Humans can only see light at wavelengths from 400 nm (violet) to 700 nm (red). Shorter wavelengths (ultraviolet) are invisible to us. Not so for reindeer. According to research led by Glen Jeffrey of Moorfields Eye Hospital, reindeer can see in the ultraviolet.
Our corneas and lenses block UV light from entering our retinas. Excessive amounts of UV light absorption by the cornea results in a painful condition known as photokeratitis, or snow blindness. As the name implies, this condition is common in regions with large amounts of snow, which can reflect up to 90% of UV radiation back into the eyes. Coincidentally, these are also regions where reindeer live.
Perhaps uniquely among mammals, reindeer corneas do not block UV. Rather, that UV light is allowed to pass into the reindeer retina where it triggers the photoreceptors. Reindeer can respond to light at wavelengths down to 320 nm.
It has long been assumed that UV rays would permanently damage the mammalian retina, and that the temporary sacrifice of the cornea to snowblindness was a way to prevent that damage. This reindeer data calls that assumption into question. The reindeer don’t experience snowblindness nor does UV light damage their retinas.
Why do reindeer have the ability to see UV light? Here are a few items that absorb UV radiation and thus would stand out blackly on a snowy field—if anyone could see them: urine, fur and lichens. The former two items could indicate predators or rivals, the latter, lunch.