Soil hydric characteristics and environmental ice nuclei influence supercooling capacity of hatchling painted turtles Chrysemys picta

Hatchling painted turtles (Chrysemys picta) hibernate in their shallow nata l nests where temperatures occasionally fall below -10 degrees C during col d winters, Because the thermal limit of freeze tolerance in this species is approximately -4 degrees C, hatchlings rely on supercooling to survive exp osure to extreme cold, We investigated the influence of environmental ice n uclei on susceptibility to inoculative freezing in hatchling C. picta indig enous to the Sandhills of west-central Nebraska, In the absence of external ice nuclei, hatchlings cooled to -14.6+/-1.9 degrees C (mean +/- S.E.M.; N =5) before spontaneously freezing, Supercooling capacity varied markedly am ong turtles cooled in physical contact with sandy soil collected from nesti ng locales or samples of the native soil to which water-binding agents (cla y or peat) had been added, despite the fact that all substrata contained th e same amount of moisture (7.5 % moisture, w/w). The temperature of crystal lization (T-c) of turtles exposed to frozen native soil was -1.6+/-0.4 degr ees C (N=5), whereas turtles exposed to frozen soil/clay and soil/peat mixt ures supercooled extensively (mean T-c values approximately -13 degrees C), Hatchlings cooled in contact with drier (less than or equal to 4 % moistur e) native soil also supercooled extensively, Thus, inoculative freezing is promoted by exposure to sandy soils containing abundant moisture and little clay or organic matter, Soil collected at turtle nesting locales in mid an d late winter contained variable amounts of moisture (4-15 % w/w) and organ ic matter (1-3 % w/w). In addition to ice, the soil at turtle nesting local es may harbor inorganic and organic ice nuclei that may also seed the freez ing of hatchlings. Bulk samples of native soil, which were autoclaved to de stroy any organic nuclei, nucleated aqueous solutions at approximately -7 d egrees C (T-c range -6.1 to -8.2 degrees C), Non-autoclaved samples contain ed water-extractable, presumably organic, ice nuclei (T-c range -4.4 to -5. 3 degrees C), Ice nuclei of both classes varied in potency among turtle nes ting locales, Interaction with ice nuclei in the winter microenvironment de termines whether hatchling C. picta remain supercooled or freeze and may ul timately account for differential mortality in nests at a given locale and for variation in winter survival rates among populations.