Invisible Polar Bears

The Bush administration may not want scientists talking about them, but polar bears are real, in plenty of danger, and, interestingly, so efficient at retaining body heat that they are nearly invisible to infra-red imaging:

The thick undercoat does, however, insulate the bears: they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography; only their breath and muzzles can be easily seen.[7]