Cold acclimation enhances cutaneous resistance to inoculative freezing in hatchling painted turtles, Chrysemys picta

Abstract

Summary Neonatal Painted Turtles, Chrysemys picta (Schneider 1783), typically spend the first winter of their life inside the shallow, subterranean nest where they completed incubation the preceding summer. This behaviour commonly causes hatchlings in northerly populations to be exposed in mid‐winter to life‐threatening conditions of ice and cold. Neonates apparently withstand such exposure by remaining unfrozen and becoming supercooled. Recently hatched turtles are unable to resist freezing by inoculation (i.e. freezing that is induced by the penetration of ice crystals through the integument and into body compartments from frozen soil). Consequently, newly hatched animals that make contact with ice are highly susceptible to freezing when temperature goes below the equilibrium freezing point for their body fluids (c. −0·7 °C). Freezing is fatal, even when temperature does not go below −2 °C, so newly hatched turtles have only a very limited tolerance for exposure to ice and cold. Acclimation (or acclimatization) by neonatal turtles to low temperatures in the interval between hatching and the start of winter elicits an increase in effectiveness of the cutaneous barrier to the penetration of ice into body compartments from the environment. Acclimated hatchlings coming into contact with ice in frozen soil usually remain unfrozen and supercooled, and survival by supercooled turtles is uniformly high. This thermally induced enhancement of the cutaneous barrier to inoculation is key to an overwintering strategy based on supercooling.