Reindeer Eyes Change Color When It's Cold

Study suggests it's to help them find their favorite food in the dark
By Gina Carey,  Newser Staff
Posted Dec 24, 2023 12:20 PM CST
Reindeer Eyes Change Color When It's Cold
Reindeer in Norway.   (Getty / Dmitry Chulov)

Forget the red, shiny nose—reindeer have a more fascinating story to tell through their blue-shining eyes. The reindeer are unique in that their eyes change color depending on the season, from a golden reflective hue in warmer months to a blue one in winter, per the Washington Post. Now, a new study in Sage Journals suggests that happens so they can more easily find food when it's cold and dark outside. More specifically, the light-enhancing tissue in their eyes—tapetum lucidum—changes with the season so they can more easily find their favorite food, lichen. "They're the only large mammal that is known to eat such high amounts of lichen," says study co-author Nathaniel Dominy of Dartmouth.

As part of the study, researchers shone UV light on various lichen species in the Scottish Highlands, then photographed them. The researchers learned that animals' preferred lichen, known as "reindeer moss" (despite not being a moss), absorbs UV light strongly, making it stand out as the reindeer forage. "To our eyes, just like for most mammals, pale lichen in white snow is incredibly hard to spot," says fellow co-author Dr. Catherine Hobaiter of the University of St Andrews' School of Psychology and Neuroscience, per the Guardian. "But to reindeer eyes, the species they need to feed on would stand out as dark patches in the much more reflective snowy landscape."

This is because the animals' blue tapetum changes how their surroundings look, casting everything in shades of purple during winter. Anything that reflects UV light glows, while surfaces that absorb it appear darker. Reindeer eyesight likely adapted in this manner to better weather frigid Arctic winters, where spotting food more easily means spending less time wandering in the cold. The Guardian likens it to wearing night-vision goggles. "If they can see lichens from a distance," Dominy says, "that gives them a big advantage, letting them conserve precious calories at a time when food is scarce." (The reason for Norway's border fence with Russia? Keeping reindeer in.)

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