Supercooling in the Iguanid Lizard Sceloporus jarrovi

Abstract

Throughout the year Sceloporus jarrovi is capable of significant supercooling of the deep-body temperature below the freezing point of its tissues. For S. jarrovi there was no seasonal acclimatization of the supercooling limit (SL). In fact, during the more severe winter of this study the ability to supercool decreased significantly from the levels measured during the summer and second, more moderate winter. There was a significant and positive linear regression of SL on freezing point (FP), and this relationship has a higher elevation than that reported for reptiles as a whole. There appears to be an optimum constant acclimation temperature for maximum supercooling (i.e., about 25 C). Below this level the ability to supercool decreases. Although constant low-temperature exposure decreased supercooling ability, there was no such effect following daily cycled-temperature exposure (-2 C to 30 C). There was a significant lowering by dehydration of the SL and FP at constant temperatures from 15 to 25 C. At 25 C a 5% water loss resulted in significant reduction in the SL and FP. Further dehydration beyond 5% did not lower either parameter. For animals measured directly from the field there is a significant relationship between both the SL and the FP and the body water content. The predicted effect on the SL may be overridden by exposure to constant low temperature. An animal of higher water content held at 30 C, for example, may actually have a lower SL than an animal of lower water content but kept at 15 C. It appears quite likely that supercooling is important for survival during the winter of populations of S. jarrovi at high elevations. At an air temperature of about -10 C, hibernaculum temperatures could be as low as -5 C, which approaches very closely the field SL for the populations studied.