Seasonal variation in freeze tolerance and ice content of the tree frog Hyla versicolor

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Abstract
Freeze tolerance and ice content of Hyla versicolor showed pronounced variation between summer (June) and winter (December). Summer frogs survived freezing at −3°C for up to 9 hr and ice accumulation up to 50% of their total body water. A time course of ice formation indicated that an equilibrium level was reached in approximately 15 hr. Thus, the lethal ice content was less than the equilibrium ice content for these conditions (63.1%). A second group was induced to enter an overwintering condition by holding them through the summer and then subjecting them to a progressive reduction in temperature and photoperiod for 2 months. These frogs survived freezing for 48 hr at −3°C. Their equilibrium ice content at this temperature was significantly lower (52.5%) than comparably treated summer animals. In the winter acclimatized group, frozen frogs had substantially higher blood glucose levels than unfrozen frogs (22.7 μmol/ml vs. 1.33 μmol/ml), but glycerol levels were not elevated after freezing. Freezing frogs conditioned for overwintering at −7°C resulted in a higher equilibrium ice content (62.6%), but none survived. It is evident that in preparation for overwintering, frogs reduce the amount of ice formed at a given subzero temperature, but there is little indication of a substantial change in the total amount of ice tolerated.