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Endogenous and exogenous ice-nucleating agents constrain supercooling in the hatchling painted turtle
Department of Zoology, Miami University, Oxford, OH 45056, USA
* Author for correspondence (e-mail: costanjp{at}muohio.edu)
Accepted 28 October 2002
Hatchlings of the painted turtle (Chrysemys picta) commonly
hibernate in their shallow, natal nests. Survival at temperatures below the
limit of freeze tolerance (approximately -4°C) apparently depends on their
ability to remain supercooled, and, whereas previous studies have reported
that supercooling capacity improves markedly with cold acclimation, the
mechanistic basis for this change is incompletely understood. We report that
the crystallization temperature (Tc) of recently hatched
(summer) turtles acclimated to 22°C and reared on a substratum of
vermiculite or nesting soil was approximately 5°C higher than the
Tc determined for turtles acclimated to 4°C and tested
in winter. This increase in supercooling capacity coincided with elimination
of substratum (and, in fewer cases, eggshell) that the hatchlings had
ingested; however, this association was not necessarily causal because turtles
reared on a paper-covered substratum did not ingest exogenous matter but
nevertheless showed a similar increase in supercooling capacity. Our results
for turtles reared on paper revealed that seasonal development of supercooling
capacity fundamentally requires elimination of ice-nucleating agents (INA) of
endogenous origin: summer turtles, but not winter turtles, produced feces
(perhaps derived from residual yolk) that expressed ice-nucleating activity.
Ingestion of vermiculite or eggshell, which had modest ice-nucleating
activity, had no effect on the Tc, whereas ingestion of
nesting soil, which contained two classes of potent INA, markedly reduced the
supercooling capacity of summer turtles. This effect persisted long after the
turtles had purged their guts of soil particles, because the
Tc of winter turtles reared on nesting soil (mean ±
S.E.M.=-11.6±1.4°C) was approximately 6°C higher than the
Tc of winter turtles reared on vermiculite or paper.
Experiments in which winter turtles were fed INA commonly found in nesting
soil showed that water-soluble, organic agents can remain fully active for at
least one month. Such INA may account for the limited supercooling capacity
(Tc
-7.5°C) we found in turtles overwintering in
natural nests and may therefore pose a formidable challenge to the winter
survival of hatchling C. picta.
Key words: painted turtle, Chrysemys picta, cold hardiness, hibernation, acclimation, supercooling, ice nucleation, gut, yolk
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  G. C. Packard and M. J. Packard To freeze or not to freeze: adaptations for overwintering by hatchlings of the North American painted turtle J. Exp. Biol., September 1, 2004; 207(17): 2897 - 2906. [Abstract] [Full Text] [PDF]
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