Don’t it make my Brown eyes blue…...

Don’t it make my brown eyes blue…….

A recent edition of the Los Angeles Times ran a story that caught the seasonal, optometric imagination. That of the phenomenon of reindeer eyes changing colour from brown in the summer to blue in the winter.

Was it really about this that the American, Country pop singer Crystal Gayle sang about in her 1977 song?

It appears that, in the summer, the reflective layer behind the retinae of reindeer, the tapetum lucidum, an interior part of the eyes of Arctic reindeer appears gold, and around Christmas it turns a deep blue. Biologists have discovered that it is a unique adaptation that helps these animals deal with the strange light conditions at the top of the world.

The reindeer’s world is one of extremes. Above the Arctic Circle, Christmas falls in the midst of a 10-week period of perpetual twilight in which the sun never rises and the landscape is cast in bluish hues. But from mid-May to late July, the sun never sets, creating a long, endless day.

Biologists at the Norway's University of Tromso, in one of the largest cities situated north of the Arctic Circle, wondered how the reindeer managed the transition from a world of near-total darkness to one of blinding light, when springtime sunlight reflects off still-unmelted snow.

To find out, they collected reindeer eyes from the Sami, indigenous herders who often slaughter the animals around the solstices. The Norwegian researchers collected eyes during both the winter and summer months, then mailed them off to Glen Jeffery, a neuroscientist who studies vision at University College London.

Humans don’t have this structure, but lots of other animals do. It helps nocturnal animals see at night by bouncing light back inside the eye, giving the light receptors in the retina a second chance to be stimulated. The tapetum lucidum is responsible for the flash of “eye shine” you see when a cat looks into a car’s headlights.

Scientists had always assumed that this piece of ocular anatomy's color was fixed.

“This is the first time that a change in color in the tapetum has been shown in a mammal,” Jeffery said.

In a study published in the Proceedings of the Royal Society B, Jeffery and his colleagues in Norway explained that when a reindeer’s tapetum is blue, 50% less light is reflected out of the eye than when the tapetum is gold. A reindeer with a blue tapetum sees less clearly than one with a gold tapetum, but its eyes are 1,000 times more sensitive to light.

“Clinically, the reindeer become glaucomic,” Jeffery said.

Perhaps Santa should find animals of another species to pull his sled on one of the darkest nights of winter.

But the scientists argue that losing acuity and gaining light sensitivity is probably a worthwhile trade-off for reindeer on the ground because it allows them to detect a moving predator in the darkness — even if they can’t see it clearly.

“Reindeer are very plastic, so it is not surprising the eye would change,” said Perry Barboza, who studies Arctic animals at the University of Alaska at Fairbanks and was not involved with the study. “Many of their external characteristics change as winter approaches — their coats fill out and go from brown to white, they put on a lot of body fat. The eye color change is just another part of that story.”

The scientists determined that the color of light reflected by a reindeer’s tapetum likely depends on how much fluid pressure there is in the eye itself. It took Jeffery and his colleagues in Norway nearly 10 years to figure this out.

In the dark winter months, the reindeer's pupils dilate completely to let in as much light as possible. That action also causes a flap to descend over the back of the eye where fluid normally drains out. Since the fluid has no way to escape, the pressure inside the eye increases. That, in turn, causes collagen fibers in the tapetum to squish together, which changes its color from gold to blue.

But blue and gold are not the only colors in the reindeer tapetum spectrum. The researchers also checked the eyes of a small herd of reindeer that lived on the campus of the University of Tromso, and who were exposed to permanent distant urban lighting. Instead of turning blue in the winter, they became green.,0,1233478.story#ixzz2oIlGzyJh

© Dr Rob Hogan 2017