Reindeer are the only mammals whose eyes are known to change color. The eyes are gold during the summer when the reindeer experience almost constant sunlight. During the darkness of winter their retinas become less reflective and their eyes appear blue.
They are also the only known mammals able to see in ultraviolet.
During the Northern winter, when the sun barely rises above the
horizon, snow reflects about 90 percent of UV. While that can cause
snow blindness in humans, reindeer use it to their advantage.
Reindeer need their excellent eyesight when they run, because they
can approach 50 miles per hour (80 km) at full run.