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.
北极驯鹿的眼睛会随季节而变色，夏季金黄色，冬季深蓝色。
驯鹿眼睛能够显示出颜色的主要原因是因为光线射在反射层的胶原纤维上。在夏季，驯鹿为了适应北极连续的极昼状况，眼睛变成金色就能够适应阳光无休止的照射。而到了冬季，眼球内的压力增大，导致这些胶原纤维被压缩得更加紧密，纤维之间的间距变小，所以驯鹿的眼睛变成了蓝色。蓝色的眼睛让驯鹿可以感应到更多的光线，从而更加适应极夜的环境。
