Snow looks beautiful in sunshine, softening the landscape into a uniform white blanket. This lack of visual cues can make survival more difficult, but nature finds ways of adapting.
Snow reflects ultraviolet light as well as visible. Skiers know that, as well as sunblock, they need goggles: the ultraviolet is so intense it can sunburn the cornea of your eyes, causing snow blindness. However, the reflection of UV from snow may be helpful for some animals.
Reindeer are unusual in their ability to see beyond the normal spectrum into the ultraviolet, and researchers have speculated why they evolved this capacity. One suggestion was that it helped them spot polar bears.
Animal fur does not reflect ultraviolet, so a bear would appear clearly as a dark spot against snow, even when its white-on-white camouflage made it invisible to humans.
Researchers in Norway, working with colleagues at Moorfields Eye Hospital and the Institute of Ophthalmology in London, may have discovered a more significant use for reindeer’s ultraviolet vision. They took pictures of vegetation in snowy fields under different conditions, with and without a UV filter. They found that the ultraviolet filter greatly increased the contrast between lichen or leaves and the snow. This applies particularly in the near-darkness that characterises long stretches of the polar winter.
It seems that UV vision is helpful for reindeer seeking forage in the dim Arctic winter, helped by what the researchers poetically describe as “ultraviolet-enriched twilight luminance.”
