Post date: Dec 19, 2013 4:1:50 PM
Scientists have discovered that the colour of Arctic reindeers' eyes change colour dramatically between summer and winter, as they adapt to extreme changes of light intensity. The researchers say it's part of the animals' defence strategy, although for Santa's reindeer, they say it could also pose problems during next week's round-the-world present delivery marathon.
THE ARCTIC CIRCLE, ROVANIEMI, FINLAND (REUTERS) - Scientists have discovered that when Rudolph and Santa Claus's eight other trusty reindeers undertake their annual round-the-world journey on Christmas Eve, they'll have to overcome a previously unknown factor of evolution.
Research by a British-Norwegian team of scientists has established that the eyes of Arctic reindeer change colour through the seasons from gold to blue, adapting to extreme changes of light levels in their environment which helps them detect predators. The team says it's a potential evolutionary survival mechanism.It's the first time such a colour change has been shown in a mammal, a fact uncovered by accident by the team from University College London (UCL), and the University of Tromso in Norway. They showed that the colour change helps reindeer to see better in the continuous daylight of summer and the 24-hour darkness of Arctic midwinter, by changing the sensitivity of the retina to light.
Interested in how the reindeer adapts to such extremes of light, the team have made yearly trips to the Arctic to buy batches of eyes from Sami herder slaughterhouses. Reindeer are the main source of meat nutrition for the Sami who use every part of the body, except the eyes. However, during their most recent trip, the team arrived earlier than normal, which led to their unexpected discovery.
Arctic reindeer, like many animals, have a layer of tissue in the eye called the tapetum lucidum (TL) which 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. During examination of reindeer TL the researchers were astonished by what they saw, according to lead researcher, ProfessorGlen Jeffery from UCL.
"In summer it's the same colour as most other mammals," said Jeffery. "It's a golden colour and you find that in cows, you find that in sheep, you even find it in your cat. However, in winter it turns into a very very deep blue that was far less reflective and there was no ambiguity in this. There was a clear distinction between summer and winter eyes and we were completely shocked by that. We'd never expected it, it was the last thing we were looking for, but it was the most obvious thing that you saw when you opened the eyes. Big colour change."
They found that in the bright light of summer the TL in Arctic reindeer is gold, similar to many other mammals, which reflects most light back directly through the retina. However by winter it has changed to a deep blue which reflects less light out of the eye. This change scatters more light through photoreceptors at the back of the eye, increasing the sensitivity of the retina in response to the limited winter light
Jeffery and his colleagues repeated the experiments on live reindeer under sedation.
"We sedate the animal quite lightly and we put a very small gold electrode underneath the eyelid, and this is a standard procedure you actually get in ophthalmic hospital. The people that did this research with us were actually a group of electrophysiologists from Moorfields Eye Hospital. So we record the electrical activity to very low lighting situations and we find the visual threshold of the animal. What's the minimum thing it can see in summer, what's the minimum thing it can see in winter," he said.
The team believes blue TL is an advantage in the prolonged murk of winter, allowing reindeer to more easily detect moving predators and forage.
"What the animal is concerned about is whether he can detect a small amount of movement out on the left or the right of its visual field because if he sees it on the left he's got to run to the right. That is the primary problem for the reindeer. 'Can I see something moving? That is a potential threat. I've got to run, I've got to move.' So he needs to be sensitive to low light levels and he needs to be sensitive to movement at low light levels. That's what this animal does," said Jeffery.
The colour change may be caused by pressure within the eyes. In winter, the pressure in the reindeers' eyes increases, probably because of pupil dilation, which prevents fluid in the eyeball from draining naturally. This compresses the TL, reducing the space between collagen in the tissue and thus reflecting the shorter wavelengths of the blue light common in Arctic winters.
The team was funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and their findings published in October's issue of journal 'Proceedings of the Royal Society B'.
Previous work from Professor Jeffery and his colleagues from Tromso has shown that Arctic reindeer eyes can also see ultraviolet, which is abundant in Arctic light but invisible to humans, and that they use this to find food and see predators. The blue reflection from the winter eye is likely to favour ultra-violet sensitivity.
Jeffery says he fears that when Santa takes to his sleigh, his nine reindeer - Dasher, Dancer,Prancer, Vixen, Comet, Cupid, Donder, Blitzen and Rudolph - will face difficulties in adapting to the bright lights they will encounter while circumnavigating the globe. But he does have some advice that will make their mission easier.
"When Rudolph comes down from the Arctic he's going to be very very dark adapted and he's going to have a blue eye and the one thing we don't want Rudolph to do is to come up against a whole load of street lights and a whole load of front room lights, so a good thing if you want Rudolph to call in at your place might be draw the curtains, so he doesn't get glared by all the light."