Continuing with winter themes, I’m sure that many readers have lost sleep wondering why polar bears don’t freeze during long arctic winters. Actually, it is an interesting aspect of biology and one that has recently had some practical applications.
Polar bears live in a harsh unprotected environment where they are often exposed to air temperatures of -40 degrees and unrelenting winds. They live on the ice and spend much of their time hunting seals, which means that sometimes they find themselves swimming in ice-cold water. Yet they thrive under such circumstances. How do they do it?
Survival for a polar bear depends upon a large number of physiological adaptations, some of which will just be touched upon here. Their main protection, however, depends upon three factors – 1) a thick layer of insulating fat located just below the skin, 2) black skin, which absorbs the warm rays of the sun and 3) unique properties of their hairs, which will be the focus of this blog.
Like many animals that live in the cold, polar bears have two kinds of hairs – a dense inner coat of 2-inch-long soft hairs and an outer coat of coarse 4-inch-long guard hairs. Although polar bears look white or yellowish, their hairs are actually colorless. The reason they look white is that their hairs reflect all wavelength of visible light, which gives them their white appearance.
The outer hairs have a hollow core that serves as an insulator. This protects them from the worst of the outside cold. The inner layer of finer hairs serves a thermal insulator, which prevents the loss of body heat. This combination of hairs, along with their black skin and layer of subcutaneous fat allows polar bears to function quite comfortably in the worst of weather. As an aside, some polar bears in zoos take on a greenish tint because tiny algae cells invade the hollow cores of the guard hairs.
In the early 1970s, some scientists, working from the structure of the guard hairs, theorized that these hairs act as solar collectors that direct ultraviolet rays from the sun toward the skin very much like fiber optic cable. This theory has largely been disproven, but it still persists as an urban legend.
Amazingly, the study of polar bear hair has led to some exciting recent practical applications. Material scientists and engineers have designed futuristic fabrics built along the lines of polar bear guard hairs. The fibers making up these fabrics have aerogel cores surrounded by a layer of hard material, giving them much the same properties as real polar bear hairs. These fibers take up heat from the sun or artificial lights. The fabrics are permeable to air, but they act as thermal insulators while yet allowing damp air to escape from the body. One problem with earlier versions of such fabrics was their delicate nature so that they couldn’t be washed easily. More recent versions are more robust. Look for them when they finally enter the general marketplace.
