Resistance to freezing in hatchling painted turtles (Chrysemys picta)

We assessed the ability of hatchling painted turtles (Chrysemys picta) from northern Minnesota to resist freezing when exposed to conditions like thos e occurring in natural hibernacula (natal nests). We placed animals individ ually into artificial hibernacula constructed in jars of damp, loamy sand a nd then lowered the temperature to approximately -0.4 degrees C, which was below the equilibrium freezing point for water in the soil (approximately - 0.1 degrees C) but above that for body fluids of the turtles (approximately -0.7 degrees C). Next, ice was used to initiate freezing of water in the so il, after which the substratum was allowed to freeze to an equilibrium temp erature of -0.4 degrees C. The minimum temperature then was reduced by 1 de grees C/day to either -2.5 or -6.5 degrees C. The minimum was maintained fo r 9 days in the former treatment and for 5 days in the latter, so that turt les in both treatments were exposed for the same length of time to temperat ures below the equilibrium freezing point for their body fluids. Some anima ls in each treatment were inoculated by ice crystals that penetrated their integument, and these turtles froze and died. However, many other animals r esisted inoculation, remained unfrozen, and survived the test of tolerance. We conclude that hatchling painted turtles from northern Minnesota, like t hose from North Dakota, north-central Nebraska, and northern Illinois, with stand the cold of midwinter by supercooling and not by tolerating freezing.