Polar bears are nearly undetectable by infrared cameras

Explanation:

Thermal cameras detect the heat lost by a subject as infrared, but polar bears are experts at conserving heat. The bears keep warm due to a thick layer of blubber under the skin. The polar bear's layers of fur and blubber trap their body heat well below their skin, leaving the outer layer to be roughly the same temperature as the snow around them. The bear's face and breath can be seen in infrared, but that isn’t enough to go on. 

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