Arctic Reindeer Have Color-Changing Eyes

For all that fuss about his nose, Rudolph ended up having a colorful surprise in store for biologists after all.

In a perfectly timed bit of wildlife news, scientists have discovered that Arctic reindeer change their eye color from season to season in order to adapt to the region’s extreme daylight variation. The polar region is currently cloaked in total darkness for a 10-week period where the sun won’t rise at all. During summer, the sun never sets.

As The Los Angeles Timesreports, researchers at Norway’s University of Tromso were curious about how any animal could adapt to such diverse lighting conditions, so they studied the eyeballs of reindeer harvested by a local indigenous tribe. Surprisingly, the eyes from the animals killed during the summer appeared gold, while eyes from animals killed in winter appeared blue.

The color change occurs not on the iris, but on a reflective surface behind the central retina that’s known as the tapetum lucidum, Karl-Arne Stokkan and his colleagues in Norway discovered.

Humans don’t have this structure, but lots of other animals do. It helps nocturnal animals see at night by bouncing light back inside the eye, giving the light receptors in the retina a second chance to be stimulated. The tapetum lucidum is responsible for the flash of “eye shine” you see when a cat looks into a car’s headlights.

Scientists had always assumed that this piece of ocular anatomy’s color was fixed.

“This is the first time that a change in color in the tapetum has been shown in a mammal,” [neuroscientist Glen] Jeffery said.

The reindeer’s blue “winter eyes” don’t see as well as well as their summer eyes, but are 1,000 times more sensitive to light. The researchers speculate that this trade-off allows the animals to detect predators in total darkness more effectively, even though their eyes can’t form as clear of an outline.

Read more: Los Angeles Times