Reindeer may not have noses that glow red at night, but they do have a notable body part that changes color in the winter darkness — sort of like “sunglasses” for reindeer.
In a published in 2013, an international team of scientists discovered that reindeer eyes change tone depending on the seasons, reflecting the color of the Arctic sky.
If we look into the eyes of an Arctic reindeer (Rangifer fencing) in summer, when the days are long and the sun is intense, we see a golden and turquoise shine returning the gaze — similar to the emerald reflection of cats’ eyes at night.
In winter, however, when darkness dominates, the reindeer’s eye does something unique: stays in one color deep blue, impressive.
The animal did not turn into a “white walker”the frightening creatures of Game of Thrones that arrive with winter, not at all fanciful. It is a metamorfose natural — a singular adaptation that, according to scientists, can function as a kind of “sunglasses” for reindeer.
As is purified feed at dusk and during the arctic winter, twilight can last for more than a third of the daycasting an intensely blue light over the icy landscape. It is no coincidence that artists call twilight “at blue hour”, note or .
Scientists believe that for help the reindeer see lurking wolves and discover the appetizing lichen in the dim light, the animal’s eyes may have evolved to reflect more blue light in winter. This causes the weak light to once again pass through the retina, allowing the photoreceptors to collect more information.
This way, the reindeer gets brighter vision of the twilight landscape (up to a thousand times brighter), but at a cost: the image has a significantly lower resolution, as if looking through fogged glass.
Like many mammals, reindeer have a layer that reflects lightcall bright carpetinside each eye, located behind the retina. But no other mammal is known with a carpet like reindeer. Its structure seems to depend on the season.
In an article published in 2022 on , Robert Fosburyprofessor at the Institute of Ophthalmology at University College London compared the process to a tire change.
“In very cold conditions, some air is let out of the tires to increase traction on ice,” he explained. “The reindeer lets out fluid of your carpet to get a better view of your surroundings”.
The exact mechanisms behind this structural change are not yet known, but researchers think it is related to fluid in the eye. In 2022, Fosbury and colleagues discovered the difference between the eyes of reindeer that had died in the summer and those of reindeer that had died in the winter.
Fossbury, Jeffery / Proc Biol Sci
The color of the tapetum in summer and winter compared to ambient light, with the Arctic Sun above and below the horizon
The findings support the idea that constant pupil dilation in low-light conditions affects fluid balance in the eyepossibly causing structural changes in the carpet.
Unfortunately, the scientists were unable to compare the structures of the same tapetum at the two stations, but based on the results, they say it is “clear that any difference is small”.
“We still cannot understand how seasonal structural change in the eye is triggered and what biochemical pathways make it possible”, admitted the researchers. “But whatever the mechanism that triggers it, it is clear that must be reversible”.
To solve the mystery, it may be necessary to study reindeer eyes in autumn and spring, to understand how they gradually transition between winter and summer tones.
For now, The eyes of Arctic reindeer appear to be unique. Still, it is quite possible that there are other mammals living in low-light conditions with similar adaptations.
