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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