PACKARD, G.C.; PACKARD, M.J.; Colorado State University, Fort Collins: Effect of Cold-Acclimation on Capacity for Supercooling in Hatchling Painted Turtles

Neonates of the North American painted turtle (<1>Chrysemys picta) typically spend their first winter of life inside the shallow, subterranean nest where they completed embryogenesis the preceding summer. This behavior commonly causes overwintering hatchlings in northerly populations to be exposed to ice and cold. Turtles in mid-winter withstand exposure to such conditions by resisting freezing and becoming supercooled. Newly hatched animals, however, have only a limited, intrinsic capacity for supercooling, owing apparently to the ingestion of quantities of soil and eggshell by turtles in the process of hatching. The ingested materials harbor numerous bacteria with ice-nucleating activity, and these bacteria seemingly cause water in the gut to freeze (by heterogeneous nucleation) at temperatures near -5&degC. The resulting crystals of ice penetrate the wall of the gut and initiate freezing of water in body compartments. Consequently, newly hatched turtles can be cooled only to about -5&degC before they begin to freeze spontaneously. Between the time of hatching and the arrival of wintery weather, turtles purge their gut of its contents and thereby reduce the population of ice-nucleating bacteria. This purging of the gut enables animals to become more deeply supercooled before they begin to freeze spontaneously. We will present evidence to show conclusively whether this purging of the gut is a result of maturational processes occurring in the weeks after hatching, or whether it is elicited specifically by acclimation to low temperature. Supported by the NSF (IBN-0112283).