Reindeer can see UV light—and we may know why

Scientists think that their special eyes have evolved for foraging in the snow.
A reindeer stands on snowy ground with hills in the distance.
Reindeer from Britain’s only reindeer herd at The Cairngorms National Park in Scotland. The animals were introduced to Scotland in 1952 by Swedish Sami reindeer herder, Mikel Utsi. Jeff J. Mitchell/Getty Images

To survive the dark and snowy Arctic winters, reindeer have evolved unique visual systems. Their eyes change color to adjust to the huge swings in sunlight between Arctic summer and winter, but may do even more to help them forage. A study published December 15 in the journal i-Perception found that their eyes may have evolved to see light in the ultraviolet spectrum to help them find their favorite food in their desolate  home.

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Reindeer primarily eat Cladonia rangiferina (C. rangiferina), which is appropriately nicknamed reindeer moss. This plant is not a moss, but a species of algae-fungus called lichen. It forms a thick and crunchy blanket on the ground across the Earth’s northern latitudes and helps play an important role in the ecosystem as a food source. 

In the study, the team worked in the Cairngorms mountains in the Scottish Highlands, home to Britain’s only reindeer herd. Reindeer were locally hunted to extinction, but began to be reintroduced from Scandinavia in 1952. The Cairngorms are home to more than 1,500 species of lichen, but the reindeer here only rely on C. rangiferina during the winter months

“A peculiar trait of reindeer is their reliance on this one type of lichen,”  study co-author and Dartmouth College anthropologist and evolutionary biologist Nathaniel Dominy said in a statement. “It’s unusual for any animal to subsist so heavily on lichens, let alone such a large mammal.”

When up against snow, the white lichen is invisible to the human eye.. However, co-authors Catherine Hobaiter and Julie Harris from the University of St. Andrews found that C. rangiferina and some other lichen species that supplement the reindeer diet absorb ultraviolet (UV) light. The team used spectral data from the lichen and light filters that were made to mimic reindeer vision and found that the plants may look like dark patches against a bright landscape to the reindeer. They stand out like Dalmatian spots and are easier for the reindeer to locate.

According to Dominy, this is one of the first studies to use a visual approximation of how these mammals may see their world. 

“If you can put yourself in their hooves looking at this white landscape, you would want a direct route to your food,” Dominy said. “Reindeer don’t want to waste energy wandering around searching for food in a cold, barren environment. If they can see lichens from a distance, that gives them a big advantage, letting them conserve precious calories at a time when food is scarce.”

Some animals that can see on the UV spectrum include dogs, cats, pigs, and even ferrets. They generally do this with the short blue photoreceptors called cones present in their eyes. 

Earlier studies have shown that reindeer eyes change from golden in the summer and a vivid blue in the winter. The light-enhancing membrane that gives many animals a shiny eye called the tapetum transitions every season. The blue hue of their eyes is believed to amplify the low levels of sunlight present during polar winters. 

[Related: How do animals see the world?]

“If the color of the light in the environment is primarily blue, then it makes sense for the eye to enhance the color blue to make sure a reindeer’s photoreceptors are maximizing those wavelengths,” Dominy says.

However, the blue tapetum also lets up to 60 percent of UV light pass through to the eye’s color sensors. The reindeer likely see the winter world as a shade of purple the way a human may see a room with a black light. Snow and other UV-reflecting surfaces then shine brightly while surfaces that absorb UV light are dark.

Scientists have investigated why an Arctic animal that is active during the day would have eyes that are so receptive to UV light that reflects off of the snow. This study suggests that the answer to this question is tied to C. rangiferina and other lichens, since UV light doesn’t reflect from those organisms. The team believes that it is possible that reindeer eyes are optimized to single out lichens during the times of year where it is most difficult to find since it is a food staple.