Original Article: https://blog.theanimalrescuesite.greatergood.com/reindeer-eye-color/

Reindeer, or caribou as they’re also known, are a type of large deer belonging to the genus Rangifer. Famous for their yearly migrations, Americans predominantly know them as the creatures behind Santa’s miraculous, 24-hour, whirlwind present drop. But when they’re not keeping his sleigh aloft, they’re navigating cold, harsh climates in the far north that sees months of darkness blanket the frozen tundra during winter. Interestingly, it is this very phenomenon that influences the color of reindeer eyes, which can be blue or gold, depending on the season.

From Gold to Blue & Back Again

After studying the reflective layer behind reindeer retinas, scientists discovered that in winter months their eyes’ tapetum lucidum receives more pressure than it does during the summer. That’s because in winter months the animal’s pupil remains permanently dilated and the constant pressure squeezes fluid out between the tapetum lucidum’s collagen fibers. This, in turn, causes the light-reflecting fibers to bunch together, giving their eyes a blue appearance from behind when examined with instruments.

And there’s a reason for it. Pupil dilation lets in more light, allowing us to see better in low-lighting conditions. In some sense, the process helps reindeer to see better in the nearly continuous darkness of the brutal far northern winters. However, Caroline Zeiss, a professor at the Yale School of Medicine with a joint appointment in the Department of Ophthalmology and Visual Sciences, notes the change isn’t about the sharpness of vision in winter, but that the mechanism helps reindeer detect motion, which is helpful to prey animals.

When summer rolls around, the process reverses itself and their eyes revert to a yellow-to-green wavelength due to the increased light reflecting through their retinas. This allows the animals to better cope with the nearly nonstop daylight. Some researchers believe that the blue eye coloring in winter intensifies ultraviolet light, while the golden color of summer muffles the constant stream of it.

Reindeer Fun Facts

The occurrence is completely unique to caribou, as no other mammal is known to experience this type of natural ocular change as of yet. Another reindeer fun fact is that Santa’s fur-covered helpers are also the only known species of deer on the planet where both males and females are able to grow antlers. Now that you’re armed with some great pub trivia to share, enjoy the holiday season!

Source: The Animal Rescue Site Blog