Responses of infrared-sensitive tectal units of the pit viper Crotalus atrox to moving objects.

Rattlesnakes perceive IR radiation with their pit organs. This enables them to detect and strike towards warm-blooded prey even in the dark. In addition, the IR sense allows rattlesnakes to find places for thermoregulation. Animate objects (e.g., prey) tend to move and thus cause moving IR images across the pit membrane. Even when an object is stationary, scanning head movements of rattlesnakes will result in moving IR images across the pit membrane. We recorded the neuronal activity of IR-sensitive tectal neurons of the rattlesnake Crotalus atrox while stimulating the snakes with an IR source that moved horizontally at various velocities. As long as object velocity was low (angular velocity of ~5°/s) IR-sensitive tectal neurons hardly showed any responses. With increasing object velocity though, neuronal activity reached a maximum at ~50°/s. A further increase in object velocity up to ~120°/s resulted in a slight decrease of neuronal activity. Our results demonstrate the importance of moving stimuli for the snake's IR detection abilities: in contrast to fast moving objects, stationary or slowly moving objects will not be detected when the snake is motionless, but might be detected by scanning head movements.