Reindeer Eyes Change Color To Adapt To Arctic Seasons' Crazy Light Changes
The eyes of reindeers change color depending on the season, an adaptation that helps the animals see predators during extreme changes in light across Arctic seasons. With an Arctic summer that never sees the sun go down and a stretch of winter when the sun never comes up, an interior part of the reindeer's eye called the tapetum lucidum shifts from gold in summer to blue in winter. The eye color shift is a fairly unique trick, says Glen Jeffery of the University of London College, who led a recent study on the reindeer eyes.
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"This is the first time a color change of this kind has been shown in mammals," said Jeffery. "By changing the color of the [tapetum lucidum] in the eye reindeer have flexibility to cope better with the extreme differences between light levels in their habitat between seasons."
The tapetum lucidum is a tissue layer behind the retina of many vertebrates. It reflects light back through the retina, a sort of mirror that gives photoreceptors a second chance to catch light that might not have properly reached photoreceptors the first time. Objects appear to be blurrier, but they are seen with an increased light sensitivity. For reindeers, an increase in light sensitivity during extremely dark winters is helpful at spotting predators, even if they appear as blurry predators.
Jeffery and his team collected reindeer eyes from Scandinavian herders who kill the animals around solstices. Jeffery started with 10 "summer" eyes and 10 "winter" ones, and after slicing them open he was shocked by the obvious differences in colors of the tapetum lucidums.
"When I opened them, I had the biggest shock I've ever had in science--the winter ones were clearly blue and the summer ones were clearly gold," said Jeffrey. "I wished I had someone sitting next to me to exclaim to."
Reindeers' eyes are dilated for months on end during dark winter months. When that happens, a flap drops down over reindeers' eyes, blocking fluid from draining and increasing pressure in the eye. This compresses the tapetum lucidum, which then reflects shorter wavelengths of Arctic winters' blue light.
"Reindeer are very plastic, so it is not surprising the eye would change," said Perry Barboza of the University of Alaska at Fairbanks, who was not involved in 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."
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