Did You Know That Reindeer's Eyes Change Color for Christmas?

Rudolph the red-nosed reindeer is more likely to crash into your house than glide gracefully onto the lawn.

People need contact lenses to do a trick that Arctic reindeer do naturally: change eye color. In their case, the beautiful deer’s eyes change color dramatically between summer and winter to adapt to extreme changes in light intensity, scientists discovered.

In summer, when the days are long and the light is strong, the reindeers’ eye color is a golden brown. Come winter, when the days are short and the light dim, their eyes turn deep blue.

This evolutionary peculiarity is part of the animals’ defense strategy, say scientists from University College London and Finland’s University of Tromso. More specifically, the reindeer’s winter eye allows it to detect predators moving in the darkness.

“That is the primary problem for the reindeer: ‘Can I see something moving? That is a potential threat. I’ve got to run, I’ve got to move,’” says lead researcher Professor Glen Jeffery. “So he needs to be sensitive to low light levels and he needs to be sensitive to movement at low light levels.”

The discovery was quite a surprise. “There was a clear distinction between summer and winter eyes and we were completely shocked by that,” Jeffery said. “We’d never expected it, it was the last thing we were looking for, but it was the most obvious thing that you saw when you opened the eyes. Big color change,” he said.

No other mammal is known to change eye color with the season, let alone so dramatically.

Jeffrey’s team makes annual winter trips to the Arctic to examine reindeer eyes bought from slaughterhouses. This year they travelled earlier than usual and discovered the color change by accident.

The physical mechanism lies in the layer of tissue called the tapetum lucidum, which is behind the retina. It is the layer that reflects light back through the eye to enhance night vision.

By changing color, the tapetum can help the animal see in conditions of either extreme darkness or extreme light.

The team also placed gold electrodes underneath the eyelids of live, sedated reindeer in order to see their visual threshold. The findings suggested the color change may be caused by pressure within the eyes.

In winter, permanent pupil dilation compresses the tapetum, allowing the shorter wavelengths of the blue light common in Arctic winters to be reflected.

Cats are also famous for having light-reactive eyes – but they don’t change color. Their pupils contract in bright light and expand in the dark, as is the case in other animals.

Save a reindeer, close your curtains

Of course, the ground-breaking evolutionary adaptation is a problem for Santa’s reindeer.

“When Rudolf comes down from the Arctic he’s going to be very, very dark-adapted and he’s going to have a blue eye. The one thing we don’t want Rudolf to do is to come up against a whole load of street lights and a whole load of front room lights,” Jeffrey quips. “So a good thing, if you want Rudolf to call in at your place, might be draw the curtains, so he doesn’t get glared by all the light.”