Reindeer Change Their Eyes for Summer and Winter, Study Finds

James Owen
for National Geographic News

March 13, 2007

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].)

Eye Reflections

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.

Yet the eyes of reindeer, polar bears, and Arctic foxes are able to cope.

For reindeer, the key may be changes in the reflective membrane behind the eye's retina called the tapetum lucidum ("bright carpet").

The tapetum boosts image information in dark conditions by collecting light and directing it back through the retina. The membrane is particularly well developed in animals with good night vision, such as dogs and cats.

Stokkan's team says evidence for seasonally altered reindeer vision was supported by structural changes in the tapetum.

The spacing of collagen fibers in the tapetum determines which part of the spectrum is being reflected.

These collagen fibers were found to be spaced further apart in winter than in summer, closely matching the shift in color reflections given by the reindeer eyes.

Similar shifts in tapetal reflection have previously been noted only in some marine animals, such as deep-diving sharks that adjust their vision to changing light levels, Stokkan said.

The researchers are still in the dark, however, as to exactly how this optical adaptation benefits reindeer.

"We have observed something which is absolutely real, but we have difficulties in explaining it," Stokkan said.

Light Sensitive

The leading theory to explain the adaptation, Stokkan said, is that in winter the animals increase the sensitivity of their vision at the expense of sharpness.

This is done by scattering the predominant blue light over receptors in the retina known as rods, which are responsible for night vision, instead of focusing light on cone cells that function better in bright conditions.

"The cones are responsible for high-resolution vision, but the rods are more light sensitive," Stokkan said.

"So the reindeer may improve their sensitivity to low-light conditions by sacrificing their cone vision in winter."

The blue of winter reindeer eyes "helps the retina make the best of its signals when the lights go down," added co-researcher Glen Jeffery of the Institute of Ophthalmology at University College London.

"The blue is favoring a dark environment," Jeffery said. "What you're seeing in the summer eye is the default state.

"It's just a bizarre adaptive mechanism—totally and utterly novel," he added.

The study team now hopes to replicate its findings in living reindeer, which has thus far eluded them.

The researchers also plan to study polar bear eyes collected from nuisance animals occasionally shot on the Arctic archipelago of Svalbard (see another Svalbard find: a dino-era sea monster).

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