The sensory systems of some animals allow them to perceive environmental stimuli that are not detectable by other organisms. Certain lineages of snakes, including rattlesnakes, are able to detect infrared (IR) radiation, yet there has been little investigation on how this thermal vision shapes the behavior and ecology of these predators in nature. My dissertation research uses field experiments to determine the role of rattlesnake IR sensing in prey evaluation, ambush site selection, and navigation. First, I established that free ranging kangaroo rats undergo slight changes in body surface temperatures after encountering rattlesnakes. These temperature changes could be used by rattlesnakes to assess the state of vigilance of potential prey animals. Second, I presented prey models of different temperatures to wild hunting rattlesnakes to test whether snakes use information contained in prey surface temperature to evaluate prey animals in addition to simply detecting them. The behavioral responses of rattlesnakes were most affected by prey-background temperature contrast, and not absolute prey temperature, suggesting that snakes likely do not evaluate prey animals based on temperature. Third, I evaluated the importance of thermal backgrounds to hunting rattlesnakes. I quantified natural ambush sites and calculated the thermal contrast that a prey animal would present against all possible backgrounds. Snakes tended to face backgrounds that, according to current understanding of IR receptor physiology, would facilitate detection of a moving prey animal. Finally, I determined the relative importance of vision and IR sensing in basic navigation. Through experimental occlusions of both sensory channels I found that snakes rely on vision rather than IR sensing for nocturnal navigation despite the presence of thermally salient environmental features. Overall, my research highlighted the diversity of thermal information available to IR-sensitive observers and found that the effects of IR sensing on rattlesnake behavior vary across different ecological contexts.