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 -8 degrees C for periods lasting at least 2 wk. These frogs hibernate in leaf litter where subz ero temperatures commonly occur during the winter. The onset of freezi ng triggers liver glycogenolysis and the production of high concentrat ions of glucose or glycerol (to 100 x normal) that functions as a cryo protectant against freezing injury. Concomitantly the release of the l atent heat of crystallization as body water freezes promotes the conti nued function of the cardiovascular system for many hours and serves t o distribute glucose throughout the body. The water content of major o rgans is reduced by 50% or more during the first 24 h of freezing, wit h the water being relocated and frozen in other body spaces. Organ deh ydration functions to concentrate cryoprotectant and to reduce mechani cal damage by ice during freezing. As freezing progresses, breathing, heart beat, and most other vital functions cease, but reanimation occu rs 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.