In the summer when days and nights are marked by nearly constant brightness, the eye tissue is tinged gold, which is common for ungulates such as reindeers. But once the darkness of winter hits, reindeer eyes change their hue to blue. The change of color is also associated with a reduction in light reflected, and an increase in captured light. The increased retinal sensitivity comes at a cost, though—the acuity of the reindeers sight is reduced, meaning the sharpness or clearness of what is seen is lower. But, just having the ability to see—no matter how blurry—could help the reindeer spot predators, or lead the way for Santa and his sleigh.