Most efficient insulation in a bear

The combined pelage and blubber (up to 10 cm thick) of a polar bear Ursus maritimus provides the most efficient insulation in any species of bear. This is determined by the fact that even when the external, environmental temperature drops to as low as -37°C, the polar bear's body temperature and metabolism remain normal. Moreover, it is virtually invisible under infrared photography.

The skin of the polar bear is black, whereas its fur's outer guard hairs appear white but are actually transparent. A recent study disproved the long-held assumption that the polar bear's guard hairs, being hollow, serve as fibre-optic tubes that conduct light to its black skin.

Additionally, polar bear fur contains several different sizes of hair, with a large density of interfaces. This produces the scattering needed to retrodiffuse heat.

If body of a polar bear with its black skin is at the constant temperature (e.g. 37°C) and the atmosphere is at −37-40°C, the change of temperature in the thickness of the “fur” is almost linear.