Reindeer have turned out to have an unexpected ability.
Like many animals (but no primates) Reindeer have a tapetum lucidum - a reflective layer in the back of the eyeball that helps them see better in dim light. This is why animal eyes reflect green when photographed with a flash.
Reindeer, though, take this one step further. They live so far north that some of them have to endure 24 hours of light in the summer and 24 hours of darkness in the winter. How do they deal with this?
They change the color of the tapetum lucidum. In the summer, reindeer eyes glimmer gold - a color which reflects most of the light back out through the eye, protecting them from continuous daylight and snow blindness. In winter? The eyes turn light blue, which captures more light and allows them to handle near darkness.
Now the scientists plan on studying the eyes of other arctic animals to see if any of them do the same thing.