What color are reindeer eyes? Depends on the season.
An astrophysicist and an eye expert teamed up to solve the mystery of how and why reindeers’ eyes switch from yellow to blue in wintertime.
By Alejandra Borunda
Published December 20, 2022
• 6 min read
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On Christmas Eve, a team of reindeer will crisscross the sky towing Santa and his present-laden sleigh.
But Rudolph’s team won’t be the only reindeer doing something special. Back in the highest Arctic, their cousins will be performing an optical feat not observed elsewhere in the animal world: changing their eye structure to better find food and escape predators during the long, dark months of polar twilight.
In the summer, reindeer’s tapetum lucidum—a mirror-like layer at the back of their eye—is a luminous gold streaked through with turquoise, iridescent like a golden opal. But in the winter, that layer turns a deep, rich blue.
Scientists have spent years untangling the mystery of why and how they make the dramatic change, but it took an astrophysicist, a neuroscientist, and bags of reindeer eyes to disentangle the elegant optics. (See National Geographic‘s favorite reindeer pictures.)
“What we’ve found is a really great biological mechanism that’s totally unique and bizarre—and makes perfect sense,” says Glen Jeffery, a neuroscientist at University College London and author of a recently published study explaining the phenomenon.
Winter adaptation
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At 70 degrees north, near Tromsø in Norway or Utqiagvik (formerly Barrow) in Alaska, the sun doesn’t even cross the horizon for more than 60 days in winter, leaving reindeer drenched daily in 12 to 24 hours of deep twilight.
“Even in winter, in the Yukon or northern Manitoba, you’ve got a day-night cycle. We don’t have that,” says Nicholas Tyler, a researcher at the Centre for Sami Studies, at the Arctic University of Norway in Tromsø. “That’s a really unique thing.”
Winter twilight is at least 100,000 times fainter than is summer daylight. It’s also tinted a rich blue. That’s because when the sun is below the horizon, its rays cut upward through the atmosphere before being deflected down toward Earth; they travel through an exceptionally long pathway full of ozone. That ozone absorbs nearly all the orange and red light—leaving behind just the blue, which bounces down to Earth and drapes the landscape in ultramarine.
“It’s like a filter arching across the sky,” says Fosbury, “which filters out the orange light and leaves the blue.”
Plenty of animals deal with dim light. One common adaptation is the tapetum lucidum, which sits behind the light-absorbing retina. When living in near-darkness, every photon matters: Sometimes, one will cross into the eye but miss the retina’s little catchers’ mitts of light-absorbing pigments. The tapetum bounces that photon back outward, giving it another chance to be absorbed. For some nocturnal animals like cats, tapetum reflectance can more than double the light hitting photoreceptors, says Braidee Foote, a veterinary ophthalmologist at the University of Tennessee, Knoxville.
Tapeta vary in color, but are often yellowish gold like a bronzed mirror or greenish, explains Foote—the tapetum is the reason cat or raccoon eye reflect eerily at night.
So why do reindeer tapeta turn blue in winter? The answer probably has to do with maximizing light absorption in the blue and below-blue color range during the long, dark winter twilight.
Humans perceive light from blue wavelengths of about 400 nanometers to the reddish 700-nanometer realm, but reindeer see well into the shorter ultraviolet (UV) range, which causes snow blindness in humans.
That UV vision could help two ways, says Fosbury. First, it’s likely to help them find food in the snowy winter. Lichens, a staple of reindeer winter diet, absorb UV, so they show up dark against UV-reflecting white snow. Wolf and polar bear fur also absorb UV, so instead of disappearing against snow they pop out in high contrast, allowing reindeer to spot predators more easily.
It’s likely other far-northern animals do something similar—but “we just haven’t looked yet,” says Nathaniel Dominy, an anthropologist at Dartmouth.
Colour in the collections: reindeer eye
One of the more unusual colourful specimens at the Museum is a reindeer eyeball that has been sliced in half.
The iridescent layer is a colour-changing marvel that transformed with the seasons.
When it is carved in two like this, the startling colours of the reindeer eye become obvious.
Part of the national eye collection housed at the Museum, this specimen shows the interior of the creature’s eye in extraordinary detail.
Not only is the reindeer’s eye beautiful, it is a scientific marvel. The animal’s eyes change from a golden colour to a deep blue as summer fades to winter, helping it to see in dramatically different light levels.
What is happening?
The part that changes colour is called the tapetum lucidum. It is a shiny, mirrored layer behind the retina that helps some animals to see in the dark.
When light enters the eye much of it hits the sensitive cells in the retina. But sometimes it misses the mark. The tapetum lucidum gives the eye a second chance to detect the light by reflecting it back towards the retina again.
It is the reason some mammals, including cats, have eyes that seem to glow when light is shined on them.
In many animals this reflective layer shines gold, permanently. While reindeer eyes are also gold in the summer months, in winter their layer turns blue.
Why the colour change?
Scientists think the blue colour helps to capture even more light. Animals that live in the Arctic experience dramatic changes in light levels, with long hours of bright light in the summer and almost total darkness in winter.
One theory is that in the winter, pressure inside the animal’s eye builds due to the effort of keeping the pupils dilated and large for months on end.
The pressure squeezes fluid out of the tapetum lucidum, which is formed mostly of collagen fibres. As result, the fibres pack together more tightly and start to reflect blue wavelengths of light instead of yellow.
The blue eyes become over a thousand times more sensitive to light than the yellow summer ones, making reindeer vision perfectly adapted to its unforgiving habitat.
