Cold Tolerance and Overwintering of an Introduced New Zealand Frog, the Brown Tree Frog (Litoria ewingii)

The overwintering strategy of Litoria ewingii in Otago, New Zealand, was studied under laboratory and field conditions. Microhabitat temperature measurements showed that the frogs were often exposed to subzero temperatures. In the laboratory, L. ewingii tolerated freezing for up to 6 hrs at -1°C, and after the completion of the freezing event (ca. 1 hr) at -2°C. Frogs frozen with insulation survived freezing for 12 hrs at -1°C. Frogs supercooled to -1.2 ± 0.1°C and -1.7 ± 0.3°C on wet and dry substrates respectively. L. ewingii tolerated up to 47.5% of its body water frozen. Plasma glucose levels and osmolality were not increased during freezing. It is concluded that L. ewingii cannot avoid freezing and is sufficiently freeze tolerant to survive the subzero temperatures encountered during winter in Otago.