Polar bears are tremendously impressive — the very definition of charismatic megafauna. They’re cute and fluffy but also huge and powerful.
Polar bears in Alaska, courtesy of Alan D. Wilson via wikimedia commons.
Of those four factors, though, it’s not cuteness, size, or strength that impresses most: it’s fluffiness. Fluffy fur is what lets polar bears survive the frigid temperatures of the Arctic.
Just how does fur work to keep polar bears a balmy 37 degrees Celsius in -40 degree weather? A new paper by physicist Priscilla Simonis and colleagues illuminates the insulating power of polar bear fur.
As a general rule, insulation limits heat transfer. Perfect insulation for a polar bear, therefore, would mean that heat transfer between warm polar bear skin and the cold Arctic air is zero. The polar bear stays 37 degrees without cooling down, and the air remains -40 degrees without warming up.
Often it is presumed that fur works as an insulator primarily by trapping pockets of warm air with very low heat conductivity. (Until now, some version of this explanation is probably what I would have told my not-quite-three year old had she thought to ask how fur keeps animals warm.) But Dr. Simonis recognized that this model was too simplistic — the low heat conductivity of air couldn’t fully account for keeping polar bears with 5-inch-long fur a full 77 degrees Celsius higher than the -40 degree background temperatures of the arctic.
Conduction is only one of three methods of heat transfer. The other two are convection and radiation. With polar bears, there’s probably not much heat transfer occurring via convection. This is because convection requires air movement and the air beneath a layer of polar bear fur is generally pretty still. (This is another insulating benefit of fur.) That leaves radiation.
Dr. Simonis figured out that, in a universe stripped of several possible confounding variables, polar bears would likely suffer about ten times as much heat loss from radiation as they would from conduction. So for polar bears to survive in the Arctic, their fur must be countering that radiative effect in some very significant way.
Gradually extrapolating from simple to more complicated mathematical models, Dr. Simonis showed that two elements are necessary to create this type of significant insulating effect against heat loss by radiation: First, she noted that there is a “rapid decrease in heat transfer rate with [an] increasing number of intermediate absorbers.” In other words, to achieve insulation against radiated heat, there should be lots of objects — individual strands of fur, say — between polar bear skin and the cold, cold air. Second, for each of these intermediate objects there should be “small absorption with high reflectances.” What does this mean? Well, radiative heat loss occurs via infrared radiation (this is the principle that allows for thermal imaging). And white surfaces reflect all colors of light, including infrared. Therefore, to best reflect infrared radiation and insulate a polar bear, its fur should be white.
(It helps that polar bear skin is black and quite able to reabsorb infrared radiation reflected by the white fur.)
The takeaway: polar bears’ white fur serves a dual purpose — it camouflages them in the snow, as you already knew, and it traps their radiated body heat. Or, to use Dr. Simonis’s words, “The structure of polar bear or snow fox fur is actually multifunctional, providing both visual camouflage and good thermal insulation.” Impressive.
(So impressive, in fact, that Dr. Simonis proposes using what I’ll call the “polar-bear-fur principle” for improving insulation in such high-tech applications as thermal shields for satellites.)
P. Simonis, M. Rattal, E. M. Oualim, A. Mouhse, and J. Vigneron (2014) Radiative contribution to the thermal conductance in animal furs and other wooly insulators. Optics Express, Vol. 22, Issue 2, pp. 1940-1951. doi: http://dx.doi.org/10.1364/OE.22.001940