According to scientists, reindeer “adjust” the structure of their eyes to better find food and escape predators during the long, dark months of winter.
On Christmas Eve, a team of reindeer “soars” through the sky, pulling Santa Claus and his sleigh brimming with gifts. However, Rudolph’s team (the red-nosed reindeer) isn’t the only group of reindeer doing something extraordinary.
In the Arctic, their “relatives” possess a unique “optical ability” unseen in any other species: Reindeer alter the structure of their eyes to enhance their ability to locate food and evade predators during the prolonged darkness of the polar twilight.
In summer, the reindeer’s tapetum lucidum – a mirror-like layer behind the eye – is a brilliant yellow mixed with turquoise, shimmering like a golden opal. But in winter, the tapetum lucidum turns a deep blue.
Scientists have spent years deciphering this subtle optical phenomenon.
“What we found is a wonderful, unique, and bizarre biological mechanism – and it makes perfect sense,” said Glen Jeffery, a neuroscientist at University College London and author of the study, in a statement from National Geographic.
Adapting to winter
At latitudes of 70 degrees north, near Tromsø in Norway or Utqiagvik (formerly Barrow) in Alaska, the sun doesn’t even appear above the horizon for more than 60 days in winter. This leaves reindeer with 12-24 hours of twilight each day.
“Even in the winter, in the Yukon or northern Manitoba, you have a day-night cycle. [At that latitude] we don’t have that,” says Nicholas Tyler, a researcher at the Sami Research Centre at the Arctic University of Norway in Tromsø. “It’s really unique.”
Winter twilight is at least 100,000 times dimmer than summer daylight, and is tinged a deep blue. That’s because when the Sun is below the horizon, its rays bounce upward through the atmosphere before “bending” back down to Earth.
The rays travel a particularly long path of ozone. That ozone absorbs almost all the orange and red light—leaving only the blue, which reflects back down to Earth and casts the landscape a deep blue.
“It functions like a filter in the sky, blocking orange light and allowing blue light to pass through,” Fosbury explains.
Numerous animals must survive in low-light conditions. A typical adaptation for this is the tapetum lucidum, located behind the retina, which helps absorb light.
When living in the dark, every photon counts: Sometimes a photon will enter the eye but miss the retina’s small light-absorbing pigment layer. The tapetum reflects that photon back out, giving it another “opportunity” to be absorbed.
For some nocturnal animals, like cats, the tapetum reflex can double the amount of light hitting the photoreceptors, says Braidee Foote, a veterinary ophthalmologist at the University of Tennessee, Knoxville.
Foote explains that tapetum comes in a variety of colors, but is typically a mirror-like metallic gold or a green. The tapetum is the reason why a cat’s or raccoon’s eyes look strangely reflective at night.
So why does the tapetum of reindeer turn green in winter? The answer probably has to do with the maximum absorption of light in the blue and sub-blue ranges — during the long, dark twilight of winter.
Humans perceive light from about 400 nanometers of blue wavelengths to 700 nanometers of red wavelengths, but reindeer can see clearly in the shorter ultraviolet (UV) range. The UV range is what causes snow blindness in humans.
Fosbury says UV vision could help reindeer in two ways.
First, it could help reindeer find food during the winter when the snow falls. Lichens — a staple of the reindeer’s winter diet — absorb UV rays, so they appear as “dark patches” against the white, UV-reflecting snow.
Wolf and polar bear fur also absorb UV light, so instead of disappearing into the snow, it stands out with high contrast. This allows reindeer to spot predators more easily.
Nathaniel Dominy, an anthropologist at Dartmouth, says it’s possible that other Arctic animals “do the same thing,” but “we don’t know yet.”
“Eye tuning”
The next question is more difficult: How do reindeer “tune their eyes?” That’s where the astrophysicist comes in.
Fosbury studied the optical conditions during Arctic twilight and found that the reflective film “tunes” itself to that frequency of light.
He and Jeffery went into the lab to dissect and test a large number of reindeer eyes. The bags and jars of eyes were collected over many years — from reindeer herds of the Sami, an indigenous people of Scandinavia.
Reindeer tapetum is made up of tiny collagen fibers suspended in fluid, forming a reflective crystal that can change shape. The collagen fibers in summer eyes “float loosely” in the fluid, creating a mirror crystal that best reflects red light.
But in eyes collected in winter, the collagen fibers are much more “tightly packed,” changing the shape of the crystal and causing it to reflect mostly blue light.
In the dark, reindeer can dilate their pupils, blocking a small drainage hole for fluid in the eye. This causes the pressure inside the eye to increase, compressing the tapetum collagen and changing the shape of the crystal. In summer, reindeer pupils return to normal.
