Reindeer use ultraviolet vision to find food in the winter

In the icy expanse of the Arctic winter, where the landscape is cloaked in snow and darkness, reindeer possess a remarkable adaptation that aids in their survival: the ability to see ultraviolet light. 

This unique trait enables reindeer to locate their primary food source, lichen, in the harsh winter months.

Arctic survival

Researchers from Dartmouth and the University of St. Andrews in Scotland have shed light on the long-standing mystery of why reindeer, including the legendary Rudolph, can perceive ultraviolet (UV) light. The study, led by Professor Nathaniel Dominy, reveals a fascinating aspect of reindeer biology.

“Reindeer are so cool, but many people think about them only at Christmas,” said Dominy. “Now is a good time to alert people to their extraordinary visual system.”

The lichen connection

Reindeer moss, or Cladonia rangiferina, is not moss but a type of algae-fungus fusion known as lichen. This lichen forms extensive mats across northern latitudes and is critical for reindeer survival, so much so that its name is derived from the scientific term for reindeer, Rangifer.

In the Cairngorms mountains of the Scottish Highlands, home to Britain’s only reindeer herd, the researchers observed the reindeer’s reliance on C. rangiferina, especially during winter. This preference for a singular food source is uncommon among large mammals.

Ultraviolet light

The researchers discovered that C. rangiferina and a few other lichen species absorb UV light. 

While invisible to the human eye against the snowy backdrop, these lichens appear as distinct, dark patches in reindeer vision, making them more identifiable in the vast white landscape.

How reindeer see the world

“Getting a visual approximation of how reindeer might see the world is something other studies haven’t done before,” said Dominy, who published a paper in 2015 on how Rudolph’s red nose would have acted as an effective foglamp in the haze of winter.

“If you can put yourself in their hooves looking at this white landscape, you would want a direct route to your food.”

“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.”

Visual adaptations 

Reindeer not only have unique visual capabilities but also undergo seasonal changes in their eyes. During winter, their tapetum (a light-reflecting layer) shifts from a golden to a vivid blue hue. This transition is thought to enhance low light visibility in polar winters. 

“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,” explained Dominy.

The blue tapetum also allows about 60% of UV light to reach the retina, painting the winter world in shades of purple, akin to a room under a black light.

Beyond human perception 

This study offers an answer to why reindeer, despite being diurnal Arctic animals, have evolved to perceive UV light. It seems their vision is specially adapted to distinguish food sources like lichen from the reflective snow-covered surroundings.

Given the importance of lichens in their diet, it is possible that reindeer eyes are optimized to single out this food staple at the time of year it would be most difficult to find, noted the researchers.

While the red-nosed reindeer may guide Santa’s sleigh with his luminous nose, it is, in fact, the blue eyes of reindeer that play a crucial role in their survival. These eyes, attuned to a world beyond human perception, allow them to efficiently locate their food in a landscape where every calorie counts.

The study is published in the journal i-Perception.

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