Freeze tolerance and intolerance as strategies of winter survival in terrestrially-hibernating amphibians.

The ability to tolerate extracellular freezing as an adaptation for winter survival was tested in seven species of terrestrially-hibernating amphibians found in eastern Canada. All species had only moderate supercooling abilities, with whole animal supercooling points of -1.5 to -3 degrees C. Two salamander species, Plethodon cinereus and Ambystoma laterale, and the toad, Bufo americanus, were freezing intolerant and were killed when frozen for 24 hr at temperatures just below their supercooling points. The major winter strategy of these animals appears to behavioural avoidance of subzero temperatures. Four species of frogs Rana sylvatica, Hyla versicolor, Hyla crucifer and Pseudacris triseriata, survived extracellular freezing at moderate subzero temperatures (-2 to -4 degrees C) for periods of time ranging up to 2 weeks. All four frog species accumulated low molecular weight carbohydrates as cryoprotectants, glycerol being the major cryoprotectant in adult H. versicolor, while immature adults of this species as well as the other three species all produced high levels of glucose as the cryoprotectant.