Supercooling and Freezing Tolerant Animals

Subzero temperatures may adversely affect animals by their direct lethal effects and by the damage caused by ice formation (Ramløv, 2000). Animals deal with the latter by three basic strategies. Freeze avoiding animals prevent ice formation in their bodies and supercool, keeping their body fluids liquid at temperatures below their melting point, but die if freezing occurs. In contrast, freezing tolerant animals can survive ice forming inside their bodies (Lee, 2010). Although both these categories of cold tolerance have been further subdivided (Bale, 1993; Sinclair, 1999) freeze avoidance and freezing tolerance are still recognised as fundamental cold tolerance strategies (Wharton, 2011a). In the third mechanism, cryoprotective dehydration which is found mainly in soil-dwelling animals, the body fluids remain unfrozen whilst surrounded by frozen soil. Since ice has a lower vapour pressure than liquid water at the same temperature the animal dehydrates and lowers the melting point of its body fluids, thus preventing freezing (Lee, 2010). In this review I examine the role of ice nucleation and supercooling in the main groups of freezing tolerant animals.