Thermoregulation by Bullsnakes (Pituophis catenifer sayi): do burrows make life easier on the prairies?

Reptiles living in the grasslands of Western Canada cope with extremely variable environmental temperatures. We studied body temperatures (Tb) and operative environmental temperatures (Te) for Bullsnakes (Pituophis catenifer sayi (Schlegel, 1837)) to address uncertainties regarding challenges posed by the thermal environments of northern grasslands, and to evaluate conflicting hypotheses regarding thermoregulatory strategies. Despite potentially extreme surface temperatures (meanmin. = 9.3 degrees C, meanmax. = 31.4 degrees C), mammal burrows remained within voluntary limits (15-35 degrees C) for 93% of the active season and created thermal gradients (up to 27.2 degrees C) which may facilitate thermoregulation by creating high thermal heterogeneity. This evidence suggests that grasslands may actually be less challenging for thermoregulation than comparable forests. Fifty-eight percent (57.8%) of the variation in observed Tb (meanmin. = 20.1 degrees C, meanmax. = 29.1 degrees C) was explained by three simple variables: time of day, day of year, and bare ground Te. Bullsnakes thermoregulated by conforming to Te near their preferred range (21-27 degrees C), selecting locations that enabled heating up below this range and avoiding warmth above this range. Our results support broad hypotheses of reptilian thermoregulation that predict increased thermoregulation when (i) environmental temperatures deviate farther from preferred ranges and (ii) costs of thermoregulation are lower due to thermal heterogeneity.