Like flies to polarised light

White horses may seem to be at an evolutionary disadvantage: they are easily spotted by predators and suffer from higher incidences of skin cancers and eye problems due to their greater sensitivity to UV light than darker coloured breeds. However, scientists have recently discovered an unexpected advantage due to their optical properties.

The coats of white horses reflect light with lower degrees of polarisation than that of brown or black horses. This observation is significant as tabanid flies, blood-sucking insects which can transmit diseases, are strongly attracted to horizontally polarised light. Researchers tested this hypothesis with a number of experiments, including one in which they built models of horses of different colours that they coated with a sticky substance. The brown and black models collected 15 and 25 times more tabanids respectively in comparison to the sticky white horse model.