Average January temps in the Arctic range from -40 to 32 degrees Fahrenheit. Lows can push down to -58 F. A quick swim in the sea will register 28.8 F, since saltwater has a lower freezing point than freshwater. Polar bears don’t sleep off the cold like their hibernating black and brown brethren. How do they do it? Why don’t polar bears get cold?
Despite all photographic evidence to the contrary, the charismatic mammal of the great north cannot get by on good looks alone. Thank you – with a shout out to you, Mr. Darwin – for a particular set of adaptations that helps a healthy Ursus maritimus thrive in the Arctic.
Polar bears are clearly built for the cold, and it’s not just because they look so good in white. First and foremost, they’re big. In real terms, this means they lose less heat through their skin due to a high ratio of volume (body heft) to surface (skin) area. Their top three physical points of vulnerability to cold have evolved as well. Polar bears have adorably small ears and tails, plus particularly large and furry feet. Cute factor aside, these adaptations are about minimizing heat loss.
Then, things get more technical. Polar bear skin and fur kick ass over any techy, $1,000 parka a human could envision. The easiest way to understand it is to pet the family pooch. Polar bears have two types of fur, which is similar to double-coated dog breeds, like huskies, malamutes, Newfoundlands, and Bernese mountain dogs, among others.
The outer layer of fur is made of long, oily guard hairs, ranging from two to six inches in length. The oil helps to shed water during a dip in the glacial pool. Again, just like your best dog buddy, polar bears will shake and roll as soon as they’re out of the water. Even though the oily fur layer is a reliable tool, bears (and dogs) want to get dry as soon as possible because cold water takes away more than its fair share of warmth compared to air.
The coolest adaptation in these guard hairs is that they’re hollow, enabling them to act like any good sleeping bag by trapping a layer of warm air around the bear. They also help to perpetuate one of the biggest myths of the natural world. You see, polar bears are not white. The hollow guard hairs are transparent, and it’s the reflection of the sunlight that makes them appear white. Looked at a certain way, polar bears are invisible.
In a curious afterthought to that invisibility argument, polar bears don’t show up well on infrared photography, which detects heat. They’re so well insulated that they blend into the rest of the frigid environment.
Back to warmth. Inside the guard fur, there is a dense mat of short underfur on jet black skin. The underfur provides the primary source of external insulation, while the black skin absorbs whatever sunlight is available.
Underneath the skin is where the real magic happens. A healthy polar bear has a subdermal adipose layer up to four inches thick. You can also call it fat or blubber. The fat layer certainly doesn’t hurt on land, but it’s particularly effective as an insulator in the water.
For the true deep dive into how polar bears stay warm, a study from 2012 examined the polar bear genome sequences as compared to those of their brown and black cousins to the south. The primary finding was that polar bears have an enhanced genetic function to produce nitric oxide. Nitric oxide is used by cells to convert food into energy or heat. It follows that the more abundant nitric oxide production is, the more efficient the animal will be at utilizing its resources to stay warm, and not insignificantly, will have enough energy to hunt for more food.
On the flip side, there is the question of how much heat a polar bear can handle. Not much. Polar bears are prone to overheat above 50 F. For perspective, average temps in July range from 14 to 50 degrees F in the Arctic, which doesn’t leave much wiggle room for optimum health.
It seems impolite, then, to not mention that a warming climate is the most imminent threat to this cold weather bear. Attempting to capture the challenges to polar bear survival in a few, flip sentences is neither thorough nor accurate. As with every ecological question, there are many interconnected criteria at play. Still, the National Oceanic and Atmospheric Association (NOAA) recently released some objective climate figures from 2015 that do not bode well for the bear.
Rick Spinrad, chief scientist at NOAA, announced in December 2015 that climate change is causing the Arctic region to warm twice as fast as the rest of our orb. In a 12-month period from October 2014 to September 2015, Arctic temperatures were 2.3 degrees F above average, since recording began in 1900. Overall, that represents a 5.4 degree F increase since the early 1900s.
Sea ice, which polar bears rely on for hunting, is disappearing at an alarmingly fast rate. The quantity of sea ice varies throughout the year, as can be expected. In 2015, the maximum sea ice measurement in February was 7 percent below average since recording began in 1979, and was reached 15 days earlier than an average year. The minimum sea ice measurement, recorded in September, was the fourth lowest since 1979, and was 29 percent below average. Sea ice is melting at a 13.4 percent rate, decade by decade.
Those numbers have potential to put a sobering spin the notion of polar bear invisibility.