• It's because reindeer eyes change with the seasons that their vision adapts to the dim blue light of the long winter twilight, according to our partner

    The Conversation

    .

  • Confronted with certain specific light conditions, the eyes of reindeer see their sensitivity improved by the addition of a kind of "mirror" located behind the retina.

  • This analysis was conducted by

    Robert AE Fosbury

    , former astrophysicist and honorary professor at the Institute of Ophthalmology at University College London (UCL - England).

Many childhoods were ruined by this terrible day when the hard and cold truth about how gifts arrive under the Christmas tree was revealed... But whatever the big brothers and big sisters, or the friends of class of the most cynical, even after such a sad revelation, reindeer retain a special place in our imagination.

There is no doubt that their large dark eyes - just like their majestic antlers and the memory of their magical complicity with Santa Claus - are not for nothing in the love that is dedicated to them all over the planet.

But beyond the soft gaze they give them, the eyes of the reindeer are above all evidence of their great adaptation to their snowy habitat, just like the second layer of fur they have, or their wide shaped hooves. of crescent, which guarantee their stability and allow them to dig the snow efficiently.

Our work has indeed revealed that the eyes of these deer change with the seasons, which allows their vision to adapt to the weak blue light of the long winter twilight.

​Twilight, such a special moment

In arctic regions, winter temperatures can drop to -50°C.

At this time, when the landscapes are not plunged into night darkness, they are bathed in twilight light, as the sun is below the horizon all day.

It is precisely at dusk that the reindeer (

Rangifer tarandus

, also called caribou ) leave in search of their meal of lichens.

Abundant in the Arctic, the latter are an ideal food source, as they are available wherever the herds travel.

Reindeer discover them by brushing the snow-covered ground with their hooves, antlers and snouts (the term "caribou" may have been borrowed from the indigenous people of the Micmacs, who called the reindeer "xalibu", which would mean “shovelling beast”).

At a time when the reindeer digs to find its food, the wolf, its predator, also goes hunting.

Both are incredibly well adapted to the winter conditions of the Arctic, and in particular to the very particular light which halos these regions at this time of the year: extremely blue, it contains only very little green, yellow and orange.

This bluish color is due to the fact that the sun darts its rays below the horizon.

At dusk, when the day star is low, its rays travel a great distance through the atmosphere, and pass horizontally through the ozone layer.

However, this acts as a filter: it absorbs almost all the wavelengths of the light spectrum, with the exception of blue light.

Note that the blue color resulting from this ozonic filter is different from that observed in the sky on a beautiful sunny day, which results from the interactions between sunlight and the molecules present in the atmosphere (this is called Rayleigh scattering phenomenon).

Although artists refer to this special time of day as the "blue hour", we usually don't notice it, as our eyes adapt to this gradual color change.

Indeed, as the darkness progresses, our vision, which during the day relied on specialized neurons, the photoreceptors of the retina in cones (involved in color vision), becomes dependent on other more sensitive photoreceptors, the rod photoreceptors (which are unable to distinguish colors).

In winter, in the polar regions, twilight can last more than a third of the day.

Confronted with these particular conditions of luminosity, the eyes of wolves and reindeer see their sensitivity improved by the addition of a kind of "mirror", called

tapetum lucidum

(shiny carpet), located behind the retina.

Some of the light that enters their eyes and passes through the retina is detected and absorbed by photoreceptors.

Only part: the

tapetum lucidum

reflects the rest, which passes back through the retina a second time, allowing more light to be detected.

The images perceived by the reindeer are therefore brighter, but slightly more blurred, because the mirror diffuses part of the light laterally, much like a fogged window would.

However, this is an advantage when the light is poor, because for reindeer the perception of contrast and movement is more important than visual acuity.

The

tapetum lucidum

evolved independently in many animals.

But not everyone: humans and raptors, for example, don't have it, because they have a vital need for sharp images.

Eyes that change with the seasons

During our work, we compared the eyes of reindeer who died during the summer with the eyes of their congeners who died in winter.

Our results revealed that the reindeer tapetum undergoes a specific seasonal change, which has the effect of modifying the reflected colors: it reflects a golden-turquoise light in summer, and a deep blue in winter.

Lichens, like wolf fur, reflect less blue than other colors and therefore appear darker on the snowy landscape.

The tapetum of reindeer has a structure similar to that which gives their iridescence to the feathers of the peacock, the brilliant blue wings of the Morpho butterfly or the bursts of color of the opal.

This is called “structural coloring”.

The structures present in the tapetum of reindeer are made up of very narrow collagen fibers, too small to be seen under an optical microscope.

They resemble the fibers found in muscle, but are thinner.

These fibers are aligned like countless pencils that would be stored in a transparent box, in a hexagonal pattern.

Add enough water to fill the gaps, change the scale - reducing it by a factor of about 40,000 - and the box will reflect blue light.

This corresponds to the winter tapetum.

To switch to summer tapetum, multiply the amount of water tenfold and double the depth of the box.

At this tiny scale, the fibers will retain roughly their hexagonal pattern, but there will be more spaces between them.

Our hypothesis is that the transformation to move from one tapetum to another is triggered by a change in pressure inside the reindeer's eye, a change that occurs in summer and winter.

In winter, in very cold weather, drivers lower their tire pressure to increase traction on ice.

The reindeer let liquid come out of their tapetums to better see what surrounds them...

This finding could help engineers design products that can alter reflected color.

The possibilities are limitless.

One can imagine changing the color of a surface covered with a reflective nanostructure similar to that of the tapetum of the reindeer (rather than a paint based on pigments) by modifying the space between the "pencils" that constitute it.

You could thus modify the color of your car as you wish… In addition, unlike many paints based on pigments, these structural paints would not deteriorate over time.

OUR “ARCTIC” FILE

After having long inspired Christmas tales and traditions, reindeer could now inspire science and technology...

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