Rudolph’s red nose isn’t the only thing helping him navigate this Christmas

When Rudolph leads the reindeer touch down at the North Pole on December 26, it won’t be his red nose helping them find their breakfast.

Instead, researchers have found that reindeer eyes may have evolved to spot their favourite food during the dark and snowy Arctic winters.

No, it’s not a tasty carrot!

The species of lichen key to the reindeer diet is easier to spot against the UV-reflecting snowy backdrop because it absorbs ultraviolet (UV) light, according to a new study in the journal i-Perception.

“Getting a visual approximation of how reindeer might see the world is something other studies haven’t done before,” says Nathaniel Dominy, professor of Anthropology at Dartmouth College in the US, and first author of the paper.

“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.”

A 2014 study previously suggested that reindeer use their UV vision to detect vegetation amid snow, finding evidence that vascular plants, but not lichens, are visually distinctive in snow in UV-only images. However, they only looked at a species of lichen that is not eaten by reindeer, Ophioparma ventosa.

Lichen is a plant-like organism made up of a symbiotic association between a fungus and an alga or cyanobacteria. Different species of lichen can vary in how much UV light they absorb and reflect, so the researchers wanted to study a specific species of lichen Cladonia rangiferina – also known as reindeer moss – that forms a major part of reindeer diet.

For this study, researchers simulated reindeer vision to visualise what reindeers see when looking for their favourite food.

To humans, the white-ish coloured reindeer moss tends to disappear against a snowy backdrop. But using spectral data and light filters that mimic reindeer vision revealed that these organisms appear to reindeers as dark, distinct patches against the landscape.

They appear dark because they absorb more UV light and reflect less of it back like the rest of the snow-covered landscape, making them easier to locate.

The finding also helps to explain why reindeers’ eyes change from summer to winter. The reindeer’s tapetum – the light-enhancing membrane that gives many animals “shiny” eyes – changes from a golden colour in summer to a vivid blue in winter.

These golden tissues usually block UV wavelengths, protecting the eyes of day-active mammals.

The reindeer is the only mammal with a colour-shifting tapetum lucidum and it has been thought that this shift to blue helps to amplify the low light of polar winter.

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

But the blue tapetum also allows up to 60% of ultraviolet light to pass through to the eye’s colour sensors, which is enough to excite the cones responsible for colour vision.

The result? The reindeer’s eyes are optimised to single out the lichen at the time of year when it is most difficult to find.

So, while Rudolph’s red nose helps him to guide Santa’s sleigh on Christmas eve, the authors conclude that it is his “blue-eyes that allow him to find dinner after a long Christmas season.”