Natural freeze tolerance in ectothermic vertebrates.

Abstract

Amphibians and reptiles living in seasonally cold regions of the earth face several challenges to their continued survival. These include short summer seasons for the development of eggs and juveniles, long periods of fasting when food supplies are interrupted by winter, and months of cold exposure at environmental temperatures often well below the freezing point (FP) of their body fluids (21, 41, 64). This review focuses on one strategy of winter cold hardiness: freeze tolerance. A number of terrestrially-hibernating reptiles and amphibians have developed the ability to endure the freezing of extracellular body fluids and can revive after days or weeks spent with as much as 65% of their total body water locked in ice (41, 52, 61). All ectothermic vertebrates living in seasonally cold regions of the earth seek the shelter of thennally-buffered, relatively wann hibernacula. For most species, hibernation sites underwater or deep underground position the an­ imals to spend the winter without fear of freezing (21, 41, 64). Various species, however, hibernate at sites that are less well insulated; for example, several species of frogs hibernate on the forest floor. Here, coverings of leaf litter and snow insulate animals from the most extreme winter air tempera­ tures, but extended periods of subzero exposure, sometimes as low as -8°C, may occur (32, 35, 36). Species hibernating in such sites need additional