This Is Why … Zebras Have Stripes

(“IMG_2095” by expat696969 is licensed under CC0 1.0)

Ok, I’ll admit that I was more than a little skeptical when my 15-year-old daughter asked me to buy a zebra-print flysheet for her horse. I may even have scoffed, saying something along the lines of “Why would you want to make your horse look so silly?” When Hannah replied: “Because – SCIENCE!”, I knew that I had to do a little digging. Well played, Young Grasshopper, well played.

Imagine my surprise when I learned that, as recently as 2012, the purpose of zebra stripes was still a highly-contested issue. Such a simple question, asked by biologists for centuries (including Charles Darwin), with no definitive answer. Sure, there were plenty of theories (camouflage, confusion of predators, social interaction, heat control, sign of overall health, protection from insects), but nothing had been experimentally confirmed.  

And then along comes a team of scientists from Hungary and Sweden, conducting field experiments to test the theory that stripes protect zebras from biting, disease-carrying insects. Starting with pans of oil with different black and white patterns at the bottom, and then moving on to 3D animal models covered in mouse-trap glue, there was a significant difference in the number of horseflies collected for stripy surfaces versus solid. Of the four “horses” tested, the black one collected 61% of the flies, the brown horse accounted for 36%, followed by the white model at a little more than 2% of the total. The model that was painted like a zebra collected less than 1% of all the flies stuck to the gluey surfaces!

Our Hungarian-Swedish crew returned to the same Hungarian horse farm a few years later. This time, however, they brought mannequins that were either brown, beige, or brown with white stripes, to test whether body painting typically seen in tribes living in Africa, Australia, Papua New Guinea, and North America deterred horseflies. Their results, again, were compelling. The brown mannequin was 10 times more attractive to horseflies than the body-painted model, whereas the beige figure was twice as attractive as the one with stripes. It might be time to outfit the family in stripes for our next spring fishing trip into the backcountry of Algonquin Park!

From https://royalsocietypublishing.org/cms/asset/eb95900e-83f8-452b-9ed6-66cf4e4bd563/rsos181325f03.jpg

But why? Why would a horse or a person that is half black and half white (approximately) attract significantly fewer horseflies compared to the same models that were either all black or all white? The answer to this was addressed by another team led by Tim Caro from University of California, Davis. By painstakingly observing horses and zebras grazing in adjacent paddocks in Somerset, England, they concluded that the stripes confuse the poor little flies when they get close – they tend to come in too fast, land less often, and sometimes just bounce off haphazardly – compared to the more frequent successful landings on their non-stripy neighbours. (Get it? “Neigh-bours”)

The Caro team also covered the horses in three different sheets: one black, one white, and one zebra-stripy (just like the one Hannah wants to buy) to control for everything except the coat colour. A team of grad students was sent into the fields to stand there and count fly landings for hours on end. Their graph says it all – there was a huge difference when the horses were dressed up like zebras except on their heads, which were not covered with the stripy fabric.

These flying pests are looking for their next snack based, in part, on a specific property of the light when it bounces off a surface, whether it’s water, a blade of grass, or a zebra-striped sticky mannequin out standing in its field. Light that bounces off dark surfaces like a black stallion is highly polarized, which attracts the flies, whereas the light reflected from white ponies is hardly polarized at all. The mixture of highly polarized and not polarized light from the zebra stripes seems to be the cause of all the confusion.

(Hannah’s horse (Ella), on the left, looking super suave in her new flysheet)

So, science wins and Hannah gets a zebra-print flysheet. A centuries old mystery is solved and it’s not at all about camouflage as Rudyard Kipling would have had us believe all those years ago:

Zebra to The Ethiopian and the Leopard, from inside some little thorn bushes where the sunlight fell all stripy: “And where is your breakfast?” But all they could see was stripy shadows in the forest. (in How The Leopard Got His Spots, Just So Stories, Rudyard Kipling, 1902)

References:

Benefits of zebra stripes: behaviour of tabanid flies around zebras and horses, Tim Caro et al, PLoS ONE 14(2): e0210831 (2019) https://doi.org/10.1371/journal.pone.0210831

Striped bodypainting protects against horseflies, Gábor Horváth et al, R. Soc. Open Sci. (2019) 6: 181325 http://dx.doi.org/10.1098/rsos.181325

Stripes disrupt odour attractiveness to biting horseflies: Battle between ammonia, CO2, and colour pattern for dominance in the sensory systems of host-seeking tabanids, Miklós Blahó et al, Physiology & Behavior (2013) 119 168 – 174 https://doi.org/10.1016/j.physbeh.2013.06.013

Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes, Ádám Egri et al, Journal of Experimental Biology (2012) 215 736 – 745 https://doi.org/10.1242/jeb.065540

Published by joanneomeara

Professor, Department of Physics, University of Guelph

2 thoughts on “This Is Why … Zebras Have Stripes

  1. A horse in the field near us has a zebra striped cover! I thought it was horse-fashion. So really great to hear that there is science behind it!

    Like

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