GRACE, M.S.*; VANDYKE, J.U.; Florida Institute of Technology: Infrared Imaging in Pit Vipers: Complex Behavior from a Thermal Contrast Detector

Crotaline snakes (pit vipers) form images of their environments based upon input from either the eyes or infrared (IR)-imaging pit organs alone, or from the two systems together. We hypothesize that the IR imaging system, like the visual system, is fundamentally a contrast detector, operating on the basis of differential emission of IR energy from thermally distinct objects in the environment. To test this hypothesis, we investigated how thermal contrast influences IR imaging and response behavior in the copperhead (Agkistrodon contortrix). Temporarily-blinded copperheads were presented with targets creating positive, negative, or zero thermal contrast differentials with respect to controlled background temperature. Behavioral responses (tongue flicks, head turns and strikes) to positive and negative thermal differentials were greater than responses to zero differentials, and snakes preferentially targeted warm aspects of thermal differentials (behaviors were expressed in phase with motion of warm targets against a cool background, but in exact antiphase with motion of cool targets against a warm background). Snakes often exhibited defensive behavior such as rapid tail vibration in response to positive and negative differentials, and sometimes struck at moving targets. Strikes were always directed at the warm aspect of thermal differentials (strikes never occurred in zero contrast situations). These results show that thermal contrast is the basis of IR imaging in pit vipers, and suggest that IR-imaging snakes may be hard-wired to respond preferentially to warm objects. In addition, this is the first direct experimental evidence that pit vipers utilize the IR system for defensive behavior. Moreover, these results demonstrate that fundamental properties of IR imaging and vision are similar.