Jon Costanzo, costanjp@muohio.edu

Projects in Vertebrate Cryobiology

The historical view that ectothermic vertebrates cannot survive exposure to subzero body temperatures was challenged within the last quarter century by demonstrations of extensive supercooling and freeze tolerance in various amphibians and reptiles. A growing literature suggests that freeze tolerance and/or supercooling, a phenomenon in which a solution remains unfrozen at temperatures at or below its equilibrium freezing/melting point, are crucial elements in the cold hardiness strategies of certain temperate, boreal, and subarctic species of amphibians and reptiles.

 

Natural Freeze Tolerance

About a dozen species of amphibians and reptiles are known to tolerate the freezing of their tissues under thermal and temporal conditions that mimic frost exposure in nature (i.e., slow cooling to relatively high subzero temperatures). 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 dead - muscle contraction, heartbeat, and breathing have completely ceased. The frozen tissues become depleted of oxygen and the cellular energy status declines sharply. Remarkably, these animals arouse after thawing and can soon resume normal physiological and behavioral functions. Freeze tolerance is promoted by special physiological adaptations, including an accumulation of certain cryoprotective compounds, a redistribution of bulk water within the body, and an innate tolerance of cells to dehydration.

 

Freeze Avoidance by Supercooling

Contrary to what is taught 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 may be chilled nearly 40 degrees below 0°C before they spontaneously freeze. The capacity of a solution to "supercool" diminishes with increasing volume, which is why large animals cannot depend on supercooling for freeze avoidance. Also, supercooled solutions are metastable and will freeze instantly if brought into contact with ice crystals or various ice nucleating agents, which catalyze the nucleation event. Freezing of a deeply supercooled animal is lethal, regardless of whether the species has evolved freeze tolerance, because the ice propagates rapidly throughout the body. 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.

 

Research Aims

Our long-term goal is to elucidate the physiological, ecological, and evolutionary determinants of vertebrate cold hardiness. We are particularly interested in the roles of supercooling and freeze tolerance in promoting overwintering survival and shaping the geographic ranges of 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. Wherever possible we take an integrated field and laboratory approach in our studies.