According to scientists, reindeer “adjust” their eye structure to better search for food and escape from predators during the long twilight and dark months of winter.
On Christmas Eve, a team of reindeer “flies” across the sky, pulling Santa Claus and his sleigh filled with gifts. But Rudolph’s group (the red-nosed reindeer) is not the only reindeer team doing something special.
Back in the Arctic, their “cousins” have an “optical miracle” not seen elsewhere in the animal kingdom: Reindeer change their eye structure to better search for food and evade predators during the long dark months of polar twilight.
Reindeer can change their eye structure to find food better and escape from predators. (Source: National Geographic).
In summer, the tapetum lucidum of reindeer – a mirror-like layer at the back of the eye – appears bright yellow mixed with turquoise, shimmering like golden opal. But in winter, this tapetum lucidum turns dark blue.
Scientists have spent years decoding this subtle optical phenomenon.
National Geographic quotes Glen Jeffery, a neuroscientist at University College London and author of the study, stating: “What we found is an amazing, unique, and strange biological mechanism – and it makes perfect sense.”
Adapting to Winter
At 70 degrees North latitude, near Tromsø in Norway or Utqiagvik (formerly Barrow) in Alaska, the sun does not even rise above the horizon for more than 60 days in winter. This causes the daily life of reindeer to be “submerged” in 12-24 hours of twilight.
“Even in winter, in Yukon or Northern Manitoba, you still have a day-night cycle. [In the mentioned latitudes] we do not have that cycle,” says Nicholas Tyler, a researcher at the Sami Research Centre at the Arctic University of Norway in Tromsø. “This is truly unique.”
Winter twilight is at least 100,000 times dimmer than summer daylight, and is “stained” a deep blue. This is because when the sun is below the horizon, sunlight filters through the atmosphere before “bending” down to Earth.
The rays travel through a particularly long path filled with ozone. This ozone absorbs almost all orange and red light – leaving only blue, which reflects down to Earth and blankets the landscape in deep blue.
“It’s like a filter in the sky, removing orange light and keeping blue,” Fosbury explains.
Many animals must navigate through dim light. A common adaptation is the tapetum lucidum – located behind the retina to absorb light.
In the dark, every photon is crucial: Sometimes, a photon enters the eye but does not reach the small light-absorbing pigment layer of the retina. The tapetum reflects that photon back out, creating another “opportunity” for absorption.
For some nocturnal animals like cats, the tapetum’s reflection can double the light hitting the light receptors, says Braidee Foote, a veterinary ophthalmologist at the University of Tennessee, Knoxville.
Foote explains that the tapetum can vary in color, but is often golden or greenish. This layer is the reason why cat or panda eyes reflect strangely at night.
So why does the tapetum of reindeer turn blue in winter? The answer likely relates to maximizing light absorption in the blue and ultraviolet spectrum – throughout the long twilight and dark periods of winter.
Humans perceive light from wavelengths of blue around 400 nanometers to red 700 nanometers, but reindeer can see clearly in the ultraviolet (UV) range, which can cause snow blindness in humans.
(Source: San Diego Zoo Animals & Plants).
Fosbury notes that UV vision can benefit reindeer in two ways.
First, it helps reindeer find food in winter when it snows. Lichens – a primary food source in the winter diet of reindeer – absorb UV rays, appearing as “dark patches” against the white snow reflecting UV light.
Wolf and polar bear fur also absorbs UV, so instead of blending into the snow, they stand out with high contrast. This allows reindeer to detect these predators more easily.
Nathaniel Dominy, an anthropologist at Dartmouth, suggests that other animals in the far North may also “do the same” but “we haven’t studied that yet.”
“Adjusting Their Eyes”
A more challenging question arises: How do reindeer “adjust their eyes?” – That’s where astrophysicists come into play.
Fosbury has studied the optical conditions during twilight in the Arctic and found that the reflective layer self-“tunes” to that light frequency.
He and Jeffery went into the lab to dissect and test a large number of reindeer eyes. The bags and jars containing the eyes were collected over many years – from the Sami reindeer herds, an indigenous Scandinavian people.
The reindeer tapetum consists of tiny collagen fibers suspended in liquid, forming a reflective crystal that can change. The collagen fibers in summer eyes are “loosely floating” in the liquid, creating a crystal mirror that reflects the best red light.
But in the winter-collected eyes, the collagen fibers are “packed tightly” together, altering the crystal shape and primarily reflecting blue light.
In the dark, reindeer can dilate their pupils, blocking a small drainage hole for the liquid in their eyes. This causes internal eye pressure to rise, compressing the tapetum collagen and changing the shape of the crystal. In summer, the reindeer’s pupils return to normal.
Tyler states: “If you add all these factors together, the sensitivity of reindeer eyes in winter is at least 1,000 times greater than in summer.”
However, the unique adaptations of reindeer may also pose dangers to them.
Today, high-voltage power lines run across the traditional grazing territories of the Sami, emitting UV rays. Reindeer perceive these UV rays as “fireworks” and “they won’t come close,” says Jeffery.
