R. Willard et al., The role of the integument as a barrier to penetration of ice into overwintering hatchlings of the painted turtle (Chrysemys picta), J MORPH, 246(2), 2000, pp. 150-159
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.