Coping with the Cold: Strategies of Arctic Birds and Mammals

Back in the 1960s when I was a young biology student, Dr. Bill Pruitt, a specialist in Arctic ecology, became a bit of a hero for me. He was a short burly man with an impressive beard and passion for all things northern. He taught me much, but more importantly, his enthusiasm inspired me.

I remember him saying once, “Imagine a raven flying into the wind at -40℃. Why don’t its eyeballs freeze?” I never did figure that out, but ever since then, I have been keenly interested in how plants and animals adapt to the severe conditions of the Far North.

Imagine that it is mid-winter in the Arctic. It’s dark. The sun remains below the horizon from October to March. It’s very cold. In February the temperature dips to negative fifty degrees Celsius and stays there for three weeks. A fierce wind drives stinging ice crystals across the snowpack. It’s dry. There’s not a lick of liquid water to be found.

How do you possibly survive this, let alone find food? Yet that’s the world of the polar bear (nanuq), Arctic fox (tiriqaniaq), snowy owl (ukpik), redpoll (hakhagiaq), and about thirty other land mammals and birds that live year-round in the Arctic.

These stalwart souls all face the twin perils: cold and hunger. They survive these tough conditions through the evolution of clever anatomical, physiological, and behaviour adaptations.

Insulation

On some winter days, the difference between the surrounding air temperature and a body’s core temperature can be up to ninety degrees Celsius. That tells you something about the fur and feather coats of these hardy animals! They all have good insulating coverings; most are doubled-up with a coarse, outer layer that sheds water and works like a windbreaker, and a more insulating softer underfur or downy layer.

No animal illustrates the importance of good insulation better than the muskox (umingmak), a supremely adapted Arctic specialist. Its insulating coat of coarse outer guard hairs and inner coat of fine qiviut is so good that it seems oblivious to the cold and wind! The outer fleece hangs nearly to the ground so that even its legs are protected. In contrast, the fur of caribou (tuktu) is shorter, but each hair has an air-filled chamber that traps heat.

Winter Whiteness

Some adaptions are shared by many Arctic animals, and snowy whiteness is one example. Polar bears, Arctic fox, ermine (tiriaqpak), Arctic hare (ukaliq), ptarmigan (aqilgiq), gyrfalcon (kilgavikpak), snowy owl, and ivory gull (naujavaaq) are either white all year round or turn white in the winter.

How does this help? The obvious answer is camouflage, but there is more to it than that. White hair and feathers lack pigment, which makes room for air cells, a great insulator. All layers of fur or plumage trap air, but each individual fibre and feather also holds a pocket of air, enhancing the overall insulating qualities.

Brown Fat

Muskoxen and caribou, especially the calves, and some other small Arctic mammals have reserves of brown fat. Humans, babies in particular, have small deposits too. Brown fat contains more iron-rich mitochondria (which gives it its colour) than other fat and works like an engine. When activated in response to cold, it creates an internal heat source without shivering, which is another way of producing heat.

Caribou Quirks

Caribou have a few additional tricks to deal with dry Arctic conditions. In the winter, their hooves grow longer while their softer foot pads shrink. This improves traction and creates feet that are better for pawing through hard, crusted snow.

Lichens, an important winter food source for caribou, do not contain many nutrients and are almost impossible to digest by most animals, but they are abundant and widespread in the Arctic. Caribou have the singular ability to produce lichenase, an enzyme that helps break lichens down. While the digestion of proteins requires a lot of water, lichens are protein-poor, thus lessening a caribou’s need for liquid water during the frozen months.

Caribou have another adaptation to conserve water, especially in the winter. They are the only ruminant (a suborder of mammals that includes deer, cattle, goats, sheep, antelopes, giraffes, and camels) who can recycle their nitrogenous waste back through their stomachs, thereby reducing their need to urinate.

Allen’s Rule

In a cold climate it is better to be chunky than gangly. That, in essence, is Allen’s Rule, learned by most ecology students as a basic tenet of the science. In other words, animals with a greater surface area, long limbs, ears, tails, and snouts, lose more heat than those with compact bodies and stubbier appendages.

For example, Arctic foxes have shorter ears, limbs, and snouts than their more temperate cousins. Arctic hares show a similar physique. Allen’s rule is generally true but cannot be applied universally. Polar bears, for example, have longer snouts than their more southern brethren, but there is a different reason for that.

Heat Exchange

Another adaptation shared by many Arctic animals is a countercurrent vascular system that works much like a heat exchanger. As the heart pumps warm blood out to the extremities, the arteries pass through a network of veins that carry cooler blood back to the body core. The result is that the arterial blood warms the cooler venous blood, so that by the time it reaches the body core it has warmed up. Similarly, by the time the arterial blood reaches the extremities it has already cooled down, so little heat is lost to the environment.

A caribou can be near forty degrees Celsius at is core and only ten degrees in its lower legs. The polar bear has such a system in its legs and its Allen's Rule-defying long snout, and many Arctic birds have one in their legs as well. Therefore, gulls can perch on an iceberg for long periods without losing much heat, and polar bears can walk over the ice and snow all day.

Bulking up and Shrinking

Polar bears and caribou go through significant cycles of weight gain and loss over the yearly seasons. They feed heavily when the food supply is available and store the gained energy in their fat. During lean times, they burn that fat to get through the cold winter. There is one documented case of a female polar bear that put on four hundred kilograms in just one hunting season.

Using Microclimates

For animals as small as voles and lemmings (avin'ngak), staying warm above the snow is practically impossible. Their little bodies have too much surface area compared to their ability to produce heat internally. The answer for them is to create a cozy winter world beneath the snow with cached food, sleeping chambers, and runways. They will huddle together to share their body heat and are even capable of giving birth in the subnivean world.

The ermine, who finds these little creatures rather tasty, has a long, slinky body that can take advantage of the burrows created by the rodents. For a vole or lemming, it can be a scary world down there, but the benefits outweigh the hazards.

Hibernation

Surprisingly, few Arctic species hibernate deeply. In fact, the Arctic ground squirrel (siksik) is the only one to do so. Its metabolism slows down remarkably, as does its heart rate and breathing. Its body temperature goes way down but stays above freezing. The only way to wake a hibernating ground squirrel is to warm it up first. Not so for female polar bears, who create birthing dens and sleep in them for long periods. They are dormant, but not in a state of true hibernation.

Sea Ice—A Plus

Despite all the challenges these animals face, some parts of winter are beneficial for their survival, especially sea ice. During the summer months, Arctic animals are confined mostly to land and to the northernmost sea ice which does not melt (an area that is shrinking disturbingly fast). However, in winter, their world expands substantially with the formation of sea ice. It provides a highway and hunting ground for polar bears and the creatures, such as Arctic fox, ravens (tulugaq), and ivory gulls, who follow in their wake and feed on the discarded carcasses of their prey.

Perhaps you cannot imagine the winter world of these cold-hardy denizens that dwell above the Arctic Circle, but for them, it is just a simple fact of life. What does the future hold for them as the Arctic heats up at three times the average of the rest of the planet? We don’t yet know, but change is certain.