Reindeer are the only mammals whose eyes are known to change colour, going from gold in the summer, when the sun is a constant presence in the arctic, to a less reflective blue in the 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; causing pressure to build up. Consequently, this compresses the collagen fibres that make up the tapetum, a mirrored layer that sits behind the retina, and cause the eye to reflect blue wavelengths of light instead of yellow.
-Jean
Left image courtesy of little miss piccie on Flikr http://www.flickriver.com/photos/little_miss_piccie/3261701700/
Right image: Still from https://www.youtube.com/watch?v=deo3a3GcYo8
EDIT: It turns out that the second image is photoshopped and you can’t actually see the change in reindeer eyes, as it takes place on the inside of the eye.
