Why do Flies Avoid Zebras yet Love Horses?

Scientists have long been trying to figure out why Zebras have stripes. There’s been a lot of speculation/testing of theories related to confusing predators, social purpose, or heat regulation, but none of those have really panned out.

Then a study on Tsetse flies in Zimbabwe in 1992 came to the conclusion, that “the striped pattern of zebras may protect them from being bitten by blood-sucking flies.”

There was another study in 2012 in Hungary which showed that the width of the stripe is of primary importance to generate the required “differential brightness and polarization of reflected light” that confuses the tabanids (horse flies). Here’s what the different types of zebra stripes look like per species and the width of the stripes – note how the stripe width changes on different parts of the body:

They did a number of experiments to see which patterns (vertical vs horizontal) and size of stripes worked best to deter the flies: “We show that the attractiveness to tabanids decreases with decreasing stripe width, and that stripes below a certain size are effective in not attracting tabanids.” Translation: The narrower stripes repel more flies than the wider stripes.

If you look at the Red Circle line, you can see that vertical stripes work much better than horizontal stripes, and the narrow width stripe attracted the least flies.

Forget models, what about live horses & zebras?

So what happens when you test out this theory using live zebras and live horses, and then live horses camouflaged as zebras? That’s the study that was published this week by the University of Bristol and UC Davies in California.

Their findings mirrored the Hungarian study, as they used video analysis to show that the flies didn’t seem to be able to land properly on the zebras which “may be due to stripes disrupting the visual system of the horse flies during their final moments of approach. Stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes,” said Dr. Martin How of Bristol.

Fig 1. Examples of horsefly flight trajectories around domestic horses (a-c) and captive plains zebra (d-f).

Then they tested just horses wearing white, black, or zebra-striped coats (fly sheets) to see how many successful landings the horse flies were able to make:

Fig 2. Mean (and SE) number of tabanid flies (a) touching or (b) landing on cloth coats of different shade and pattern, and (c) landing on the bare head of 7 different horses.

You can see that the number of flies landing on the horses’ heads evened out – because the camouflage coat didn’t cover the head.

The Zebra camouflage coat (c) School of Biological Sciences, University of Bristol

So there you have it – if you’re buying a fly sheet for your horse this summer, choose the zebra pattern one! No doubt as this study gains recognition, fly sheet manufacturers will get on the bandwagon. Hopefully they read my post here and take note of the fact that narrower stripes work best. 🙂

p.s. Want to see these beauties in person, learn from the Maasai directly and spend way less than most safaris cost? Check out my friend Nelson’s nature conservancy and safari camp

Why do Flies Avoid Zebras yet Love Horses?

2 thoughts on “Why do Flies Avoid Zebras yet Love Horses?

  • February 24, 2019 at 6:01 pm

    Brilliant. Nice to have the scientific jargon interpreted in simple terms that tell us what to do for the best results.

    • February 24, 2019 at 9:59 pm

      Yes exactly! And when purchasing a fly sheet, to look for the zebra pattern with narrower stripes 🙂


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