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In the wild, as environments change drastically due to weather, animals learn to adapt. For some animals, they hibernate throughout the majority of the winter while others migrate to warmer climates. For the arctic reindeers, researchers discovered that their eyes change color when the seasons change.

For this study the Biotechnology and Biological Sciences Research Council (BBSRC) funded a team of researchers from the University College London and the University of Tromsø in Norway. The team observed the eye colors of the Arctic reindeers and found that in order to adapt to the changing lights within their environment due to seasonal changes, the reindeers' eyes change colors. The eyes go from gold to blue so that they will not be as sensitive to light.

According to the researchers, the arctic reindeers, like many animals, have a layer of tissue known as the tapetum lucidum in their eyes. The tapetum lucidum is located behind the retina and works by reflecting light so that the reindeers can see better in the dark. The researchers found that when the eyes change color, the tapetum lucidum is capable of reflecting varying wavelengths. During the summer when the eyes are gold, the tapetum lucidem can reflect light directly through the retina. During the winter, the eyes turn blue so that the tapetum lucidem reflects less light out of the eye, allowing the reindeer to see better in the dark since there is less sunlight in the wintertime.

The researchers explained that the eye color changes could be due to pressure in the eyes. During the wintertime, there is more pressure in the eyes, which could cause permanent pupil dilation. The dilation forbids fluids that exist in the eyeballs to drain, which compresses the tapetum lucidem. When this layer is compressed, it reflects shorter wavelengths of the blue light that is quite common during the winter.

"This is the first time a color change of this kind has been shown in mammals. By changing the color of the [tapetum lucidem] in the eye reindeer have flexibility to cope better with the extreme differences between light levels in their habitat between seasons," Lead researcher Professor Glen Jeffery from UCL, said according to a press release. "This gives them an advantage when it comes to spotting predators, which could save their lives."

The study was published in the Proceedings of the Royal Society.