Did you know that reindeer are the only known mammal whose eyes change seasonally from golden in the summer to a wintry blue? Their eyes also become a thousand times more sensitive in winter, adapting to dim light in the near darkness to avoid predators.

Part of this is because of changes in the tapetum lucidum (TL), a mirror-like layer under the retina.

Tapetum lucidum

The tapetum lucidum (TL) reflects light so that the photoreceptor cells in the retina get another chance of capturing stray photons. Most mammals have a golden TL, as do reindeer in the summer. But in the constant darkness of winter, muscles keeping the pupils dilated become strained and pressure inside the eye builds up (just as in glaucoma). This makes the TL flatten and packs the collagen fibers inside it so they diffract blue light.

If the reindeer are herded in regions with distant urban lights, their TL is partly altered and their eyes look green! This may not be the whole story and there could be changes in the retina as well.

REF: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.2451

Pop Sci: https://www.nationalgeographic.com/science/article/why-are-reindeer-eyes-golden-in-summer-but-blue-in-winter