How polar animals cope with frigid darkness for months at a time

Category: Animals
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Winters at the top and bottom of the world are nobody’s idea of a good time. The sun can disappear for months during polar night in the Arctic and Antarctic, and temperatures can plummet well below −40 degrees Fahrenheit. Food is hard to come by and the winds are fierce and constant.

And yet some animals insist on staying put, even as others migrate to escape the brutal cold and darkness. As bleak as this landscape might appear, life does not stop just because polar night has descended. On land, sea, and the ice in between, animals continue to eat, sleep, and mate.

“There’s more in the Arctic in winter than people might think,” says Grant Gilchrist, a research scientist at the National Wildlife Research Centre in Ottawa, which is run by the federal agency Environment and Climate Change Canada. “Things underneath the ice are often more active than we ever could have imagined.”

In recent years, scientists have gained a new appreciation for the hardy animals that carry on in the dark. But how does anything survive here?

The land

A few intrepid animals spend their winters out on the tundra. Muskoxen sniff out plants under the snow and sport a thick undercoat that is eight times warmer than sheep’s wool. Reindeer also rely on two coats to stay warm. And while their noses might not actually glow, they are packed with blood vessels to warm incoming air. They even have a trick for coping with darkness. “Under the low light conditions reindeer have this peculiar adaptation in their eye that allows them to use more UV light than a typical eye,” says Cory Williams, a physiologist at the University of Alaska Fairbanks. Being able to see into the ultraviolet spectrum helps the reindeer discern plants in the snow.

Polar bears depend on their sense of smell to find dinner in winter, along with what dim light remains after the sun has set. “Even though we think of this as constant darkness, it’s not really constant darkness,” Williams says. Arctic wolves can endure four months of wintry darkness by hunting under the light of the moon and stars and by using their sensitive noses to find prey more than a mile away.

Other creatures head underground as winter approaches. “You have a whole ecosystem that’s beneath the snow where we just don’t see anything on the surface,” Williams says. Ermine, lemmings, and voles retreat down to the space underneath the snowpack. The rodents tunnel through the snow to forage for seeds and vegetation left over from the year before. Others, like the Arctic ground squirrels that Williams studies, hibernate in winter. By December, the soil surrounding their little burrows will freeze solid, dropping to -13 degrees Fahrenheit (temperatures on the surface can be −76 degrees Fahrenheit before wind-chill).

There isn’t much food to go around in winter. “It’s definitely a lean time,” Williams says. “A hibernator deals with that by putting on huge fat stores in the summer.” Other critters take more drastic measures. Some rodents will shrink during the winter so they require less energy; voles can reduce the size of some of their internal organs and muscle mass, while shrews will actually shrink their skeletons, Williams says.

As punishing as the cold can be, it’s not the key element that dictates how life is spent during polar night. “Light is by far more important than temperature, especially in polar regions,” Jørgen Berge, a marine biologist at UiT The Arctic University of Norway in Tromsø, said in an email. Sunlight means warmth and photosynthesis, but it also sets our body clocks.

In the lower Arctic, the light will still change subtly from day to night. “The sun it doesn’t ever come up, but it gets closer to coming up,” Williams says. But the farther north you travel, the more difficult it is to maintain circadian rhythms. Circadian rhythms control sleep patterns, digestion, body temperature, and other functions. In Arctic winter, many animals stop adhering to a 24-hour daily schedule altogether. Arctic ground squirrels keep their circadian rhythms through the constant daylight of Arctic summers, but not while hibernating in winter. Svalbard reindeer and rodents like lemmings adopt short cycles of feeding, digesting, and snoozing. “Instead of having this consolidated period of sleep each day, it’s just kind of spread in these chunks throughout the day,” Williams says. Other animals such as the chicken-like rock ptarmigan have no obvious rhythm to their sleep and activity patterns in winter.

In people, disrupting the circadian clock messes with our metabolism and over time can increase our risk of cancer and heart disease. But scientists assume that Arctic animals are none the worse for the shakeup. “They’re well adapted to that situation, so they don’t have those same types of issues,” Williams says. How exactly polar creatures avoid health problems when they lose their circadian rhythms is unknown, however.

The sea

The ocean under polar night was once believed to be a barren, dormant place. After all, without sunlight there can be no new growth of phytoplankton. These algae and other photosynthetic organisms feed marine animals like krill, which in turn become a meal for larger and more predatory creatures.

“We all, including myself, used to think that the polar night was a period of no biological activities,” Berge says. “That perspective has now changed radically!” In 2007, Berge accidentally discovered just how busy the seas can be during polar night. That winter, his acoustic sensors captured evidence of zooplankton traveling up and down through the waters near Svalbard, an archipelago located between Norway and the North Pole.

This mass movement happens in seas around the world—the tiny creatures commute to the ocean’s surface at night to feed and then retreat to deeper waters to avoid predators during daylight. It’s called the diel vertical migration, and may be the largest daily migration on Earth. But no one had imagined it could continue in the darkness.

Berge and his colleagues have since observed that in winter, this migration depends on illumination from the moon. “As with the hunting behavior of mythical werewolves that is driven by moonlight, our data reveal a moonlight-driven reality,” they wrote in the journal Current Biology. However, what the tiny animals feed on—and why they keep returning to the surface—isn’t entirely clear.

Year after year, Berge and his team have returned to the ocean during polar night and spied marine animals in action. At depths between 20 to 40 yards below the water’s surface, the glow from bioluminescent zooplankton is strong enough to outshine moonlight and any dim glow from the sun. Krill use their sensitive eyes to make the most of the wintry gloom. Scallops keep growing at a steady clip by feeding on floating detritus when fresh phytoplankton is unavailable.

And there are larger animals, too, including hordes of jellyfish, shrimp, arctic cod, and squid. The scientists have even seen seabirds diving for dinner. What’s more, they have full bellies. “They are dependent on finding their food on a daily basis,” Berge says. “To see a little auk or a guillemot stuffed with krill or copepods, food that have been found and eaten within just a matter of hours, is in my eyes the most telling evidence of a system in full operation.”

Somehow, the birds are managing to hunt despite the darkness. That doesn’t mean they won’t take advantage of light when they find it, though—murres and guillemots seem to be drawn to the beam of scuba diver lights and harbor lamps.

For other animals, however, artificial light is actually a nuisance. Even the gentle light of small research boats is enough to send zooplankton fleeing, Berge and his colleagues reported January 10 in the journal Science Advances. “This is because they are naturally so closely tuned with their environment—they are adapted to see and respond to even the smallest change in light,” he says. “This means they are ‘pre-disposed’ to be affected by sudden and bright artificial light!”

The researchers sent a robotic kayak to record zooplankton movements both within and away from the glow cast by their boat. It turns out that zooplankton tried to escape the unnatural light down to depths of about 80 yards. This indicates that there is even more life in the Arctic Ocean during polar night than scientists have been able to track until now.

The ice

As winter descends, the pack ice that encrusts polar seas grows, and open water becomes harder to find. That’s bad news for seabirds, which need to be able to dive beneath the water’s surface to find dinner. The Gulf Stream keeps parts of the European Arctic along the Norwegian coast ice-free so eiders and other seabirds can stay in the winter, but that’s more the exception than the rule. “Most birds are migrating to get the hell out of there,” Gilchrist says.

A few, however, do not stray far. The Hudson Bay eider duck spends its winters in Canada’s Belcher Islands. “It’s south of the Arctic Circle, but the climate and conditions are certainly arctic,” Gilchrist says. The days are only a few hours long, and temperatures can drop to -31 degrees Fahrenheit with 75 mile-per-hour winds during storms.

“But there’s seabirds living out there in the dark,” Gilchrist says. That’s because the shape of the islands and strong tidal currents allow areas of open water surrounded by ice to form. These formations, called polynyas, become the eiders’ home base. By day, they hunt for mussels and sea urchins underwater. When dusk falls, they return to the edge of the ice and huddle together to conserve energy. “And then at night arctic foxes and polar bears arrive and pick off the weak, and birds sitting around the edges of the group,” Gilchrist says.

There are in fact dead eiders littering the ice, but the predators ignore them. “When they freeze solid it’s very hard for an arctic fox to actually ingest them,” Gilchrist says. “That fox would rather kill a fresh, warm bird and eat it quickly.”

He and his colleagues have also observed that snowy owls will follow the ducks out to sea and roost on the ice in wintertime. Most snowy owls of the Canadian Arctic will winter on the prairie in Ontario. But “some of these snowy owls are actually seabirds in the wintertime, feeding on seabirds and sea ducks out on the pack ice and, I’m assuming, using darkness as an advantage,” Gilchrist says.

Polynyas sometimes form far enough north to give animals a place to stay once polar night falls. The North Water polynya between Ellesmere Island and northwest Greenland is one such haven. “There are marine mammals that winter there in complete darkness for three months,” Gilchrist says. “The key is the open predictable water areas that give them access to air.” This draws walruses, belugas, narwhals, and seals.

One bird that does seem to stick around for polar night is the ivory gull, which winters in the waters between Canada and Greenland. “They’re spending a lot of time in darkness, and how they’re eking out a living is really hard to know,” Gilchrist says. The gulls are known to feed on both fish and the afterbirth of seals that whelp on the ice. This rather flexible diet likely helps the ivory gull capitalize on any feeding opportunity, even in the dark, Gilchrist says.

Down in Antarctica, emperor penguins must also endure the polar night. These birds are famous for braving the bitter long winters with males spending more than 100 days fasting while they incubate their eggs. Most penguins endure this winter retreat inland, but scientists reported earlier this month that some emperors stay near the edge of the sea ice. That means that they have time to dive for a meal before the eggs are laid and don’t have to fast as long as their peers. “They’re usually visual predators, but penguins can feed at night. And this, I think, is an example of that,” coauthor Gerald Kooyman, a biologist at Scripps Institution of Oceanography in San Diego, told Popular Science.

Mysterious territory

Berge is currently on his seventh research expedition into the polar night, during which he hopes to find out how much light pollution is needed to disturb the resident zooplankton. Several weeks ago, he and his coworkers piloted a small boat away from the larger ship that is their second home in winter. Once they escaped the artificial lights, the bioluminescence was breathtaking. “You are able to see the miracle of biological light below you—blue-green lights that shine everywhere,” he says.

It turns out that an icebreaker is both the most comfortable and safest place to study the polar night. Researchers know more about what goes on in the sea in winter than they do the land or pack ice. “Scientists are constrained by actually going out to where the wildlife is at night in the wintertime in the Arctic,” Gilchrist says. Often, the best they can do is attach satellite transmitters to animals during the warm months and use them to track their movements in wintertime.

For Gilchrist, ships are not an option. “Many coastal areas in the Arctic that are important for marine birds are ‘uncharted’ and harbor dangerous, underwater reefs and shoals,” he says. “I’m trying to go where the action is…where foxes and bears and eiders and seabirds are converging, and snowy owls, and a lot of that is happening out on the pack ice that we can’t reach.”

So he camps with experienced Inuit guides and sets up plywood observation blinds with stoves that might bring the temperature up to -4 degrees Fahrenheit. It’s “like sitting in a freezer,” Gilchrist says. “If you make a mistake or get wet you can perish; it’s really dangerous work.”

As extreme as the polar night ecosystem and surrounding areas can be, it is also fragile. The Arctic is warming more quickly than anyplace else on Earth. This means there will be less ice on the polar seas, allowing light to reach deeper waters in winter. What’s more, shipping routes will push farther north, bringing an influx of artificial light. Berge’s work suggests this could be a problem for marine critters used to operating in profound darkness.

Scientists are also concerned that winter rainfall will become more common in the Arctic. When the rain hits the ground and freezes, it locks up vegetation that rodents, reindeer, and muskoxen need to survive. “Instead of digging through snow they’re encountering ice,” Williams says. In future years, muskoxen may no longer be able to live in Siberia or Alaska, and might only be found in the remote north of Greenland.

There’s a lot we still don’t understand about how animals manage to survive the polar night, much less how vulnerable they will be to climate change. But the darkness and frigid conditions won’t stop parka-clad scientists from returning to this rugged environment—or as close to it as they can get—to find out.

Written By Kate Baggaley

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