All animals, including humans, can adapt their eyes to the changing level of light. In dark conditions, muscles in the irises contract to dilate the pupils and allow more light into the eyes. When it’s bright again, the irises widen and the pupils shrink. The same thing happens to reindeers. But when the arctic winter brings perpetual darkness for months on end, something more happens to reindeer’s eyes causing them to change color.
The arctic reindeer is the only mammal whose eyes shine a different color depending on the season. In summer, when the sun is bright, the eyes are dazzling golden, but as the sun prepares to set for the winter, the eyes become less reflective until it turns dark blue in the dark winter months.
Two dissected reindeer eyes. The left one comes from an animal killed in winter. The right one was killed in summer. Photo credit: Glen Jeffery
The part of the eye that actually changes color is the tapetum lucidum, a layer of tissue that sits behind the retina. This layer is reflective, causing light to be reflected back to the retina increasing the amount of light available to the photoreceptors. It is the tapetum lucidum that contributes to the superior night vision in many animals such as cats, dogs and other nocturnal predators, and it is the tapetum lucidum that shines in the dark when you shine a torch into a cat’s or dog’s eyes.
During the dark winter months, a reindeer’s pupils are forced to stay dilated for months, and this constant pressure on the muscles of the iris changes the structure of the eye. First, it builds up pressure inside the eyeballs and this pressure rearranges the long collagen fibers that make up the tapetum lucidum making them more tightly packed. The spacing of these fibers affects the type of light they reflect. With relaxed spacing during the summer months, the tapetum lucidum reflect yellow wavelengths and the eyes appear golden. When squeezed together, they reflect blue wavelength and the eyes appear blue. Reindeers that are kept in pens and can see sodium street-lights, as opposed to complete darkness in the wild, are shown to develop green eyes—halfway between the yellow to blue transformation.
Series A are winter eyes. Series B are summer eyes. Photo credit: University of Tromsø
Researchers found the blue winter eyes are at least a thousand times more sensitive to light than the golden summer ones. The sensitivity of green eyes lie somewhere in between.
Glen Jeffery, a neuroscientist from University College London, believes that because blue light gets scattered more, it causes the photons to keep bouncing inside the eyes increasing their chances of getting captured by the retina. But Dan-Eric Nilsson, a vision expert from Lund University, suggests that the increased sensitivity has got more to do with the levels of light-sensitive pigments in its retina.
The increased sensitivity allows the reindeer to see in the dark, which is essential for their survival during the long months of darkness.
Close up of a reindeer eye in snow, Tromso region, Northern Norway. Photo credit: aaabbbccc/Shutterstock.com
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