On Christmas Eve, a team of reindeer (驯鹿) will help Santa Claus deliver gifts to children all around the world. The reindeers, led by their fearless leader Rudolph, won't be the only ones doing something special. Back in the highest Arctic, their cousins have a remarkable ability changing their eye color.

During the summer months, when the days are long and the sun is bright, reindeer's tapetum lucidum (荧光膜), a mirror-like layer at the back of their eye, appears golden, which helps bounce the majority of light off the eyes, effectively acting like a pair of natural sunglasses. As winter comes, and the days become shorter and darker, the tapetum lucidum turns blue to absorb more light, allowing reindeer to improve their night vision and see clear in low light conditions.

With these adaptations, reindeer can adapt and thrive in one of the harshest environments on Earth. Unlike humans, reindeer can see well into the shorter Ultra Violet (UV) range. This UV vision enables them to spot food and predators more effectively in the snowy landscape. Lichens, a key part of their winter diet , absorb UV, so they show up dark against UV-reflecting white snow. Wolf and polar bear fur also absorb UV, so instead of disappearing against snow they pop out in high contrast, allowing reindeer to spot potential threats from a distance. 

Reindeer change their eyes by adjusting their tapetum lucidum, which is made of collagen fibers. In winter, the collagen fibers become packed tighter, causing the tapetum lucidum to mainly reflect blue light. This change happens when reindeer dilate their pupils (瞳孔). In summer, the reindeer's pupils return to a smaller size, which helps reindeer reduce the amount of light entering the eye.

But their unique adaptation may hurt them. Today, the increasing use of artificial lighting, especially during the winter months, poses a potential threat to their sensitive eyes. It can make reindeer lose their way, affecting their ability to survive in their challenging environment. So it is crucial for us to be mindful of our use of electricity and make efforts to minimize light pollution to ensure the well-being and survival of these magnificent creatures.