The role of the integument as a barrier to penetration of ice into overwintering hatchlings of the painted turtle (Chrysemys picta)

Hatchlings of the North American painted turtle (Chrysemys picta) spend the ir first winter of life inside a shallow, subterranean hibernaculum (the na tal nest) where they may be exposed for extended periods to ice and cold. H atchlings seemingly survive exposure to such conditions by becoming superco oled (i.e., by remaining unfrozen at temperatures below the equilibrium fre ezing point for body fluids), so we investigated the role of their integume nt in preventing ice from penetrating into body compartments from surroundi ng soil. We first showed that hatchlings whose epidermis has been damaged a re more likely to be penetrated by growing crystals of ice than are turtles whose cutaneous barrier is intact. We next studied integument from a forel imb by light microscopy and discovered that the basal part of the alpha -ke ratin layer of the epidermis contains a dense layer of lipid. Skin from the forelimb of other neonatal turtles lacks such a layer of lipid in the epid ermis, and these other turtles also are highly susceptible to inoculative f reezing. Moreover, epidermis from the neck of hatchling painted turtles lac ks the lipid layer, and this region of the skin is readily penetrated by gr owing crystals ofice. We therefore conclude that the resistance to inoculat ion imposed by skin on the limbs of hatchling painted turtles results from the presence of lipids in the alpha -keratin layer of the epidermis. Neonat es apparently are able to avoid freezing during winter by drawing much of t he body inside the shell, leaving only the ice-resistant integument of the limbs exposed to ice in the environment. The combination of behavior and sk in morphology Enables overwintering hatchlings to exploit an adaptive strat egy based on supercooling. J. Morphol. 246:150-159, 2000. (C) 2000 Wiley-Li ss. Inc.