Researchers have discovered that the eyes of Arctic reindeer change from gold in summer to blue in winter, which helps them adapt to the extreme lighting changes in their environment. Animals living above the Arctic Circle experience extended periods of 24 hour light in the summer, followed by near complete darkness in the winter. Even though the lights are out in winter months, the Arctic animals still have to eat and protect themselves from predators.
Reindeer have a light-reflecting layer of tissue in their eyes, called TL. But unlike almost any other mammal, reindeer’s TL can adjust with a change in pressure. In the winter, when it’s dark out, reindeer pupils are constantly dilated, which increases fluid pressure in the eye. The pressure causes the TL to adjust, changing the light-reflectivity and color of the eye. This allows it to see more in low light, but with less detail.
Factually, the eye-color change that takes place in winter makes the eyes between 100 to 1,000 times more light sensitive.
By changing the color of the TL in the eye, reindeer have flexibility to cope better with the extreme differences between light levels in their habitat between seasons. This gives them an advantage when it comes to spotting predators, foraging in the dark, which could potentially save their lives.