Furred and Feathered Animals Stay Warm by Reflecting Infrared Light

The exceptionally warm coats of polar bears and other animals are augmented by hairs capable of reflecting infrared light, which may contribute to their tremendous insulating power, according to new research.

Scientists at University of Namur in Belgium and the University of Hassan I in Morocco report in the journal Optics Express that this natural infrared-reflection could go on to be inspiration for next-generation insulation systems in buildings.

When studying the insulative capabilities of polar bears, researchers learned that the animals, who only have fur that's 5 centimeters thick, were able to keep their internal body temperature regulated to 37 degrees Celsius (98.6 Fahrenheit) during long, cold winters where outside temperatures were a bone-chilling minus 40 C (minus 40 F).

Polar bears' incredible insulation abilities made Priscilla Simonis, a biophotonics expert at the University of Namur, curious about why thermal insulation in buildings, usually done with a material called glasswool, doesn't work as well.

"Why do we need at least 60 cm of rockwool or glasswool to get a temperature of 20 degrees Celsius inside from about minus 5 degrees Celsius outside?" she said. "Why is the polar bear fur much more efficient than what we can develop for our housing?"

Heat can be transferred a number of ways. Simonis and her colleagues explored how two ways of heat transfer - radiation and conduction - play a role in animal insulation.

Radiation is the transfer of thermal energy through electromagnetic waves. Conduction transfers thermal energy through the vibrations of neighboring atoms and molecules.

Simonis said that one common assumption in how animals keep themselves warm is that their fur or feathers traps a layer of air that slows thermal conduction. But radiation, Simonis said, plays a bigger role than most people believe.

The researchers conducted an analysis of heat transfer between two objects called "radiative shields" that mimic the fur coats of animals. The experimental model included two thermostats that represented the temperature of cold outside air and an animal's warm body.

They found that "as the reflectivity of the radiative shields increased, the rate of heat transfer between the hot and cold thermostat was dramatically reduced. Adding more shields also dramatically reduced the energy loss," the researchers said in a statement. "All together, the model suggests that the repeated backscattering of infrared light between radiative shields, like individual hairs and barbed feathers, could be the primary mechanism for the thermal insulation properties of fur and feathers."

Simonis also noted that under certain conditions, the properties of an animal's coat can generate thermal insulation as well as scatter visible light in the white wavelength.

"This is particularly useful to animals, such as mammals and birds, that live in snowy areas," Simonis said, because it affords them camouflage against the white snow as well as added warmth.

The discovery could end up leading to a new class of building insulation materials.

"The idea is to multiply the interaction of electromagnetic waves with gray bodies -- reflecting bodies, like metals, with very low emissivity and no transparency -- in a very thin material," Simonis said. "It can be done by either a multilayer or a kind of 'fur' optimized for that purpose."