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Long before there were filters and apps to change the colour of your eyes in photos, some animals had eyes that changed colour naturally, according to the season.

Scientists have discovered reindeer use a special form of night vision to help them find food in the colder months.

The surface of their eye changes from golden-orange in summer to a rich blue colour in the winter.

Experts think their vision might have evolved to help them see in snowy conditions and explains why they are able to see food so well in the dark.

Researchers from the University of St Andrews in Scotland teamed up with Dartmouth College in America to take a closer look at why reindeer eyes change colour with the season.

They found the colour shift might help reindeer to see better in the dark Arctic winters when snowfall can be heavy - it also allows their eyes to transmit ultraviolet light.

Scientists think this is the reason why reindeer are able to see clearly in the dark and spot their favourite food lichen which is crucial for their survival in winter.

Reindeer are known to feed on a particular species of lichen which grows in sponge-like beds across the northern latitudes where many reindeer live.

Dr Catherine Hobaiter, who helped lead the research, explained: "To our eyes, just like for most mammals, pale lichen in white snow is incredibly hard to spot.

"But to reindeer eyes, the species they need to feed on would stand out as dark patches in the much more reflective snowy landscape."

Professor Dominy from Dartmouth College 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."