By Michele Berger
Scientists in 2011 discovered that reindeer have a built-in AC system to keep from overheating in their natural fur coats. Recently, researchers found these mammals also have a unique way of coping with extremely dark Arctic winters: Their eyes change color.
During the sunny time in their habitat, reindeer have yellowish-gold eyes, but during the cold season, the eyes turn a noticeable hue of blue, a new study published in the Proceedings of the Royal Society B reported.
It’s because of changes in a light-reflecting tissue layer behind the retina, according to Science. “This structure, called the tapetum lucidum (Latin for ‘bright tapestry’), gives the eye’s light-sensitive neurons a second chance to detect scarce photons in low-light conditions. (The layer also 
produces the ‘eye shine’ that can make animal eyes appear to glow in the dark.)” In other words, it makes the eyes extremely sensitive to light.
“Most people are familiar with the way a cat’s eye reflects light back to enhance night vision. In fact many animals have this reflective layer,” said lead researcher Glen Jeffery, of the University College London Institute of Ophthalmology, in a news release. “Arctic reindeer possess this reflective layer, too.”
“This is the first time a color change of this kind in the eye has been shown in mammals,” he added.