Projects in Vertebrate Cryobiology
Our ultimate research aim is to discover how temperate amphibians and reptiles thrive in seasonally cold environments. These animals can spend over half of their lives in hibernation, yet relatively little is known of their winter biology. Elucidating the physiological, ecological, and evolutionary determinants of survival in such habitats would foster a more complete understanding of factors influencing evolution of life-history traits, geographic distributions, and potential consequences of global climate change for northern amphibians and reptiles. Current understanding of the ecophysiological and evolutionary aspects of vertebrate cold hardiness is limited by gaps in our fundamental knowledge of the interactions between animals and their winter microenvironments.
The historical view that ectothermic vertebrates die at subfreezing temperatures has been challenged during the last quarter century. A growing literature suggests that certain cold-adapted species can survive these thermal extremes by virtue of their capacities for freeze tolerance or supercooling.
Natural Freeze Tolerance
About a dozen species of amphibians and reptiles are known to be “freeze tolerant,” able to tolerate tissue freezing under naturalistic thermal and temporal conditions. Generally, ice formation is restricted to extracellular spaces, as intracellular freezing is not tolerated. Some species survive freezing at temperatures as low as -6°C and endure freezing episodes lasting more than a month. Fully-frozen animals, in which up to 65-70% of the body fluid has become ice, appear lifeless: muscle contraction, heartbeat, and breathing have ceased. There is no flow of blood to the frozen tissues, which become depleted of oxygen and energy. Nevertheless, frozen specimens arouse after thawing and can resume normal physiological and behavioral functions within a day or two. Natural freeze tolerance is promoted by special molecular and physiological adaptations, including an accumulation of certain cryoprotective compounds, a redistribution of bulk water within the body, and an innate tolerance of cells to hypoxia and dehydration.
Freeze Avoidance by Supercooling
Contrary to what we learned in the elementary chemistry classroom, water does not necessarily freeze at its so-called freezing point of 0°C. In fact, small volumes of pure water can be cooled to about -40°C before freezing spontaneously occurs. Such supercooled solutions are metastable and will freeze instantly if brought into contact with ice crystals or various ice nucleating agents, which catalyze the ice nucleation event. Freezing of a deeply supercooled animal usually is lethal, regardless of whether the species has evolved freeze tolerance, because rapid, uncontrolled ice formation does not allow cells time to adjust to ensuing stress. The capacity of a solution to "supercool" diminishes with increasing volume, which is why large animals cannot depend on supercooling to avoid freezing. Despite these limitations, supercooling is an important survival mechanism for some reptiles, including lizards and the neonates of several species of turtles that are exposed to subzero temperatures in nature.