Night King ¯\_(ツ)_/¯

As its title suggests, the penultimate episode of Game of Thrones Season 7, “Beyond the Wall,” places the terrifying blue-eyed Army of the Dead in the forefront of the action. The icy blue eyes of the White Walkers and wights, however, aren’t only the stuff of Jon Snow’s nightmares. They also actually serve a practical purpose: Real-world science suggests that blue eyes, common in wintry climates, give the creatures that bear them a serious advantage during the Long Night.

In 2016, research on ancient DNA samples published in Nature showed that blue eyes were prevalent among neanderthals in Europe thousands of years ago, suggesting that there is a good evolutionary reason that trait persisted.

One explanation comes by way of a blue-eyed animal that thrives in the cold and the dark. Reindeer, researchers have found, have eyes that turn from golden brown in the warmer months to deep blue in winter in order to help them see in the 24-hour darkness.

reindeer eyes
Reindeer's eyes change color in the winter to help them see better. a) reindeer eye in winter, b) reindeer eye in summer, c,d) same eyes but flattened

Examining eyeballs collected by reindeer ranchers near the Arctic Circle showed researchers that the collagen spacing in reindeer eyes vary in response to the all-day-sun of summer and the all-day-darkness of winter. It’s suspected that this adaptation allows light to bounce around inside the eye more before exiting, allowing the animals’ eyes to absorb more light.

It would be a fantastical — and sensible — case of convergent evolution if the Army of the Dead developed blue eyes during the Long Night for similar reasons. Reindeer eyes, golden brown in the summer, turn deep blue in the winter because it ultimately translates to better, albeit less sharp night vision. The collagen spacing of their “winter eyes” reflects blue light, making them appear an eerie blue.

Early research, published in 2008 by University of Copenhagen scientists, had even suggested that all individuals with blue eyes descended from one common ancestor, in the same way that the Night King is the lone patriarch of all the White Walkers. The Nature paper’s genetic maps, however, showed that some of humankind’s blue-eyed ancestors lived in modern-day Spain, even though most of them roamed the the climes of Northern Europe, which much more closely resemble the lands Beyond the Wall.