reindeer eyes change color seasonally
October 30, 2013

Reindeer Change Eye Color As Seasonal Adaption

Brett Smith for - Your Universe Online

While strolling through the woods late at night, you may be lucky - or unlucky - enough to see a pair of glowing eyes floating in the darkness and staring back at you.

Wild animals’ glowing eyes aren’t just a great way to scare human campers or hikers, they are a byproduct of their boosted night vision, which is the result of a layer of tissue in the eye called the tapetum lucidum (TL), which sits behind the retina and reflects light back through the eye.

New research from the University College London in the UK and the University of Tromsø in Norway has found that Arctic reindeer seasonally change the color of their TL to adapt to the variations in daylight that occur over the course of the year in the Arctic.

For years, Norwegian hunters have known that reindeer eyes are golden in the winter and transition to blue in summer, but the reason behind the phenomenon has been something of a mystery.

To understand the color change, the research team dissected a number of reindeer eyes and concluded that the shift helps reindeer see better in the constant daylight of summer and the relentless darkness of Arctic winters by altering the sensitivity of the retina to light.

In the summer, Arctic reindeer have a gold TL, similar to many other mammals. The bright color reflects most light back through the retina. In winter, it shifts to a deep blue, reflecting less light out of the eye. This scatters more light throughout photoreceptors at the rear of the eye, boosting the sensitivity of the retina in response to the restricted winter light.

The team says that this change provides an advantage in the prolonged darkness of winter, meaning reindeer can more easily detect moving predators and forage for food.

"This is the first time a color change of this kind has been shown in mammals,” said study author Glen Jeffery, a neuroscientist from UCL. “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, which could save their lives,” he added.

The change could also be caused by pressure inside the eye balls. In winter, pressure in the animal’s eyes is higher, most likely caused by constant pupil dilation, which keeps fluid in the eyeball from draining. This squishes the TL, eliminating the space between collagen in the tissue and therefore reflecting the blue light prevalent in Arctic winters.

According to a report on their discovery published in the Proceedings of the Royal Society B, the researchers “have not proved functional relationships” among pressure within the eye, color change, light reflection and any visual benefit.

Previous work from the same team has shown that Arctic reindeer eyes can also see in the ultraviolet spectrum, which is plentiful in Arctic light but invisible to human eyes. The blue reflection from the reindeer's winter eye is likely to favor this ultra-violet sensitivity.