BAKKEN, G.S.; KROCHMAL, A.R.; Indiana State University; University of Houston Downtown: What might rattlesnakes see? Thermal signal strength of prey, predators, and background

Pitvipers (Viperidae: Crotalinae) derive their name from the facial pits, which are paired thermal radiation receptors that detect fluctuations in the temperatures of surrounding surfaces. Facial pits are often regarded as specializations for detecting warm prey and directing predatory strikes, but recent findings demonstrate that pitvipers can also use these organs to direct thermoregulatory movements. Thus, facial pits may be general-purpose sense organs with many uses, possibly including predator detection and general navigation. The behavioral ecology and evolution of the facial pit are closely tied to the thermal signal strength (temperature contrast × solid angle, where solid angle = target cross-section/distance) presented by the target relevant to each function, but there are few relevant data. To determine actual thermal signal strengths, we recorded thermal images of typical prey items, predators, and environmental surfaces across a range of conditions. We found the thermal signal strength presented by prey items to be less than that resulting from the assumption of a 37°-40°C surface temperature used by many studies. For example, the average surface temperature of an Ord’s kangaroo rat (Dipodomys ordi) differed from ambient temperature by only 2° to 6°C under typical active conditions. Predators have similar surface temperatures, but subtend larger solid angles. Environmental surfaces subtend large solid angles and can show as much as 20°C-30°C temperature contrasts in direct sunlight, which can carry over into evening hours. Thus, variation in background surface temperature during the day and early hours of the night may impair the use of the facial pit to detect prey and predators.