The School: Polar Bear Adaptations for Extreme Cold


A weekly dose of education in the ocean.

This is the second in a five-part series about the polar bear’s adaptations to the Arctic environment. 
Part one    Part three    Part four    Part five 

Polar bears are supremely adapted to their environment – they have a number of traits that help them cope with snow, ice, and below-freezing temperatures. They are more likely to overheat than to become hypothermic, because of the following characteristics.

© Florian Schulz, from his book, To The Arctic.

Large Size. Polar bears are the world’s largest land-based carnivore. Males can get up to 1500 lbs (almost 700 kg) – even bigger than their hulking grizzly bear cousins – and this helps them retain their body heat. As Dr. Andrew Derocher explains in his recently published book, Polar Bears, A Complete Guide to Their Biology and Behavior,  this is because large animals have a higher volume to surface-area ratio, so they lose less heat through their skin. This is an adaptation common to many species in cold habitats.

Small Ears and Tail. Appendages that stick out also affect the volume to surface-area ratio and cause more heat loss as discussed above. The polar bear’s small ears and tail offer less surface area than large ears or tails, and help maintain body warmth. 

Fat. Polar bears have a layer of fat that can be four inches (10 cm) thick. Although their fur is the primary source of insulation, the fat helps keep heat in as well. Like other marine mammals, the milk of polar bears is extremely high in fat and protein compared to other bears or land mammals. The fatty milk helps small cubs, who are more vulnerable to the cold, put on critically important body weight quickly.

Small cubs are more vulnerable to cold than adults, but quickly put on weight thanks to mom’s fat-rich milk. Photo by tableatny, via Flickr, Creative Commons License

High Calorie Diet. Most bears are opportunistic omnivores – they eat whatever they can find, and plants figure largely in their diet. While polar bears will eat seaweed when stuck on shore, they only do this in desperation. Their true diet is pure meat and fat, from seals and whale carcasses. Meat and fat have more calories than plants, so polar bears get the most caloric intake out of every bite, and every ounce of hunting effort expended, to build their important fat layer discussed above.  

Thick Fur. Except for the tip of the nose, polar bears are entirely covered in fur. They have a very thick undercoat, which is even denser than the coats of other bears, as well as longer guard hairs. This insulates them from the cold, even when they are in water. Their fur is also hollow and transparent. (It was once thought that polar bear fur could also conduct UV light to the animal’s skin but this has been proven to be false.)  Their fur has another use as well: the hairs reflect light, making polar bears look white and helping them camouflage among the snow and ice.

Water Repellant Fur. Polar bear fur sheds water like a Gore-Tex jacket, so even after a swim, they don’t stay wet long. According to Andy Derocher, oils in the hair help with this, and the polar bears are very diligent in drying themselves by shaking and rolling in the snow when they come out of the water. Water has a very high heat capacity, which causes it to channel heat out of the bear’s body, so the faster they dry, the faster they stop losing heat.

Large Anti-Slip Paws. Polar bears have enormous paws! On land, they are like snowshoes, spreading the bear’s weight and helping them move across soft snow and thin ice. On ice, they keep the bear from slipping: they have footpads covered by papillae (small soft bumps) and claws that are short, thick, and curved. The paws also are covered in fur for both traction and warmth. 

A polar bear’s paws serve as snowshoes with anti-slip soles, and double as paddles for swimming! Photo by Srvban via Wikimedia Commons, Creative Commons License

 

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