Apex predator, cuddly toy inspiration or climate change icon: however you see polar bears (Ursus maritimus), their camouflaged fur is what makes them stand out. Dominating the frigid arctic tundra, polar bears indulge in routine rough and tumble and arctic swims, yet their fur remains ice free even when the temperature plummets to −40°C. So how do polar bears prevent themselves from turning into an icicle after swimming? This is the question that Julian Carolan from Trinity College Dublin, Ireland, and colleagues sought to answer as they compared the freezability of polar bear fur with state-of-the-art anti-icing ski wear.
When designing an anti-icing material, there are several key strategies: prevent ice from sticking, repel water and delay freezing. With these in mind, Carolan and colleagues tested the fur of six adult polar bears from the Svalbard polar bear population, which had been snipped from anaesthetised bears as part of an annual Polar monitoring project, to see whether nature uses the same anti-icing toolkit as we do. Texture plays a major role in preventing ice formation, so the researchers washed some samples to remove the oil or dirt covering the hair, then used a bespoke microscope to observe the movement of tiny water droplets balanced on individual hairs to test the fur's ability to repel water and the length of time it took for the droplet to freeze at −15.5°C. The prouder the droplets sat on the surface, forming the roundest droplets, the faster water would run off the fur. Surprisingly, some droplets froze in seconds whilst others remained liquid for nearly 10 minutes and neither the washed nor the unwashed fur was able to repel the water before the droplet froze, so if polar bears cannot stop ice forming they must have some other strategy to keep themselves ice free. Are they shaking it from their coats?
Fortunately, the researchers had access to a 20×20 cm2 piece of fur-covered skin which had been taken from the neck of a young male bear that had sadly been euthanised to protect the residents of Svalbard. To test whether grease from the skin was the source of the fur's ability to free itself of ice, the researchers cut the fur into small squares and placed them fur-side down over individual tubes (cuvettes) of water, which were left to freeze for 2 h in a custom-built icing chamber. They then measured the force required to pull the ice block off the fur. The team could never fully dislodge the ice from the washed fur, but amazingly the ability to remove ice from the unwashed fur was comparable with that of race-level synthetic ski wear. The secret to the polar bears frost-free fur is the greasy coating, which allows the fur to slough off ice that forms. But how does the fur prevent ice from sticking?
The researchers took the liquid left over from washing the fur and extracted the grease to discover what keeps the fur ice free. Identifying the key ingredients revealed that it contains large amounts of cholesterols and acids, which ice cannot stick to. Neubi Xavier, Jr, and colleagues from the University of Surrey, UK, then used computer simulations to discover how cholesterols and acids prevent ice from sticking and realised that the compounds form shapes that ice is unable to stick to. In addition, they also repel water to prevent it from reaching the fur beneath.
Although the de-icing power of the polar bear's natural hair grease is a revelation for us, this is a secret that the Inuit peoples of the Polar regions have known for generations; traditionally they don't wash the fur when preparing polar bear skins for clothing and hunting equipment. The fur's ability to slough off ice also silences the polar bear's movements on ice while hunting for keen-eared seals below, by reducing friction and preventing noise. Inuit hunters mimic the polar bear's successful hunting technique by using a combination of polar bear ‘sandals’, trousers and hunting stools with fur-covered feet to become audibly invisible to the seals they also hunt beneath the ice.
