for National Geographic News
Reindeer have a different set of eyes for summer and winter, a new study suggests.
Scientists say the animals change their eye color and structure with the seasons in Arctic regions where permanent summer sunlight is replaced by 24-hour darkness in winter.
The visual alterations appear to be an adaptation to deal with polar light extremes, according to the researchers from Norway and the U.K., who add that the phenomenon has never before been recorded in mammals.
The researchers studied reindeer from the Lapland region of northern Scandinavia and found that eyes removed from animals culled in winter appeared deep blue when light was shined into them.
But the eyes of summer reindeer were yellow in color.
This difference suggests the reindeer alter their vision seasonally to match prevailing light conditions, said study leader Karl-Arne Stokkan, an Arctic biologist at Norway's University of Tromsø.
"It seems that the eye is reflecting what is the dominant light in the surroundings at the time, because in mid-winter the environment is predominantly blue," he said.
In summer, however, the reindeer's eyes reflect predominantly in the yellow part of the visible spectrum, Stokkan said. (Related story: "Fly Eyes Inspire Better Video Cameras, Motion Detection" [September 7, 2006].)
How mammals deal with the dramatic extremes in brightness at polar latitudes has been a long-standing puzzle.
The research team noted, for instance, that the eyes of test rats are "completely ruined" when exposed to conditions similar to the permanent brightness of an Artic summer.
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