Hibernation: Poikilotherms

Abstract Poikilothermic, or cold‐blooded animals face a risk of death due to cold or freezing over the winter and have evolved multiple strategies for survival. Some are unique options include migration by some butterfly and dragonfly species whereas honeybees heat their hive by shivering. Many animals are insulated from deep cold by hibernating underground or under water. For lung‐breathing turtles and frogs, under water hibernation requires novel adaptations: skin breathing by frogs and biochemical adaptations to survive without oxygen by turtles. Other poikilotherms manage to endure temperatures below 0°C. Many insects can prevent themselves from freezing with the use of antifreeze proteins and high concentrations of sugar alcohols that keep their body fluids liquid down to −40°C or lower. Other insects as well as some intertidal mollusks, and some frogs, turtles and lizards endure whole body freezing with adaptations that regulate ice formation in extracellular spaces, protect the intracellular environment and ensure the reactivation of heart beat, breathing and other vital functions after thawing. Key Concepts: To survive the winter, cold‐blooded animals need strategies that allow them to elude or endure exposures to environmental temperatures that are below the freezing point of their body fluids. Some animals elude winter cold by migrating to warmer climates, others can dig down below the frost line or spend the winter in an aquatic environment that will not freeze. Winter survival under water by lung‐breathing animals such as frogs and turtles often requires new strategies for acquiring oxygen, such as oxygen uptake across the skin by frogs or across the epithelial lining of the throat by some turtles. Ice‐locked ponds and lakes often become oxygen‐depleted so many species have developed biochemical adaptations that allow them to survive without oxygen for weeks at a time. Cold‐blooded animals that spend the winter on land have two choices for dealing with exposure to temperatures below 0°C: use antifreezes to prevent themselves from freezing or develop strategies to endure and regulate ice formation in their bodies. The freeze avoidance strategy of survival used by many insects combines the production of special antifreeze proteins with the accumulation of high concentrations of glycerol or other polyhydric alcohols to keep body fluids liquid often to −40°C or lower. Specialised antifreeze proteins are also used by many marine fish that live in polar regions or that come in contact with sea ice. The freeze tolerance strategy of survival involves using sugars or polyhydric alcohols to protect the inside of cells while allowing specialised ice nucleating proteins to direct the formation of ice in body fluids cavities. Freeze tolerant animals include many insects, some snails and barnacles that live in the intertidal zone and a few frog and reptile species that spend the winter on land; most can survive days or weeks frozen with 50–65% of their total body water frozen. The molecular adaptations that allow animals to survive freezing have multiple potential applications for improving or developing methods for the cryopreservation of human cells, tissues and organs for use in medical transplantation.