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Three experiments investigated the effects of thermal background on orientation of western diamondback rattlesnakes (Crotalus atrox) to prey. In Experiments 1 and 2, the snakes responded more strongly to rodent pretty in front of cool backgrounds than to identical prey in front of warm backgrounds. Accordingly, we suggest that the pitviper infrared system exhibits contrast effects such that rodent prey at approximately 37°C are detected more readily if the background is of significantly lower temperature than it if is of higher temperature. Experiment 3 created a thermal background containing an edge between panels at different temperatures (15°C and 21°C). Models of prey were placed in front of each panel and at the border between them. Snakes oriented more to the border model than to either of the others, suggesting that the edge was of special significance. In human visual studies, edges between panels varying in brightness give rise to Mach Bands which enhance the salience of the edges. Perhaps an analogous phenomenon occurs in the pitviper infrared system.