Rudolph the blue-eyed reindeer, he can see the lichen glows…

Sadly, its purpose isn’t to guide Santa’s sleigh once a year but to serve a crucial everyday mission: finding sufficient food in cold weather.

Scientists from the University of St Andrews, working alongside a colleague from Dartmouth College in the USA, were examining why reindeer are the only daytime mammals whose eyes change colour according to the season.

In summer, the surface of the animal’s eyes – the tapetum – is golden orange; come winter this turns to a rich blue.

Writing in the journal i-Perception, the researchers said that while the blue hue may help their vision during the almost permanent twilight conditions of northern winters it also means the creatures' eyes will transmit potentially dangerous ultraviolet light. And when coupled with snowy conditions, the risk increases because sunlight reflects off the ground, doubling the UV dose.

The study, whimsically titled 'Reindeer and the quest for Scottish enlichtment' – led by Professor Nathaniel Dominy, Charles Hansen Professor of Anthropology at Dartmouth College, and  Dr Catherine Hobaiter and Professor Julie Harris, both of St Andrews' School of Psychology and Neuroscience – considered previously posited theories that the adaptation could be related to diet.

Reindeer consume large amounts of lichen and one species in particular; Cladonia rangiferina or reindeer moss (despite its name, not a moss) which grows in dense, spongy and pale beds not easily visible to the human eye in snowy weather.

Evidence for a link was scarce, however, as the species is just one among 13,000 and had been subject to little testing. However, Scotland’s Cairngorms region offered promising conditions to observe reindeer and lichen populations in tandem, said St Andrews’ Hobaiter.

She said: "One of the amazing things about Scotland is that we have some of the richest lichen diversity in the world: the Highlands host over 1500 different species, and the Cairngorms are home to a local herd of reindeer, reintroduced to Scotland after being hunted to extinction. So, Scotland is the perfect place to try and finally solve the mystery."

By photographing the lichen under UV light, the team identified different species of lichen absorb or reflect UV light differently, with the reindeer moss being a particularly strong absorber.

Therefore, Hobaiter explained, reindeer eyes are able to see the species they feed on because the reindeer moss lichen appear to them as  dark patches in the far more reflective snow.

Professor Dominy added: "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." 

Pic: Nicholas Lafargue