Thermal modeling of snake infrared reception: evidence for limited detection range.

For more than 40 years, information has circulated with regard to the sensitivity of infrared pit organs in both boid and crotaline snakes (pythons and pit vipers, respectively). The most often quoted sensitivity is 0.003 degrees C and this value is based on the work of Bullock and co-workers (1956). Missing from previous work was a quantitative model of radiation transfer that would report sensitivity not in terms of degrees Celsius, but rather sensing distance. Since prey detection is often cited as the function of the infrared pit organ, quantification of this sensing distance seemed to be an important value that was missing from the literature. In this paper, we model the radiation transfer process from a 37 degrees C object, i.e. warm-blooded prey, to an infrared pit organ. The model tries to answer a very basic question-at what distance does the thermal signature of a 37 degrees C object blend into the background for a non-imaging biological infrared sensor? The output of the model, the sensing distance, is of particular interest in comparing biological infrared sensors to current semiconductor-based infrared (IR) detectors-largely because of inappropriate comparisons between the temperature sensitivity of IR snake reception and imaging IR cameras. The purpose of the presented work to make more appropriate comparisons, i.e. sensing distance. This sensing distance output indicates an extremely short detection distance (<5 cm)-contradictory to what is observed experimentally. This dichotomy raises further questions regarding how the biological system amplifies this weak signal.

The Python pit organ: imaging and immunocytochemical analysis of an extremely sensitive natural infrared detector.

The Python infrared-sensitive pit organ is a natural infrared imager that combines high sensitivity, ambient temperature function, microscopic dimensions, and self-repair. We are investigating the spectral sensitivity and signal transduction process in snake infrared-sensitive neurons, neither of which is understood. For example, it is unknown whether infrared receptor neurons function on a thermal or a photic mechanism. We imaged pit organs in living Python molurus and Python regius using infrared-sensitive digital video cameras. Pit organs were significantly more absorptive and/or emissive than surrounding tissues in both 3-5 microns and 8-12 microns wavelength ranges. Pit organs exhibited greater absorption/emissivity in the 8-12 microns range than in the 3-5 microns range. To directly test the relationship between photoreceptors and pit organ infrared-sensitive neurons, we performed immunocytochemistry using antisera directed against retinal photoreceptor opsins. Retinal photoreceptors were labeled with antisera specific for retinal opsins, but these antisera failed to label terminals of infrared-sensitive neurons in the pit organ. Infrared-receptive neurons were also distinguished from retinal photoreceptors on the basis of their calcium-binding protein content. These results indicate that the pit organ absorbs infrared radiation in two major atmospheric transmission windows, one of which (8-12 microns) matches emission of targeted prey, and that infrared receptors are biochemically distinct from retinal photoreceptors. These results also provide the first identification of prospective biochemical components of infrared signal transduction in pit organ receptor neurons.