Reindeer’s eyes are golden in the summer, but turn a deep blue in the winter to capture more light making them the only mammal that does this. pic.twitter.com/IlPAPqD7ob— Lars-Johan Larsson (@LarsJohanL) December 6, 2021
Is it true? I had to look it up. Yes, reindeer have developed a system for changing their eye color in order to see better during the dark winter months in the Arctic and cold northern regions. But it has nothing to do with the animal's irises, except for pupil dilation. A reindeer's irises are always brown, but you see much less of the iris in winter when the eyes are always dilated.
The part that changes color is actually on the back of the eyeball, a layer called the tapetum lucidum that lies underneath the retina. This layer reflects light back out of the eye. In summer, it is golden colored. In winter it turns blue, which reduces the amount of light reflected back out of the eye and enables the reindeer to see in low light. However, some vision sharpness may be sacrificed.
What causes the change is the light itself. When the sun goes away, the pupils dilate, which causes a pressure change in the eye. The collagen fibers that make up the tapetum lucidum become rearranged and the tissue turns blue. Now isn't that a neat adaptation for winter? Read more about reindeer eyes at LiveScience. -via Nag on the Lake