Anti-icing properties of polar bear fur

Abstract

The polar bear (Ursus maritimus) is the only Arctic land mammal that dives into water to hunt. Despite thermal insulation provided by blubber and fur layers and low Arctic temperatures, their fur is typically observed to be free of ice. This study investigates the anti-icing properties of polar bear fur. Here, we show that polar bear fur exhibits low ice adhesion strengths comparable to fluorocarbon-coated fibers, with the low ice adhesion a consequence of the fur sebum (hair grease). Lipid analyses reveal the presence of cholesterol, diacylglycerols, anteisomethyl-branched fatty acids, and the unexpected absence of squalene. Quantum chemical calculations predict low ice adsorption energies for identified lipids and high adsorption for squalene, suggesting that sebum composition is responsible for the observed anti-icing properties. Our work enhances understanding of polar bears and their interactions with their environment and builds on Inuit knowledge of natural anti-icing materials.

Fig. 1.. Properties of polar bear fur used in the wild.

(A) Polar bear sliding down a snow slope at Sallyhamna, Spitsbergen Island, Svalbard, Norway. (B) Infrared image showing the temperature of the polar bear fur and the surrounding snow taken with an FLIR E75 24o (FLIR Systems OÜ, Estonia) Svalbard Archipelago, Norway. (C) Adult female polar bear shakes water from her fur after climbing out of the water onto sea ice. Olgastretet, Barents Sea, Svalbard Archipelago, Norway. (D) An adult male polar bear cleans his fur by rolling and rubbing against snow-covered sea ice. Arctic Ocean, near 81° N, north of Svalbard Archipelago, Norway. Image credits: (A) P. Souders, Getty Images; (B) J. Aars, Norsk Polarinstitutt; and (C) and (D) J. E. Ross, LifeOnThinIce.org.

Fig. 2.. The main anti-icing results and testing setups.

(A) Ice adhesion measurements of unwashed (PB) and washed (WPB) polar bear fur compared with unwashed (HH) and washed (WHH) human hair and racing (RSS) and non-racing (NRSS) ski skins. (B) A 2 cm–by–2 cm–by–2 cm block of ice in a mold on the polar bear fur sample before removal of this ice to characterize ice adhesion strength. (C) Water contact angle measurements for individual PB, WPB, HH, WHH, and fluorocarbon-treated RSS and NRSS samples. (D) An example of a water droplet on an unwashed polar bear hair. Error bars in (A) and (C) represent the SD of measurements for each sample type. Raw measurements are shown as gray diamonds on the plots.

Fig. 3.. Box plots of lipid quantification of sebum.

(A) GC-MS in microgram of fatty acids (FAs) per gram of hair and (B) NMR in micromole per gram of hair. Acylglycerols represent the sum of mono-, di-, and tri-acylglycerols, whereby 1,2 diacylglycerol are the dominant class (also shown in plot). (C) Total fatty acid composition of the sebum as identified by GC-MS. The central line indicates the median, and the whiskers extend to the most extreme data points that are not considered outliers. Outliers are plotted individually.

Fig. 4.. Molecular structures analyzed and adsorption energies to ice.

Schematic of the fully saturated structures of (A) TG, (B) DG, (C) cholest-5-en-3β-ol, (D) wax, (E) PFAS, (F) eicosanoic acid, and (G) squalene adsorbed on ice. These are among the most relevant molecular species in sebum analysis, and their adsorption energy is plotted in (H). The star index “*” denotes the presence of a methyl group in the anteiso position of the respective carbon chain.

Fig. 5.. Inuit artifacts and polar bear hunting behavior.

(A) Adult male polar bear still hunting. Sea ice on Hudson Bay, Manitoba, Canada. (B) Adult male polar bear setting out on an aquatic stalk. Note the marks from the hind paws indicating how the polar bear slides into the water. Sea ice near 81° N, Arctic Ocean north of the Svalbard Archipelago, Norway. (C) Inghuit hunting stool (nikorfautaq). According to the original inventory, it is made of driftwood with polar bear fur wrapped around the ends of the legs. Cordage of de-haired seal skin. Collected by T. Thomsen during a visit to North Star Bay, north of Kap York, Uummannaq (Thule) district, 1909. It is stated in the inventory that the stool used to belong to Manigosok. (National Museum of Denmark, inventory number L. 4326). (D) and (E) Inughuit sandals (tuterissat) for ice hunting, 29 cm by 12 cm. Made from polar bear fur and attached to the soles of the boots (kamiit) by straps of de-haired seal skin. The fur side is turned downward, the hair direction pointing backward. Image credits: (A) and (B) J. E. Ross, LifeOnThinIce.org; (C) L. E. Hansen, Nationalmuseet; (D) and (E) donation from D. Rasmussen (wife of polar explorer K. Rasmussen) 1935. National Museum of Denmark (inventory number L17.9). Nationalmuseet. R. Fortuna, National Museum of Denmark.