CONCORD, N.H. — Rudolph, the red-nosed reindeer, may have millions of carrots waiting for him on Christmas Eve, but what about the rest of the year?
Finding food in a cold, arid landscape is a challenge, but researchers from Dartmouth College in New Hampshire and the University of St. Andrews in Scotland report that reindeer eyes may have evolved to easily recognize their favorite meal.
It's further evidence that while reindeer are famous for pulling Santa's sleigh, it's their vision that really sets them apart, says Nathaniel Dominy, a professor of anthropology at Dartmouth and co-author of a recent study published in the journal i-Perception.
“They are quite obscure and unheralded in the annals of visual neuroscience, but they are having their moment because they have a very fascinating visual system,” he said in an interview.
Scientists have known for years that mirror-like tissue in reindeer's eyes changes color from greenish-gold in summer to vivid blue in winter, a process thought to enhance the dim light of the Arctic winter. But they weren't sure what to make of another curious fact: unlike other mammals, reindeer can see light in the ultraviolet spectrum.
“Most animals that are active under daylight want to avoid UV light. UV light is harmful,” Dominy said. “Snow reflects UV light, which is a problem, and that's why people get snow blindness.”
Some scientists believe that reindeer's vision evolved to protect the animals from predators, allowing them, for example, to spot white wolves against a snowy landscape. The new study points to another possibility: food.
Reindeer live largely on light-colored reindeer moss, which is not actually a moss but rather a type of lichen that grows in crunchy, carpet-like patches in northern latitudes.
Researchers traveled to the Cairngorms mountain range in the Scottish Highlands, home to more than 1,500 species of lichen, as well as Britain's only reindeer herd. They found that reindeer moss absorbs UV light, meaning the white lichen that is difficult for humans to see against the snow stands out as dark spots for the animals.
“If you are a reindeer you can see it and you have an advantage because you are not wandering through the landscape. You can walk in a straight line and get to that food, and you save energy in the process,” Dominy said. “These animals are desperate for food, and if they can find enough lichens, they have an advantage.”
Juan Jose Negro specializes in evolutionary ecology and conservation biology at the Spanish Council for Scientific Research. Although his focus is mainly on birds of prey, he found the new reindeer research intriguing.
“I love any piece that has to do with colors and vision,” he said. “Every time I read the work of others, there is something that sparks new ideas. … And in the case of the reindeer, this makes me want to pay more attention to this part of the spectrum.”
Although he didn't see an immediate biomedical benefit to the research, such work is useful for increasing understanding of how animals cope with difficult environments, he said.
Dominy echoed that point, but said it also has human implications. Much pharmacological research has been done on lichens because they have antioxidant properties. Reindeer eyes that let in UV light suggest there may be a mechanism to protect them from damage, he said.
“Reindeer eyes are full of ascorbic acid, or vitamin C, and vitamin C is just great for repairing damaged cells,” he said.
With that in mind, Dominy updates the advice he gave after writing a 2015 article exploring why a reindeer's red nose would be ideal for guiding Santa's sleigh.
At the time, he recommended that children leave out Rudolph cookies and other high-calorie foods to compensate for the body heat he loses through his nose. Now focus on his eyes and save the milk and cookies for Santa, he says.
“The best thing you can give them to protect their eye health is something rich in vitamin C,” he said. “Orange juice, carrots, these would be perfect treats for reindeer on Christmas Eve.”