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Caribou perceive UV light in the dim Arctic night

Doug O'Harra

Reindeer — the domestic cousins of the wild caribou that roam Alaska, Canada and Russia by the hundreds of thousands — can perceive ultraviolet light invisible to humans, perhaps lending the species a stark evolutionary edge in its struggle to find food and avoid predators during the long, dark night of Arctic winter.

A new study by a team of European and Australian scientists suggests that these “wandering deer of the North” would be able to see lichen — an essential winter food — as black against the lighter background of snow because the plant absorbs UV radiation. Even more dramatic, they would also see splashes of urine on the ground — a startling if not splotchy signal that predators likes wolves may be prowling nearby.

"Since migrating to the Arctic 10,000 years ago, these animals have adapted incredibly quickly," lead author Glen Jeffrey explained in a story published last month by New Scientist. "Very few mammals see UV light. Rodents do and some species of bat do but we have no idea why they have developed this capability. This is the first time we have got a real handle on why a mammal uses UV light."

It’s as though caribou eyes come equipped with their own CSI-style bodily fluid detectors that take advantage of the blue-hued, UV-rich Far North environment. Likewise, they would see the sign and fur of other animals, as well as subtle variations in snow conditions — all very helpful to a critter trying to make it through the dim Arctic winter without starving or getting consumed.

"We discovered that reindeer can not only see ultraviolet light, but they can also make sense of the image to find food and stay safe,” explained Jeffrey, a neuroscientist with the Institute of Ophthalmology at University College London, in this online story. “Humans and almost all other mammals could never do this as our lenses just don't let UV through into the eye.”

But the results raise some questions. Do the other Arctic mammals immune to UV-triggered snowblindness have the same perceptual abilities? And how would UV-seeing critters like caribou react when new human oil field development floods their formerly dark winter environment with light — perhaps including UV light that people don’t realize they’ve produced?

“Human activity is often associated with UV production that we do not see,” Jeffrey told Alaska Dispatch in an email. “But these animals will see it. Having spent time with the Sami up in the Arctic and their animals, I have serious doubts about industrial activity and these animals.”

Arctic evolution: Let the UV light come in

In the study, published recently in the Journal of Experimental Biology, Jeffrey and his seven co-authors from London, Norway and Australia bought 18 Norwegian male reindeer from Sami herders, put the animals under anesthesia, and then measured how their eyes reacted to different wavelengths of light.

Humans perceive electromagnetic radiation as visible light with wavelengths falling between 390 and 750 nanometers, or one billionth of a meter. Ultraviolet radiation begins at the violet end of the visible spectrum and crunches down to 10 nanometers, where it morphs into what we call X-rays. Natural UV radiation from the Sun packs a wallop that both helps and harms people. Moderate doses of certain wavelengths of UV help humans produce Vitamin D, but overexposure can lead to DNA damage, sunburns, skin cancer and snowblindness — essentially a sunburn to the cornea.

"In conditions where there is a lot of UV — when surrounded by snow, for example — it can be damaging to our eyes,” Jeffrey explained here. “In the process of blocking UV light from reaching the retina, our cornea and lens absorb its damaging energy and can be temporarily burned. The front of the eye becomes cloudy and so we call this snow blindness. Although this is normally reversible and plays a vital role to protect our sensitive retinas from potential damage, it is very painful."

But the reindeer — like other Arctic mammals — don't suffer from snowblindness. The experiment showed that their eyes weren't blocking the UV energy from impacting the sensors inside their eyes either. Instead, UV light was transmitted through the cornea and lens down to about 350 to 320 nanometers.

“Reindeer thus extend their visual range into the short wavelengths characteristic of the winter environment and periods of extended twilight present in spring and autumn,” Jeffrey and his co-authors wrote in the abstract.

It’s not clear how the animals are immune to UV damage that would cripple or disable a person who spent too much time without eye protection in the brilliant, snowy springtime, the scientists added.

“They may have evolved retinal mechanisms protecting against extreme UV exposure present in the daylight found in the snow-covered late winter environment,” they wrote.

In any case, the caribou capacity to drink in UV radiation basically gives them what might seem like Arctic superpowers to a human — showing them signs of their favorite winter food, their predators and their kin.

Field experiments offered hinted at the richness of caribou perception.

CSI on the tundra: Caribou see wolf pee

"When we used cameras that could pick up UV, we noticed that there are some very important things that absorb UV light and therefore appear black, contrasting strongly with the snow,” Jeffrey said here. “This includes urine — a sign of predators or competitors; lichens — a major food source in winter; and fur, making predators such as wolves very easy to see despite being camouflaged to other animals that can't see UV."

Could other Arctic mammals have evolved the same ability? Jeffrey plans to repeat his reindeer experiments on seals come July to find out.

"What we are finding subsequent to our publication is that it probably applies to all Arctic mammals,” he told Alaska Dispatch. “If they do not go snow blind, then they are not blocking UV in the cornea and lens. Hence, it gets into the eye.

“The animals see UV because in winter months what little light they get is often UV rich, giving the Arctic that blue colour,” he added. “These animals are extending their visual range to make the most of what they can get. They need to see as much as they can when it gets dark.

No one knows yet whether this ability to perceive the play of UV radiation could cause distress for animals that wander near an industrial site like an oil production pad or mining operation. It’s not clear whether sensitivity to UV makes animals more vulnerable to disruption.

The long-running controversy over opening the coastal plain of the Arctic National Wildlife Refuge to oil development often hinges on whether the Porcupine caribou herd would be damaged or disturbed by the surge in human activity in the herd’s calving grounds.

“The effect that oil development may have on caribou migrations depends on many things, such as the location of the development in relation to migration paths, the density of the buildings, pipes, and roads, as well as the time of year that caribou are in the development area,” explains this federal primer on the Porcupine herd and ANWR. “For example, caribou are most sensitive at calving time, and studies have shown that caribou may be displaced from their traditional calving grounds when oil development occurs there.”

Calving occurs in the spring, when the full spectrum of "visible" light floods the tundra. As summer rolls on and the darkness starts to reclaim the night, most of the Porcupine caribou migrate south and east, away from the Coastal Plain. Still, a few Porcupine caribou winter over on the North Slope, possibly mingling with the Central Arctic herd, according to this website. Some of these animals must end up wandering near the massive Prudhoe Bay field, with its hundreds of lights, pumps and compressors, buildings and warehouses.

What might caribou see in their UV-speckled night?

To humans, it may seem dark out there. But is that how a caribou sees the winter?

During the reindeer-UV experiments, Jeffrey said, they found that reindeer kept in Tromso, Norway, during winter — with regular exposure to street lights — behaved differently than the reindeer kept outside of town in environments that remained dark.

“The two groups were very different,” he told Dispatch in an email. “This caused us great problems because we were getting data from town animals that was nothing like that from animals a long way from human habitation. Lots of arguments and discussions in the team as to what was going on until we found out it was human lighting.

 “The issue here is that these animals have evolved for 10,000 of years to not have much light in winter — change that and you will disrupt their visual systems.”

Contact Doug O'Harra at doug(at)alaskadispatch.com.