Reindeer eyes change color as holiday season approaches


Any child with an imagination can most certainly describe the legendary phenomenon of the color change that takes place in the olfactory organ of a certain reindeer each Christmas Eve. However, researchers at the United Kingdom’s University College London and Norway’s University of Tromsø have only recently discovered the more common and observable occurrence of seasonal eye color change in this mysterious species of the North Pole.
An article published last month in “The Proceeding of the Royal Society B” concluded that in the summer months, the tapetum lucidum, or light-reflecting tissue layer located behind the retinal pigment epithelium, is “golden” but changes to a “deep blue” in the winter.
Researchers conducted the study by measuring the reflection and collagen fiber spacing of in-vitro retina cells obtained from the Eurasian mountain reindeer (Rangifer tarandus tarandus).
The data suggests that the color change is a result of changes in light reflection resulting from the spacing of collagen fibers. A higher density of collagen fibers leads to the deep blue color as more light is absorbed and scattered among the photoreceptors.
According to the study, “Winter animals have significantly increased intra-ocular pressure, probably produced by permanent pupil dilation blocking ocular drainage.” This higher pressure leads to a higher density of collagen and ultimately less light reflected directly out of the eye. With less light reflected out of the eye, reindeer experience higher sensitivity to light, providing an advantage when avoiding predators in the dimly-lit Arctic winter.
With higher sensitivity, however, comes reduced acuity. Scientists see this as a fair trade-off because reindeer require less acuity in the winter when there are fewer plants in which reindeer must forage. Therefore, “Acuity loss will not be greatly detrimental to motion perception” or provide a selective disadvantage.
While researchers did not pin down the exact functional mechanism for eye color change, they hypothesize that the decreased light as the days get darker causes a more permanent ocular dilation resulting in increased pressure.
They also point out that “This is, to our knowledge, the first description of a retinal structural adaptation to seasonal changes in environmental light,” and they hope to explore similar possibilities in other Arctic animals.
This groundbreaking research also will prove to be useful to children and Santa-tracking organizations worldwide, as many will now be looking for blue eyes flying through the Christmas Eve sky in addition to the iconic red nose.

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