Seasonal changes in physiology and development of cold hardiness in the hatchling painted turtle Chrysemys picta

Hatchling painted turtles (Chrysemys picta) commonly hibernate in shallow, natal nests where winter temperatures may fall below -10 degreesC. Although hatchlings are moderately freeze-tolerant, they apparently rely on superco oling to survive exposure to severe cold. We investigated seasonal changes in physiology and ire the development of supercooling capacity and resistan ce to inoculative freezing in hatchling Chrysemys picta exposed in the labo ratory to temperatures that decreased from 22 to 4 degreesC over a 5.5 mont h period. For comparison, we also studied hatchling snapping turtles (Chely dra serpentina), a less cold-hardy species that usually overwinters under w ater. Although Chrysemys picta and Chelydra serpentina differed in some phy siological responses, both species lost dry mass, catabolized lipid and ten ded to gain body water during the acclimation :regimen, Recently hatched, 2 2 degreesC-acclimated Chrysemys picta supercooled only modestly (mean tempe rature of crystallization -6.3+/-0.2 degreesC; N=6) and were susceptible to inoculation by ice nuclei in a frozen substratum (mean temperature of crys tallization -1.1+/-0.1 degreesC; N=6) (means +/- S.E.M.). In contrast, cold -acclimated turtles exhibited pronounced capacities for supercooling and re sistance to inoculative freezing, The development of cold hardiness reflect ed the elimination or deactivation of potent endogenous ice nuclei and an e levation of blood osmolality that was due primarily to the retention of ure a, but was not associated with accumulation of the polyols, sugars or amino acids commonly found in the cryoprotection systems of other animals, Also, Chrysemys picta land Chelydra serpentina) lacked both antifreeze proteins and ice-nucleating proteins, which are used by some animals to promote supe rcooling and to initiate freezing at the high temperatures conducive to fre ezing survival, respectively.