To humans, ultraviolet (UV) radiation is a menace: we cannot see it, yet it is all around us, increasing our risks of melanoma, cataracts and other ills. It is especially harmful in the upper latitudes, where a thinning ozone layer has become less and less effective at blocking the sun’s UV rays, and ice and snow reflect them back up at us. All these facts have caused biologists to wonder: How have Arctic mammals adapted to handle acute UV exposure—not only tolerating the intense light conditions at the poles, but even using it as an evolutionary advantage?
A study of reindeer has shed some light on this question. Glen Jeffrey and his colleagues at University College London and the University of Tromsø in Norway report evidence that this iconic Arctic species is not only resistant to eye damage from the intense UV rays but is also able to perceive UV light, which is invisible to all but a few mammals, such as some species of rodents, bats and marsupials. They published their findings recently in the Journal of Experimental Biology.
Being able to see UV light confers some rich benefits on the reindeer. Its primary winter food source, lichens, and the fur of its main predator, the wolf, both absorb UV light, which makes them stand out against the UV-reflecting snowy landscape.
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UV vision actually has deep roots in the mammalian family tree: hundreds of millions of years ago early mammals had a short-wave-sensitive visual receptor, called SWS1, that could detect UV rays. That sensitivity is thought to have shifted toward longer waves—away from short UV wavelengths—because mammals were mainly nocturnal and UV vision was of little use to them at night. This shared ancestral UV sensitivity may explain why a small yet diverse set of mammals has regained the ability to see UV light. If scientists can figure out how the reindeer prevent UV rays from damaging their eyes, it could lead to new ways of treating vision loss in people. The average person loses 20 to 30 percent of central photoreceptors over the course of a life, mostly attributable to light exposure. “We might be able to better deal with age-related cell loss in the retina and perhaps age-related macular degeneration,” Jeffrey says.
In the meantime, the revelation that reindeer are able to perceive UV light while also resisting damage from these powerful rays will open a new door to understanding how Arctic animals have adapted to survive in one of the earth’s most extreme habitats.