As the cold months of winter draw near, human adaptations tend to include warm coats, turning the heat up, and turning the clock back. But for Arctic reindeer, it’s a little different. Researchers have discovered something special about how Artic reindeer prepare for the long darkness of an arctic winter. Their eyes can change color.
Reindeer eyes shift from brightly gold in the summer to a deep blue in the winter, allowing them to better see both food and predators as the seasons change. Researchers from University College London (UCL), and the University of Tromsø, Norway, have teamed up in the first study to document how reindeer adapt their eye color from the continual brightness of summer to the constant darkness of winter by altering their retina light sensitivity. The reindeer’s eyes are able to change color because, like many other mammals, they have a reflective layer of tissue behind their retinas called the tapetum lucidum (TL). This layer acts as a mirror, reflecting light back out to boost night vision. In the brightness of summer, the reindeer’s TL is gold and it reflects most of the summer light back out through the retina. But as the darker winter months approach, the reindeer’s TL changes from gold to dark blue, allowing much less light to be reflected back out and allowing the reindeer to see better.
The research team believes the color change might be caused by pressure inside the reindeer’s eyes. During the constant darkness of winter the reindeer’s pupils are continuously dilated, preventing fluid in the eyeball from draining naturally and causing increased pressure in the eyeball. This extra pressure on the TL reduces the spacing between the collagen in the tissue, resulting in the TL reflecting shorter wavelengths of blue light common in Arctic winters. “This is the first time a colour change of this kind has been shown in mammals”, says Lead researcher Professor Glen Jeffery from UCL, “This gives them an advantage when it comes to spotting predators.”
Previous research also leads the team to believe that by changing their eye color, reindeer are also able to see different wavelengths of light, such as ultraviolet light. There is an abundance of ultraviolet light in the arctic, and reindeer’s wintery blue eyes seem likely to favor a higher sensitivity to ultraviolet.
A video summary of their research provides more great information.