Cold Hardiness and Overwintering Strategies of Hatchlings in an Assemblage of Northern Turtles

Field and laboratory studies were conducted during 1989—1994 to investigate the overwintering strategies of hatchling turtles representing four families native to western Nebraska. Whereas hatchling snapping turtles (Chelydra serpentina) and spiny soft—shelled turtles (Apalone spinifera) overwinter in aquatic habitats, yellow mud turtles (Kinosternon flavescens) and ornate box turtles (Terrapene ornata) burrow below the natal nest and hibernate in sandy soil. Painted turtles (Chrysemys picta) overwinter within their shallow natal nests, but this species, and T. ornata, tolerate extensive tissue freezing. Overwintering behaviors of these species are consistent with indices of physiological cold hardiness and patterns of geographic distribution. Frost commonly penetrated and persisted below 10 cm, the soil depth at the which hatchling C. picta routinely hibernate. Field and laboratory data suggested that hatchling C. picta survive either by remaining supercooled (unfrozen) or by tolerating tissue freezing, the strategy employed depending on prevailing physiological and microenvironmental conditions. Whereas relatively lower temperatures can be survived in the supercooled state, supercooling capacity may be limited via the inoculation of body fluids by environmental ice. Alternatively, whereas freeze tolerance fortuitously is promoted by ice inoculation, this strategy may be viable only at relatively high subzero temperatures. A cold—hardness strategy based on both survival mechanisms may promote winter survival in hatchling C. picta by conferring protection under dynamic physiological and microenvironmental conditions. Physiological cold hardiness and behavior are integrated determinants of the northern distributions of temperate region turtles.