COLD-HARDINESS AND OVERWINTERING STRATEGIES OF HATCHLINGS IN AN ASSEMBLAGE OF NORTHERN TURTLES

Field and laboratory studies were conducted during 1989-1994 to invest igate the overwintering strategies of hatchling turtles representing f our families native to western Nebraska. Whereas hatchling snapping tu rtles (Chelydra serpentina) and spiny soft-shelled turtles (Apalone sp inifera) overwinter in aquatic habitats, yellow mud turtles (Kinostern on 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 beha viors of these species are consistent with indices of physiological co ld hardiness and patterns of geographic distribution. Frost commonly p enetrated and persisted below 10 cm, the soil depth at which hatchling C. picta routinely hibernate. Field and laboratory data suggested tha t 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 r elatively lower temperatures can be survived in the supercooled state, supercooling capacity may be limited via the inoculation of body flui ds by environmental ice. Alternatively, whereas freeze tolerance fortu itously is promoted by ice inoculation, this strategy may be viable on ly at relatively high subzero temperatures. A cold-hardiness strategy based on both survival mechanisms may promote winter survival in hatch ling C. picta by conferring protection under dynamic physiological and microenvironmental conditions. Physiological cold hardiness and behav ior are integrated determinants of the northern distributions of tempe rate region turtles,