Polar bear morphology and adaptability

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While brown bear have more robust limbs than polar bears pound to pound, the latter actually has denser bones.

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Robusticity is the outside of the bones, its the width divided by the length. Density is the inside of the bones, its the amount of minerals it contains.

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Polar bear paw pad surface roughness and its relevance to contact mechanics on snow


Abstract

Microscopic papillae on polar bear paw pads are considered adaptations for increased friction on ice/snow, yet this assertion is based on a single study of one species. The lack of comparative data from species that exploit different habitats renders the ecomorphological associations of papillae unclear. Here, we quantify the surface roughness of the paw pads of four species of bear over five orders of magnitude by calculating their surface roughness power spectral density. We find that interspecific variation in papillae base diameter can be explained by paw pad width, but that polar bear paw pads have 1.5 times taller papillae and 1.3 times more true surface area than paw pads of the American black bear and brown bear. Based on friction experiments with three-dimensional printed model surfaces and snow, we conclude that these factors increase the frictional shear stress of the polar bear paw pad on snow by a factor of 1.3-1.5 compared with the other species. Absolute frictional forces, however, are estimated to be similar among species once paw pad area is accounted for, suggesting that taller papillae may compensate for frictional losses resulting from the relatively smaller paw pads of polar bears compared with their close relatives.





www.researchgate.net/publication/365049937_Polar_bear_paw_pad_surface_roughness_and_its_relevance_to_contact_mechanics_on_snow

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Appearance:

Polar bears are extremely well-adapted to their arctic environment. They have the thickest fur of any bear species. It’s composed of two layers. The layer closer to the body, the undercoat, consists of thick, plush hair that retains heat. The outer layer provides extra insulation and repels cold water and ice. This layer of “guard hairs” is not hair at all but is rather composed of clear, hollow tubes. The tubes trap air for insulation and also reflect all the visible wavelengths of light in their hollow airspace, which combine to make the fur look white, though it is in fact colorless. The white coloring provides excellent camouflage for the bears, allowing them to blend in with their snowy surroundings.



Polar bears molt in the summer months and grow a new coat. By the next spring, the coat may look yellowish from seal oil stains. They sometimes wash their fur in ice or snow during or after feeding.



Underneath their fur, polar bears have black skin which absorbs the heat of the sun, and below the skin is a thick, 4-inch layer of blubber. This blubber layer is particularly beneficial while polar bears swim, keeping them warm in the cold water and increasing buoyancy.



Polar bears also have fur on the bottoms of their paws, providing extra warmth and creating traction on the ice. The bottoms of their paws are also covered with small bumps called papillae. The paws are broad to distribute their weight as they walk across ice and snow, and tipped with long, powerful claws to help grip the ice.



Compared to other bears, the polar bear has an elongated body with a particularly long neck and a narrow skull. These features streamline the bear, making it an efficient swimmer. Large, flat, and partially webbed paws, functioning like oars, also help them navigate the water.


www.pbs.org/wnet/nature/blog/polar-bear-fact-sheet/