Cooler snakes respond more strongly to infrared stimuli, but we have no idea why

The pit organ defining pit vipers (Crotalinae) contains a membrane covered with temperature receptors that detect thermal radiation from environmental surfaces. Temperature is both the environmental parameter being sensed and the mechanism by which the pit membrane detects the signal. As snakes are ectotherms, temperature also has a strong influence on neurological and locomotor responses to the signal. This study of Pacific rattlesnakes (Crotalus oreganus) systematically examined the effect of body, target and background temperatures on response to a moving target. We presented each snake with a moving pendulum bob regulated at a series of six temperatures against a uniform background regulated at one of three temperatures. Snake body temperatures varied from 18 to 36°C. As expected, we found stronger responses to positive contrasts (target warmer than background) than to negative contrasts, and stronger responses to greater contrasts. However, the effect of body temperature was contrary to expectations based on studies of the TRPA1 ion channel (believed to be the molecular basis for pit membrane temperature receptors) and typical thermal reaction norms for neural and motor performance. These predict (1) no response below the threshold where the TRPA1 channel opens, (2) response increasing as temperature increases, peaking near preferred body temperature, and (3) declining thereafter. Remarkably, this behavioral response decreased as body temperature increased from 18 to 36°C, with no threshold or peak in this range. We review various possible physiological mechanisms related to body temperature proposed in the literature, but find none that can satisfactorily explain this result.