Infrared imaging in vipers: differential responses of crotaline and viperine snakes to paired thermal targets.

Pit vipers use infrared-sensitive pit organs to accurately target homeothermic prey even in the absence of visual cues. It has been suggested that other vipers, including large ambush predators of the genus Bitis, also may use radiant infrared information for predatory targeting. We compared behavioral responses of pit vipers and snakes of the viperine genus Bitis to paired targets of different temperatures (i.e. cool and warm balloons), some scented with rodent odors. The rates of tongue flicking, head turning and approaches by pit vipers (Agkistrodon contortrix and Crotalus atrox) were significantly higher toward warm targets than toward cool ones. Moreover, they all were significantly higher in pit vipers than in the vipers Bitis arietans, Bitis gabonica, and Bitis nasicornis. Bitis sp. exhibited no significant differences in their behaviors toward warm versus cool targets. Pit vipers often struck at targets (always the warm target even when paired with a prey-scented cool target), but Bitis sp. never struck at either warm or cool targets. These results show that the behavioral correlates of infrared-based predatory and/or defensive targeting differ significantly between these two groups of viperid snakes, and suggest that the neural substrates of infrared imaging in crotaline snakes are either absent or functionally distinct in viperine snakes of the genus Bitis.