Corrected entry: Arnie covers himself in cool mud because the Predator sees heat. But, they are in the middle of hot jungle. The jungle's temperature would surely be hotter than 98.6, or at least close to that. That means that the Predator would see all the air around him as red, not just the humans.
J I Cohen
27th Apr 2004
Correction: 1) Whether the heat-vision is the predator's natural vision, provided by the helmet, or the former enhanced by the latter, it would surely be able to compensate for the 'baseline' air temperature. 2) Even for a jungle, 98.6F is pretty hot. 82-94F day and 62-73F night would be the range of temperatures to expect (see http://www.go-south-adventures.com/go-south-weather.htm). Even though it's meant to be warmer than normal, it could still be below 98.6F. Of course, the humidity would make it feel hotter, but that would not affect heat-vision.
J I Cohen