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 Proceedings of the Royal Society B: Biological Sciences 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.

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.