Rudolf’s eye adaptation copes with extremes

Reindeers’ eyes have to function in extreme conditions – continuous summer daylight, followed by continuous winter darkness – and researchers have discovered they adapt their eye colour to deal with the Arctic seasons.

Researchers from University College London and the University of Tromsø – funded by the Biotechnology and Biological Sciences Research Council – have shown that the eyes of Arctic reindeer change colour through the season from gold to blue, adapting to extreme changes of light levels in their environment.

Like many animals, reindeer have a layer of tissue at the back of the eye called the tapetum lucidium (TL), which reflects light back through the retina to enhance night vision. The wavelength of reflection depends on the TL collagen spacing – reduced spacing equals a shorter wavelength reflected.

In the summer, the eye is gold and most light is reflected through the retina. But in the winter, they eye is deep blue and less light is reflected out of the eye.

“This is the first time a colour change of this kind has been shown in mammals,” said lead researcher, Professor Glen Jeffery from UCL. “By changing the colour of the TL in the eye reindeer have flexibility to cope better with the extreme difference between light levels in their habitat between seasons.”

This shift to blue reflection may scatter light through photoreceptors rather than directly reflecting it, leading to an elevated retinal sensitivity via increased photon capture. It also leads to an increase in intraocular pressure, probably a result of permanent pupil dilation blocking ocular drainage.

The increased sensitivity leads to a reduced acuity, which might be an important trade-off to allow the reindeer to detect moving predators in Arctic winter.

“This gives them an advantage when it comes to spotting predators, which could save their lives,” said Jeffery.

The work – published in Proceedings of the Royal Society B – builds on the team’s previous work which showed Arctic reindeer eyes can also see ultraviolet. The blue reflection is likely to favour ultraviolet sensitivity.

Shifting mirrors; adaptive changes in retinal reflections to winter darkness in Arctic reindeer http://dx.doi.org/10.1089/rspb.2013.2451

Dr Glen Jeffery speaks about his research 

http://www.youtube.com/watch?v=UOT7HOuZNno