In the Lord of the Rings, Gandalf rides upon a magnificent white stallion called Shadowfax. White horses have been greatly prized in human societies as a sign of wealth and dignity, largely because their bright coats are both pretty and rare. There are reasons for that. In the wild, the same conspicuousness that inspires legendary tales also makes white horses vulnerable to predators and sensitive to skin cancer. But they have an unexpected benefit - they make horses less attractive to horseflies.
Anyone who has been bitten by a horsefly (formally, a tabanid) knows that they're much more irritating than your average midge or mosquito. Rather than puncturing skin, their mandibles are designed to rip and shear. As a result, their bites hurt and they can drive grazing animals to distraction. They can also transfer serious diseases, including Equine Infectious Anaemia, parasitic worms, and even anthrax.
Now, Gabor Horvath from Eotvos University, Hungary, has found that white coats are more horsefly-proof than darker ones. They reflect very little polarised light - light vibrating on a single plane - and it's this light that horseflies use to track down their next blood meal.
On a sunny June day, Horvath watched two horses - one brown and one white - as they grazed in a local field. Both were almost continuously attacked by horseflies and had to defend themselves by tail-swishing, kicking, shuddering, head-swinging, biting, licking and even rolling on the group. But the white horse had the better time of it - photographs revealed that, on average, the brown horse had 3.7 times more horseflies on or near it. Eventually, the attacks were so irritating that the horses were driven into a nearby shady forest, where they gained a temporary respite. Again, the brown horse was always the first to cave and spent longer in the shade.
Why did the flies prefer the brown horse and shun the white one? It wasn't to do with colour or brightness. When Horvath put up two pieces of matte cloth, one brown and one white, he found that both were unappealing to horseflies. It was only when he half-covered the brown cloth with a shiny, transparent plastic sheet did the flies start flocking.
Horvath says previous studies have shown that horseflies are attracted to polarised light. In his experiment, the shiny, brown surface reflects plenty of polarised light, vibrating in the horizontal place, regardless of the angle it's viewed from. Neither of the matte surfaces do this.
To demonstrate this even more clearly, Horvath built three life-size horse models, one brown, one black and one white, and all covered in a sticky coating. Using a polarimeter, he measured the polarised light bouncing off these models and showed that the stronger those reflections, the more flies became stuck to them. Even the parts of the models that reflected the most polarised light - the rump and hindquarters - attracted the most flies.
Horvath says that the horsefly's proclivity for horizontally polarised light probably has several functions. It could lead them to water sources where they can lay their eggs. And horseflies are usually dark and drab in colour, so landing on a white body could make them more conspicuous to hungry birds.
Regardless of the fly-proofing provided by white coats, it's clear that for wild horses, this benefit doesn't outweigh the many drawbacks. Otherwise, white horses would be far more naturally common. However, some grazing animals like the zebra and okapi are striped, and all individuals carry some degree of white banding. That hasn't escaped Horvath's attention and his paper ends on a tantalising note:
"We would like to mention that we are preparing a separate paper on the effect of stripes on host animals like zebra, okapi, etc. to the attraction of tabanid flies. We show that such stripes make the host quite tabanid-repellent."
Reference: Horvath et al. 2010. An unexpected advantage of whiteness in horses: the most horsefly-proof horse has a depolarizing white coat. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2009.2202
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As a counter-example, I can remember a town near the Argentina-Chile border, famous for its horse flies around December. Huge things, they drew blood when they bit you. But if you wore white, you were OK. Anyone in a black T-shirt had nightmares - you could see them from miles away, going frantic, trying to evade the things. As soon as you went in the shade, they stopped following.
Isn't that a supporting example? White = fewer horseflies. What am I missing?
If, unlike a King, you groom your own horse every day (and especially want it to look good for horse shows), you don't want a grey/white horse - they are such a pain to keep clean and pretty. Which may explain why they are prized - the person who has a white horse also shows off he has money to hire a groom.
Sorry, Ed, I read the article as saying that the colour itself didn't matter - it was the polarisation. I can't see your average T-shirt cloth reflecting polarised light...
In popular mythology, the Camargue horses (live in swampy areas) are white, as any brown horse would be eaten to death by insects. So the old wives tale is true!
I would like to address your assertion that white horses are more prone to skin cancer.
Horses with white hairs and black skin are considered to be "grey." Truly white horses are white-haired and pink-skinned. Truly white horses do have an increased risk of developing squamous cell carcinoma due to sun exposure. Grey horses, on the other hand, are at an increased risk of developing melanoma unrelated to sun exposure. Grey horses are actually better off than their black-haired black-skinned counterparts in terms of sun exposure since their white hairs reflect more radiation than do darker hairs. Both grey and black horses are better off than truly white horses.
Grey horse melanoma is less like to be malignant than the melanoma occurring in non-grey horses and appears to be caused by some underlying genetic problem with the melanocytes.