Depending on when you see them, the eyes of an arctic reindeer will change in color. In the summer, they're almost golden. In the winter, they seem deep blue. This is not due to a change in their mood or even a reaction to the temperature, but rather because of a change in the amount of overall light in the environment.What you're really seeing is light reflected off a thin layer of tissue at the back of the arctic reindeer's eyes, like a mirror. Called the tapetum lucidum, this tissue helps them see at night. In the Arctic, there is a significant difference in the amount of available seasonal light. While there is lots of light during the long days of summer, the Norwegian winter means darkness for months, so a reindeer's eyes are constantly dilated. Dilation increases intraocular pressure (the fluid pressure within the eye) and that pressure changes the density of collagen fibers in the tapetum lucidum. In turn, the density of these fibers affects the wavelength of the light that the tapetum lucidum reflects back, from one end of the color spectrum to the other.
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