Physiological adaptations of ectotherms to cold: Seasonal and geographic trends (Hyla versicolor, Hyla chrysoscelis, Eurosta solidaginis)

The first study provided a seasonal characterization of metabolic rate and the potential for morphological development in overwintering third-instar larvae of the goldenrod gall fly, Eurosta solidaginis. Metabolic rate (CO₂ production) was significantly lower between October 15 and February 9 than in autumn (September 9) and spring (March 1). The induction of diapause was temporally correlated with the development of cold-hardening, maximum larval mass, and gall senescence, but my experiments did not identify the specific cues triggering diapause induction. Until December 15 the larvae were incapable of morphological development, even at permissive temperatures. Remarkably, diapause development occurred in larvae frozen at −20°C.; The next study examined the hypothesis that cold and freezing reduce metabolic rates and water loss of goldenrod gall fly larvae. Metabolic rate fell consistently as the larvae were cooled below freezing but remained measurable even at −15°C. Frozen larvae had a significantly reduced metabolic rate and rate of water loss compared to supercooled larvae at the same temperature. Thus, freezing may be advantageous by reducing energy consumption and water loss.; I also examined whether mild winter temperatures are detrimental to the survival and reproductive potential of goldenrod gall flies. Larvae held at 12°C suffered high mortality (70%) and relatively low potential fecundity as adults (mean ± SEM = 199 ± 11 eggs/female), while those held at 0°C had both low mortality (11%) and high potential fecundity (256 ± 15 eggs/female). I conclude that mild winter temperatures may be detrimental to some overwintering insects, particularly species that do not feed following winter diapause.; The final study compared the degree of freeze tolerance between gray treefrogs (Hyla chrysoscelis and H. versicolor) from three different geographic locations (Indiana, Minnesota, and Missouri). There were no differences in cryoprotective responses (glycerol and glucose) nor in the minimum temperature survived. The Minnesota frogs had more liver glycogen available but did not produce more cryoprotectant. I conclude that (1) there are no genetic differences to enhance freeze tolerance in northern frogs and (2) comparisons of previously published studies have led to incorrect conclusions regarding adaptations to freeze tolerance in this group.