Zebra stripes are supposed to provide camouflage, visually confuse predators, signal to other zebras, or help control heat gain, but none of these hypotheses have withstood rigorous experimentation. An alternative - that stripes somehow reduce the likelihood of being bitten by predatory flies - has gained adherents but, until now the mechanism has been unclear.
It turns out that zebra stripes reduce horse flies' ability to make a controlled landing!
In this new, open access study by Tim Caro of the University of California Davis, Martin How for the University of Bristol and their colleagues compared the behaviour of horse flies (sometimes known as March flies) as they attempted to prey on zebras and uniformly coloured horses (white, grey, brown and black), held in similar enclosures.
The flies circled and touched horses and zebras at similar rates, but actually landed on zebras less than one-quarter as often. When horses were fitted with a striped, black and white rug, flies landed far less often on the striped coat, but just as often on the uncovered head.
The authors found that while flies decelerated prior to landing on horses, they approached zebras at a faster clip and failed to slow down as they closed the distance, often bumping into the zebra before flying away again. Additionally, zebras were at greater pains to keep flies off through tail swishing and running away.
Taken together, these results indicate that stripes do not deter horse flies from approaching zebras, but do prevent effective landing, and thus, reduce the number of flies successfully feeding. This finding provides further support for the hypothesis that the evolutionary benefit of zebra stripes is to reduce biting by predatory flies.
The authors add: "Zebra stripes are now believed to have evolved to thwart attack by biting flies. We observed and filmed the behaviour of horse flies near captive zebras and horses and found that flies failed to decelerate close to stripes preventing controlled landings. Combined with zebras' anti-parasite behavior, few flies landed successfully or probed their hosts for blood."
So, if horse flies are biting your horses, consider fitting a zebra-striped rug for protection - possibly including a hood.
Always remember that horses can easily overheat under rugs so make sensible decisions in your horse's best interest.
Tim Caro, Yvette Argueta, Emmanuelle Sophie Briolat, Joren Bruggink, Maurice Kasprowsky, Jai Lake, Matthew J. Mitchell, Sarah Richardson, Martin How. Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses. PLOS ONE, 2019; 14 (2): e0210831 DOI: 10.1371/journal.pone.0210831