Reindeer can change eye color to 'match' the long winter months

Reindeer "adapt" their eye structure to better find food and escape predators during the long, dark months of winter, scientists say.

On Christmas Eve, a group of reindeer 'fly' across the sky, dragging Santa Claus and his sleigh full of presents. But Rudolph's group (the Red-Nosed Reindeer) are not the only reindeer doing something special.

Back in the Arctic, their 'cousins ' have an 'optical feat' seen nowhere else in the animal world: Reindeer change the structure of their eyes to better find food and escape predators during the long, dark months of polar twilight.

Reindeer can change the structure of their eyes to better find food and escape predators. (Source: National Geographic).

In summer, the reindeer's tapetum lucidum —a mirror-like layer behind the eye—is a brilliant yellow tinged with turquoise, shimmering like a golden opal. But in winter, the tapetum lucidum turns a deep blue.

It took scientists years to decipher this subtle optical phenomenon.

'What we found is a wonderful, unique, and bizarre biological mechanism — and it makes perfect sense,' Glen Jeffery, a neuroscientist at University College London and author of the study, told 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 winter, in the Yukon or northern Manitoba, you have a day-night cycle. [At that latitude] we don't have that cycle,' 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 'tinted' 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 of light pass through a particularly long path filled with ozone. That ozone absorbs almost all the orange and red light—leaving only the blue, which reflects back to Earth and casts the landscape a deep blue hue.

'It's like a filter in the sky, filtering out the orange light and keeping the blue,' says Fosbury.

Many animals have to live in dim light. One common adaptation is the tapetum lucidum - located behind the retina that absorbs light.

When living in the dark, every photon counts: Sometimes a photon will enter the eye but miss the tiny light-absorbing pigment layer of the retina. 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 metallic gold like a bronze mirror or a green. The tapetum is the reason why cat or raccoon eyes look strangely reflective at night.

So why does the reindeer tapetum turn blue in winter? The answer probably has to do with maximizing light absorption in the blue and sub-blue range — during the long, dark twilight hours of winter.

Humans perceive light from blue wavelengths of about 400 nanometers to red wavelengths of 700 nanometers, but reindeer can see clearly in the shorter ultraviolet (UV) range, which is what causes snow blindness in humans.

 (Source: San Diego Zoo Animals & Plants).

Fosbury says UV vision could help reindeer in two ways.

Firstly, it may help reindeer find food during the winter when it snows. 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 absorbs 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, said it was possible other animals in the Arctic were 'doing the same thing' but "we don't know yet".

'Eye adjustment'

A more difficult follow-up question: How do reindeer 'adjust their eyes?'" That's where the astrophysicist comes in.

Fosbury studied the optical conditions during Arctic twilight and discovered that the reflective film would 'tune' 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 had been collected over many years—from the reindeer herds of the Sami, an indigenous people of Scandinavia.

Reindeer tapetum is made up of tiny collagen fibers suspended in fluid, creating a variable reflective crystal. The collagen fibers in Summer's eyes 'float loosely' in the fluid, creating a crystalline mirror that best reflects red light.

But in eyes collected in winter, the collagen fibres are much more 'tightly packed' , changing the crystal shape and causing it to reflect mainly blue light.

In the dark, reindeer can dilate their pupils, blocking a small drainage hole for fluid in the eye. This causes pressure inside the eye to increase, compressing the collagen tapetum and changing the shape of the crystals. In the summer, reindeer pupils return to normal.

"If you add all these factors together, reindeer eyes are at least 1,000 times more sensitive in winter than in summer," says Tyler .

But reindeer's unique adaptations can also work against them.

Today, high-voltage power lines run across traditional Sami grazing land, emitting bursts of ultraviolet light. The reindeer perceive these bursts as 'fireworks' and 'they won't come near them,' Jeffery says.