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.

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.

7 Comments

1. 1. Clyde Spencer 09:38 PM 7/23/11

   Everyone is familiar with the so-called "ozone hole" in Antarctica. However, I believe the statement made by Ms. Hodges, "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,...", is inaccurate. Can Scientific American provide a citation that demonstrates that there is a trend of increasing surface UV at northern latitudes?

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2. 2. mswzebo 12:13 PM 8/4/11

   Here's a peer reviewed study focused specifically on that, which finds
   
   "Zonal mean erythemal UV data show ~3-7% per decade increases in the mid-to-high latitudes."
   
   http://rpd.oxfordjournals.org/content/91/1-3/157.abstract

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3. 3. Clyde Spencer in reply to mswzebo 03:45 PM 8/4/11

   I don't have access to the full article. However, from reading the abstract I see that the data are essentially for the period prior to the Montreal Protocol. That is, they do not reflect what has been happening in the last two decades. Furthermore, the abstract says that the Total Ozone Mapping Spectrometer was used. That implies that the surface UV has been inferred from the vertical total ozone column and was not actually measured at the ground. It has been my experience that such calculations don't take into account the fact that the surface intensity decreases as the elevation of the sun decreases, typically don't account for the fact that the sun's rays are entering the atmosphere south of any depleted regions in the stratosphere, and don't take into account the longer slant range through the atmosphere when the sun isn't directly overhead, which it never is at high latitudes.

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4. 4. ConcernedCitizen 04:15 AM 8/14/11

   Fascinating article.

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5. 5. da bahstid 07:28 AM 8/14/11

   My possibly imperfect recollection is that since all the reductions in chlorofluorocarbon output, the ozone layer has slowly started to recover, though when I last heard about it things hadn't gotten back to pre-industrial levels. It's too bad thermodynamics doesn't allow a similar cheap substitute for CO2 emissions.
   
   I also seem to recall something about patients receiving lens replacement surgery reporting that they could see UV during parts of the procedure...I have no idea how reliable that information is though.

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6. 6. geojellyroll 10:21 AM 8/14/11

   Note..reindeer are caribou
   
   I doubt if few mammals can perceive UV light. Rodents make up the majority of mammals and they can perceive UV. Also, the bulk of other mammals are vegetarians and thus 'prey'. They also may have some UV perception to detect their predators (as is a possible evolutionary reason for caribou in this study).

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7. 7. Cigarshaped 05:24 PM 8/18/11

   Note that our eyes couold be adapted to previously higher levels of shorter wavelength light, too. Ever wondered why the photopic curve is much less sensitive to the blue end of the spectrum. Why does the human eye require such high green sensitivity, when the sun produces a max in G/Y?
   I wonder if it's the same reason the caribou and SWS1 have enhanced UV perception, because our light source has changed its spectral distribution?
   
   While we're at it, why are leaves green? They absorb B & R and REFLECT Green - the most abundant wavelength - now!??

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