Characteristics of nest soil, but not geographic origin, influence cold hardiness of hatchling painted turtles

We investigated environmental factors influencing cold hardiness in hatchling painted turtles (Chrysemys picta) indigenous to northeastern Indiana and the Sandhills of west-central Nebraska. In both locations, hatchlings overwinter in their natal nests. Survival of hatchlings chilled to minimum temperatures between -2.5 and -6.0 degrees C inside explanted natal nests ranged from 30 to 100%. Mortality likely was caused by freezing of the turtles that was induced by contact with ice nuclei in the surrounding soil. Susceptibility to inoculative freezing was strongly influenced by moisture content (7.5-25%, w/w) of the frozen soil in which hatchlings were cooled. When chilled in soil containing 15% moisture, turtles from Indiana resisted inoculative freezing better than hatchlings from Nebraska, but this variation was due to physical characteristics of the soils indigenous to each locale rather than genetic differences between populations. Soil in which the Indiana turtles nested contained relatively higher amounts of clay and organic matter, and bound more moisture, than the loamy sand at the Nebraska site. Soil collected from both locales contained potent ice nuclei that may constrain supercooling of the hatchlings, even in the absence of soil moisture. In addition to temperature and precipitation, local and regional variation in soils is an important determinant of overwintering survival of hatchling C. picta.