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Deer can change their eyes in winter to adapt to harsh conditions.

The Arctic reindeer, like its main predator the wolf, is incredibly well adapted to its snowy home, where winter conditions can see temperatures drop to -50°C and low levels of daylight. Reindeer have a second layer of fur, and wide crescent-shaped hooves that keep them stable and allow them to dig in the snow. And as our new study shows, their eyes undergo physical change as the seasons turn which allows them to see clearly in the long winter twilight.

Reindeer feed in twilight when wolves hunt. However, twilight has a unique property which distinguishes it from day or night: it is extremely blue, containing very little green, yellow and orange.

This is because, lit by a sun below the horizon, the Earth’s ozone layer acts as a filter spanning the sky, which in twilight absorbs almost all light except blue light. The sunlight travels a greater distance through the atmosphere, passing horizontally through the ozone layer. This ozone-blue is different from the clear daytime sky-blue , which is caused by scattering of sunlight by air molecules.

Wolves and reindeer both improve their sensitivity to Arctic twilight with a “mirror” behind the retina. When light enters the eye and passes through the retina, not all of it is detected and absorbed by specialised neurons called photoreceptors. The mirror instead reflects it back through the retina for a second time, when more light will be detected. The reindeer see an image that is brighter but slightly fuzzier because the mirror scatters some light sideways, a bit like a misted glass.

This is an advantage in dim light because the animal relies more on visual contrast and motion than visual sharpness. The mirror, called the tapetum lucidum (shining carpet), evolved independently in many animals. Important exceptions include humans and raptors, who need crisp images.

Our study compared the eyes of reindeer that died during summer to specimens from reindeer that died in the winter.

It showed that reindeer eyes undergo a unique seasonal change in its tapetum and change colour, reflecting gold-turquoise light in the summer and mirroring deep blue in the winter. Both lichen and wolf fur reflect less blue than other colours so appear dark against the snow-covered landscape.

A reindeer’s tapetum uses the same structure that makes the peacock’s iridescent feathers, the Morpho butterfly’s brilliant blue wings and the opal gem’s flashes of colour. This is called structural colouration.

In the reindeer tapetum, these structures are very fine fibres of collagen too small to be seen with a light microscope, similar to but finer than the structure of muscles. Imagine these fibres as a large number of pencils neatly stacked in a transparent box in a hexagonal pattern.

This finding may help engineers to make products that change reflected colour. The opportunities are endless. If you use a surface coated with a reflecting nano- structure, similar to that of the reindeer’s tapetum, rather than a pigment-based paint, you can change colour by changing the separation of the scaled-down “pencils” that reflect the light. For example, you could change the colour of your car by tuning the separation. Unlike many pigments, these structural paints do not fade over time.

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