NATURAL FREEZING SURVIVAL IN ANIMALS

Natural freeze-tolerance supports the winter survival of many animals including numerous terrestrial insects, many intertidal marine inverte brates, and selected species of terrestrially hibernating amphibians a nd reptiles. Freeze-tolerant animals typically endure the conversion o f 50% or more of total body water into extracellular ice and employ a suite of adaptations that counter the negative consequences of freezin g. Specific adaptations control the sites and rate of ice formation to prevent physical damage by ice. Other adaptations regulate cell-volum e change: Colligative cryoprotectants minimize cell shrinkage during e xtracellular ice formation; other protectants stabilize membrane struc ture; and a high density of membrane transporter proteins ensure rapid cryoprotectant distribution. Cell survival during freezing is also po tentiated by anoxia tolerance. mechanisms of metabolic rate depression , and antioxidant defenses. The net result of these protective mechani sms is the ability to reactivate vital functions after days or weeks o f continuous freezing. Magnetic resonance imaging has allowed visual e xaminations of the mode of ice penetration through the body of freeze- tolerant frogs and turtles, and cryomicroscopy has illustrated the eff ects of freezing on the cellular and microvasculature structure of tis sues. Various metabolic adaptations for freezing survival appear to ha ve evolved out of pre-existing physiological capacities of animals, in cluding desiccation-resistance and anoxia tolerance.