An Arctic reindeer's eyes change colour, from gold to blue, depending on the season, new research has found.
When the sun is shining brightly in the summer, their eyes are gold, but that changes to a deep blue in winter when it's almost always dark.
The changing colours help reindeer see better in extreme lighting differences. The researchers from University College London and the University of Tromso in Norway said many animals have a layer of tissue in the eye called the tapetum lucidum (TL). It lies behind the retina and reflects light back through it to enhance night vision.
By changing its colour, the TL reflects different wavelengths of light.
The exact cause of the colour change was not part of the study, but it could be caused by pressure within the eye, the researchers said.
Lead researcher Glen Jeffery said this is the first time a colour change of this kind has been shown in mammals.