Reindeer eyes change from golden in summer to blue in winter.

Eye color sounds like a fixed trait, not a seasonal setting. Arctic reindeer do something stranger in the reflective layer behind the retina, the tapetum lucidum. In summer, that layer reflects a golden color. In winter darkness, prolonged pupil dilation changes pressure inside the eye and compresses collagen spacing in the tapetum, shifting its reflection toward blue. The change helps reindeer cope with months of dim Arctic light, though with tradeoffs in sharpness. The animal is not changing iris color like a mood ring. It is tuning a mirror inside the eye for a different world.

1 discovery explored