How reindeer eyes transform in winter to give them twilight vision | The Conversation


There’s more to reindeer than meets the eye.

Reindeer are loved the world over for their dark, expressive eyes, majestic antlers and magical association with Santa Claus. The moment you learn the cold, hard truth of how Christmas presents arrive under the tree is a harrowing one that blights many a childhood. But reindeer are more special than your cynical older sibling or classmates would have had you believe.

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.

Mid-winter in the Arctic is either dark or twilight, when the sun is below the horizon, all day. Reindeer need to find and uncover their winter food, lichen, by brushing the snow-covered ground with their hooves, antlers and muzzles. Lichens are plentiful in the Arctic – an ideal food source that reindeer can find wherever they go.

Northern lichen. Shutterstock

Twilight is special

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.

Although artists call this time after sunset “the blue hour” we tend not to notice it because our eyes adapt to the slowly changing colour. As darkness approaches, our vision switches from relying on the cone receptors that give us colour vision to using the more sensitive rods, which are colour blind. In winter, the twilight can last more than a third of the day in polar habitats.

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.[…]

Honorary professor, UCL Institute of Ophthalmology, UCL

More: How reindeer eyes transform in winter to give them twilight vision

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