Reindeer Are The Only Mammals Whose Eyes Are Known To Change Colour, Going From A Gold In The Summer,

reindeer are the only mammals whose eyes are known to change colour, going from a gold in the summer, when the sun is a constant presence in the arctic, to a less reflective blue in the near perpetually dark winter months. 

in dark conditions, muscles in your irises contract to dilate your pupils and allow more light into your eyes. when it’s bright again, the irises widen and the pupils shrink. the same thing happens in reindeer, but the arctic winter forces their pupils to dilate for months at a time.

this constant effort to stay dilated ends up blocking the small vessels that drain fluid out of the eyes, which causes pressure to build up. this in turn compresses the collagen fibers that make up the tapetum - a mirrored layer that sits behind the retina (second photo).  

when compressed, these fibers in the eye reflect blue wavelengths of lights instead of the yellow which accompanies a typical spacing of the fibers, as in summer.  (photos x, x)