Arctic Reindeer Eyes Change Color Along With Seasons

Researchers from UCL (University College London) and the University of Tromso, Norway, have discovered that the eyes of the Arctic reindeer change color through seasons in order to adapt to the extreme changes in the light level in their environment and help detect predators. The eye color changes to gold in summer and deep blue during the cold winters.

Through this finding the researchers showed that this color change in the eyes of the reindeer helps them to see better in the continuous daylight of summer as well as the continuous darkness of the Arctic winters, by altering the sensitivity of the retina to light.

Similar to other animals, the Arctic reindeers carry a layer of tissue in the eye known as tapetum lucidum (TL). This layer is present behind the retina and reflects light back to enhance the vision in dark. When the light color changes, TL reflects the various wavelengths of light.

The researchers noticed that the Arctic reindeers' TL changes to gold in the bright light of summer and in winter it changes to deep blue, which reflects less light out of the eye.

When the change in TL occurs, more light is scattered through the photoreceptors at the back of the eye and this increases the retina's sensitivity in response to the limited light available in winter.

Lead researcher Professor Glen Jeffery from UCL, said in a statement, "This is the first time a colour change of this kind has been shown in mammals. By changing the colour of the TL in the eye reindeer have flexibility to cope better with the extreme differences between light levels in their habitat between seasons. This gives them an advantage when it comes to spotting predators, which could save their lives.'

 The researchers assume that the change in the color occurs due to the pressure within the eyes. The pressure in the reindeer's eye increases during winter and this probably is caused due to the permanent pupil dilation that curbs the fluid in the eye from draining naturally.  Due to this the TL gets compressed and the space between the collagen and the tissues decreases thus reflecting shorter wavelengths of blue light that is common in Arctic winters.

Prior to this, the same team had revealed how the Arctic reindeer eyes can also see ultraviolet light that is abundant in Arctic light but invisible to humans. The reindeers use it to spot predators and forage. They believe that the reflection of blue color in the eye of the reindeer is likely to favour ultra violet sensitivity.

Studies conducted earlier by researchers from Netherlands and Norway uncovered the scientific reason behind Reindeer's red nose 

The findings were documented in the Proceedings of the Royal Society B.