Reindeer are beloved all throughout the globe for their dark, expressive eyes, beautiful antlers, and mystical connection to Santa Claus. The moment you discover the cold, hard truth about how Christmas presents get beneath the tree is a terrifying one that haunts many a childhood. However, reindeer are far more unique than your jaded elder sibling or peers would have you believe.
The Arctic reindeer, like its major predator, the wolf, is extremely well adapted to its icy habitat, where temperatures can drop below -50°C and daylight hours are limited. Reindeer have a second covering of fur on their bodies and large crescent-shaped hooves that allow them to dig in the snow. And, as our latest research demonstrates, their eyes change physically as the seasons change, allowing them to see clearly in the lengthy winter gloom.
In the Arctic, midwinter is either dark or twilight, when the sun lies below the horizon all day. Reindeer must sweep the snow-covered ground with their hooves, antlers, and muzzles to discover and expose their winter diet, lichen. Lichens are abundant in the Arctic, making them a perfect food source for reindeer wherever they roam.
Twilight is special
When wolves hunt, reindeer eat around dusk. However, twilight has a distinguishing feature that sets it apart from day and night: it is highly blue, with very little green, yellow, or orange.
This is because, when lighted by a sun below the horizon, the Earth’s ozone layer functions as a filter covering the sky, absorbing practically all light except blue light in twilight. The sun’s rays travel further into the atmosphere, traveling horizontally through the ozone layer. This ozone-blue differs from the bright daytime sky-blue created by sunlight scattering by air molecules.
Although painters refer to this period after sunset as “The Blue Hour,” we rarely notice it since our eyes adjust to the gently shifting color. As night approaches, our eyesight shifts from employing cone receptors, which provide color vision, to using more sensitive rods, which are color blind. Twilight can extend more than a third of the day in arctic ecosystems throughout the winter.
Wolves and reindeer both use a “mirror” behind the retina to boost their sensitivity to Arctic dusk. When light enters the eye and travels through the retina, not all of it is caught and absorbed by photoreceptors, which are specialized neurons. Instead, the mirror reflects it back through the retina for a second time, when more light is recognized. The reindeer see a brighter but somewhat fuzzier picture because the mirror reflects some light sideways, similar to a misted glass.
This is advantageous in low-light situations because the animal depends on visual contrast and motion rather than visual acuity. Many animals developed independently of the mirror, known as the tapetum lucidum (shining carpet). Humans and raptors are important exceptions because they require sharp images.
Eyes that change with the seasons
Our research compared the eyes of reindeer that died in the summer to those of reindeer that perished in the winter.
It shown that the tapetum of reindeer eyes undergoes a unique seasonal transition and changes color, reflecting gold-turquoise light in the summer and mirroring deep blue in the winter. Lichen and wolf fur both reflect less blue than other colors, making them look darker against the snow-covered landscape.
The tapetum of a reindeer is made of the same structure as the peacock’s iridescent feathers, the Morpho butterfly’s beautiful blue wings, and the opal gem’s bursts of color. This is known as structural coloration.
These structures are very tiny collagen fibers too small to be visible with a light microscope in the reindeer tapetum, comparable to but finer than the structure of muscles. Consider these fibers to be a vast number of pencils neatly packed in a hexagonal configuration in a clear box.
Allow enough water to cover the gaps, then lower the scale by around 40,000, and the box will reflect blue light. The winter tapetum is represented by this. To convert to the summer tapetum, multiply the amount of water by 10 and twice the depth of the box. The hexagonal layout of the fibres will be essentially maintained at this microscopic size, although there will be more gaps between them.
This transition, we believe, is caused by a pressure difference in the reindeer eye that occurs in the summer and winter.
Another way to look at it is as if reindeer eyes had summer and winter tires. In really cold weather, you let some air out of the tyres to improve grip on the ice. To have a better perspective of its surroundings, the reindeer discharges fluid out of its tapetum.
This discovery might aid engineers in developing devices that modify the reflected color. The possibilities are limitless. You may modify the color of a surface coated with a reflecting nanostructure, comparable to that of a reindeer’s tapetum, rather than a pigment-based paint, by adjusting the separation of the scaled-down “pencils” that reflect the light. You could, for example, modify the color of your automobile by adjusting the spacing. These structural paints, unlike many pigments, do not fade over time.
So, while reindeer have long inspired Christmas legends throughout the world, they may now also inspire technology and science.
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