Adaptations of frogs to survive freezing
Five species of frogs from North America survive extensive freezing of their body fluids to temperatures as low as -8C for periods lasting at least 2 weeks. These frogs hibernate in leaf litter where subzero temperatures commonly occur during the winter. The onset of freezing triggers liver glycogenolysis and the production of high concentrations of glucose or glycerol (to 100x normal) that functions as a cryoprotectant against freezing injury. Concomitantly the release of the latent heat of crystallization as body water freezes promotes the continued function of the cardiovascular system for many hours, and serves to distribute glucose throughout the body. The water content of major organs is reduced by 50% or more during the first 24 hours of freezing, with the water being relocated and frozen in other body spaces. Organ dehydration functions to concentrate cryoprotectant and the reduce mechanical damage by ice during freezing. As freezing progresses, breathing, heart beat, and most other vital functions cease, but reanimation occurs within a few hours after thawing. The evolution of freeze tolerance in these animals illustrates the highly flexible capacities of frogs to adapt to stressful environments.