Polar bears are nearly undetectable by infrared cameras. Thermal cameras detect the heat lost by a subject as infrared, but polar bears are experts at conserving heat.
Polar bears living in the Arctic Circle survive in one of the harshest climates in the world. Winter temperatures can fall to -40 °C, but polar bears manage to keep their internal body temperature at a steady 37 °C.
One physical feature that helps the polar bear stay warm is its fur coat. The coat is made up of two distinct layers: a short and dense underfur layer right next to the skin, and an outer layer of longer and coarser guard hairs. The guard hairs are transparent but the polar bear's coat appears white because the hairs scatter sunlight.
Research on these transparent guard hairs has revealed a key property that helps to prevent heat loss in the cold Arctic air. The guard hairs appear to be very effective at absorbing infrared radiation, which makes up a portion of the electromagnetic spectrum that most mammals (including humans) cannot see but can feel as heat. This means that heat emitted from the polar bear's warm body could be absorbed by the hairs instead of transmitted through them, where it would be lost to the cold environment. The hairs' ability to absorb radiation is especially high at the specific part of the infrared spectrum where mammals tend to radiate heat most strongly.
An interesting consequence of this property is that a polar bear appears invisible in the infrared if the temperature at the surface of its coat matches the temperature of the ice and snow around it.