned cloths draped over horses and recording the behavior of incoming horseflies, they discovered horseflies are averse to landing on striped objects, but the specific aspects of the stripes that repel the insects were unclear, according to the release.

The results of the study revealed that large, solid dark objects were highly attractive to horseflies, while dark, broken patterns were less appealing. Further, contrasting stripes attracted fewer flies, whereas more homogeneous stripes were more attractive. The team concluded that the thin black-white stripes and sharp outlines found on zebra fur minimize the size of local features that horseflies find appealing, effectively thwarting their attacks. By reducing the overall dark outline against the sky, hoofed animals like zebras can significantly reduce the risk of ectoparasite infestation, according to the release. 

Despite significant findings, questions still remain. The researchers plan to investigate why natural selection has driven striping in equids, such as zebras, but not in other hoofed animals. Additionally, the team aims to explore why zebra pelage, or fur, is particularly short, making them susceptible to fly annoyance and the diseases they carry, according to the release. The groundbreaking research conducted by Professor Tim Caro, Dr. Martin How and their team at the University of Bristol provides valuable insights into the evolutionary adaptation of zebras and the fascinating ways in which animals develop unique defenses against predators and parasites.