The Basis for Cold Tolerance in Hatchling Painted Turtles (Chrysemys picta)

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Abstract
Hatchling painted turtles (Chrysemys picta) overwintering in subterranean hibernacula may withstand exposure to temperatures as low as - 11° C Some workers contend that this ability to survive in the cold results from a tolerance by the animals for freezing of water in the extracellular compartment, but other workers assert that hatchlings actually sustain a state of supercooling and remain unfrozen. We performed three experiments in the laboratory in an attempt to resolve this controversy. Fifty percent of hatchlings confined in artificial hibernacula in damp, clayey soil survived exposure to temperatures between - 8° and - 9° C, and some survived to near - 11° C. Clean, dry turtles that were removed from contact with ice survived in a supercooled, unfrozen state at minimum temperatures averaging - 8.5° C, so animals seem not to be at serious risk of freezing by spontaneous nucleation at temperatures spanning the range encountered in nature. The integument in the axillary and inguinal pouches of hatchlings is highly conductive to ice, but skin on the extremities resists the penetration of ice into body compartments from the environment. By withdrawing their head and limbs inside the shell and thereby removing skin of the axillary and inguinal pouches from contact with ice, hatchlings generally were able to avoid inoculation and to survive in frozen soil at - 2.6° C for over 2 wk without freezing. However, a few animals experienced delayed inoculation, and all these turtles died. Thus, hatchling painted turtles overwintering in the field probably withstand exposure to cold by undergoing supercooling and not by tolerating freezing.

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