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
It appears that the latter image is shopped. Below is the oldest instance of that picture on the internet according to Tineye: http://www.zoonar.com/photo/eye-of-a-reindeer_3909669.html
I’m looking for pictures of live reindeer with blue eyes, but it’s a bit difficult since there are a lot of subspecies of reindeer and some would just have blue eyes because their pigmentation is different across subspecies. And they change coat color with the season, so it’s difficult to control for that. I think it’s the internal part of their eye, which you can’t see, that changes color. Furthermore, reindeer experts that I’m interviewing now say that their eyes don’t change color, but it might look like they do in the harsh light of summer or the dim light of winter.
But you can certainly see the difference in these eyes removed from harvested dead reindeer, though.
