Arctic reindeer can see beyond the "visible" light spectrum into the ultra-violet region, according to new research by an international team.
They say tests on reindeer showed that the animal does respond to UV stimuli, unlike humans.
The ability might enable them to pick out food and predators in the "UV-rich" Arctic atmosphere, and to retain visibility in low light.
Details are published in the The Journal of Experimental Biology.
UV light is invisible to humans. It has a wavelength which is shorter (and more energised) than "visible" light, ranging from 400 nanometres down to 10nm in wavelength.
The researchers first established that UV light was able to pass through the lens and cornea of the reindeer eye by firing light through a dissected sample. The tests showed that light down to a wavelength of about 350nm passed into the eye.
"Humans and some other mammals are actually a minority in not having UV sensitivity”
Professor Lars Chittka Queen Mary University London
They then sought to prove that the animals could "see" the light, by testing the electrical response of the retina of anaesthetised reindeer to UV light.
"We used what is called an ERG (electroretinography), whereby we record the electrical response to light by the retina by putting a little piece of gold foil on the inside of the eyelid," co-author Professor Glen Jeffery of University College London told BBC News.
The tests showed that photoreceptor cells or "cones" in the retina did respond to UV light.
"If you're a bumblebee, you wouldn't think much of what this animal is doing because it's seeing in what's called 'near UV' (about 320 to 400nm), but that's still very high energy stuff."
The researchers believe UV vision could enable the reindeer to distinguish food and predators in the "white-out" of the Arctic winter and the twilight of spring and autumn.
Lichen, on which the animal feeds, would appear black to reindeer eyes, they say, because it absorbs UV light. The animal's traditional predator, wolves, would also appear darker against the snow, as their fur absorbs UV light.
Urine in the snow would also be more discernable in UV vision, which might alert reindeer to the scent of predators or other reindeer.
Neither did the animal appear to suffer any damage as a result of seeing in UV, say the researchers, or suffer the "snow blindness" humans can experience in the UV-rich Arctic environment.
Professor Lars Chittka of Queen Mary University London, who has explored the UV capabilities of bees, said the study showed what we call the "visible" spectrum did not apply to most of the animal kingdom.
"It's further evidence that UV sensitivity across animals is the rule rather than the exception, and that humans and some other mammals are actually a minority in not having UV sensitivity," he said.
Professor Chittka was not surprised the UV light appeared to do no damage to the reindeer retina. He said the tests suggested the eye would only admit lower-frequency UV light ("UV-A light") rather than more damaging higher-frequency light ("UV-B").
Further modelling and behavioural tests would also be needed to verify that reindeer's apparent capacity to detect UV light really did result in "better detection of predators and arctic lichens", he said.
The same research team which conducted the reindeer tests will soon repeat the same experiments on seals to see whether they can see into the UV region. Professor Jeffery believes many Arctic animals are likely to have the capacity.
"There's no evidence that Arctic foxes or polar bears suffer from snow blindness, so I bet you that most of the Arctic animals up there are seeing into UV."
BBC News - Tests show Arctic reindeer