Natural freeze tolerance in the terrestrial turtle, Terrapene carolina

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Abstract
To investigate freeze tolerance in box turtles (Terrapene carolina), we froze 19 adults to body temperatures as low as −3.6°C under controlled laboratory conditions. Our data reveal that box turtles tolerate the freezing of at least 58% of their body water and can remain frozen, without injury, for at least 73 hr. Supercooling occurred in 63% of the turtles but only lasted from 0.1 to 2.0 hr. Supercooling points (X ± SEM = −1.1 ± 0.3°C) were high, and thus supercooling is not an effective strategy for freeze avoidance in T. carolina. Body ice contents (7–58% of total body water) of turtles frozen for 0.7–50 hr, determined calorimetrically, were inversely related to core body temperature. Significant thermal gradients occurred within the turtles' bodies during prefreeze cooling, freezing, and thawing. Since T. carolina hibernates in shallow terrestrial burrows, where exposure to subzero temperatures is likely, freeze tolerance represents a remarkable adaptation that probably enhances winter survival. These turtles are the largest (up to 0.5 kg) animals, by nearly an order of magnitude, for which natural freeze tolerance has been demonstrated.