White horses are more susceptible to skin cancer and predation than their darker kin, but their coats seem to protect them from another danger: pathogen-bearing horseflies.
Gábor Horváth at Eötvös University in Budapest and his colleagues tracked the landing frequency of horseflies on horses of different colours. The flies preferred black and brown horses. When the team covered horse models in transparent glue, more than 15 times as many horseflies stuck to dark models as to white ones.
The authors found that, unlike white horses, dark-coloured horses reflect polarized light, which horseflies can detect. A brown matt cloth attracted horseflies only if it was covered by a transparent, light-polarizing sheet, demonstrating that polarized light, not dark colour, draws the flies.
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Ecology: why horses wear white. Nature 463, 852 (2010). https://doi.org/10.1038/463852a
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DOI: https://doi.org/10.1038/463852a
