ABSTRACT
Physiological aspects of freeze tolerance have received attention. Collectively these initial studies suggest that critical adaptive mechanisms consist of integrated, fundamental responses to freezing stress. Natural freeze tolerance is an adaptation that permits survival of freezing under conditions of ecological relevance. With few exceptions, freeze tolerant vertebrates likely hibernate in shallow, terrestrial hibernacula where subzero temperatures occur. Freeze tolerance is known in populations ranging from 60°N to 39°N. Freeze tolerance, rather than enhanced supercooling capacity, likely evolved in R. sylvatica because its water-permeable skin is a poor barrier to environmental ice. Variances in liver glycogen levels may largely explain the variability in cryoprotectant production, ice contents and, hence, freeze tolerance capacity. Investigations of the mechanisms of vertebrate freeze tolerance have provided novel insight into biomedical problems related to osmoregulation, hypothermia, glucose metabolism, and organ cryopreservation. Remarkable progress has already been realized by adapting key principles of natural freeze tolerance in frogs to mammalian models.
