Scientists unravel eye-colour changes that allow reindeer to cope in Arctic

Rudolph the fictional reindeer was famous for his red nose, but his true-life cousins have eyes that change colour with the seasons.

In the summer, the eyes of Arctic reindeer appear gold, and around Christmas they turn to a deep blue, biologists have discovered. It's not holiday magic, but rather an adaptation that helps these animals deal with the strange light conditions.

Above the Arctic Circle, Christmas falls in the midst of a 10-week period of perpetual twilight in which the sun never rises and the landscape is cast in bluish hues. But from mid-May to late July, the sun never sets.

Biologists at the University of Norway wondered how the reindeer managed the transition from near-total darkness to blinding light, when springtime sunlight reflects off unmelted snow.

They collected reindeer eyes from the Sami, indigenous herders who often slaughter the animals around the solstices. The eyes were collected in winter and summer and sent to Glen Jeffery, a neuroscientist who at University College London.

"That first time, they sent me 10 eyes from summer and 10 eyes from winter," Jeffery said. "When I opened them, I had the biggest shock I've ever had in science. The winter ones were clearly blue and the summer ones clearly gold. I wished I had someone sitting next to me to exclaim to."

Karl-Arne Stokkan and his colleagues in Norway discovered that the colour change occurred on a reflective surface behind the central retina known as the tapetum lucidum.