Reindeer eyes turn blue in winter

A reindeer can look gold in summer and deep blue in winter.

The color comes from the tapetum lucidum, a reflective layer behind the retina. The study measured the seasonal reflectance shift and tested a model in which changing fluid between collagen fibers changes their spacing, tuning the reflected light toward blue winter twilight. An earlier study linked the winter state to increased retinal sensitivity and proposed higher eye pressure as a possible trigger; that trigger remains unconfirmed. 1 2

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