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Journal of Experimental Biology, Vol 201, Issue 22 3105-3112, Copyright © 1998 by Company of Biologists
JOURNAL ARTICLES |
JP Costanzo, JD Litzgus, JB Iverson and RE Lee
Department of Zoology, Miami University, Oxford, OH 45056, USA. costanjp@muohio.edu
Hatchling painted turtles (Chrysemys picta) hibernate in their shallow natal nests where temperatures occasionally fall below -10 C during cold winters. Because the thermal limit of freeze tolerance in this species is approximately -4 C, hatchlings rely on supercooling to survive exposure to extreme cold. We investigated the influence of environmental ice nuclei on susceptibility to inoculative freezing in hatchling C. picta indigenous to the Sandhills of west-central Nebraska. In the absence of external ice nuclei, hatchlings cooled to -14.6 1.9 C (mean s.e.m.; N=5) before spontaneously freezing. Supercooling capacity varied markedly among turtles cooled in physical contact with sandy soil collected from nesting locales or samples of the native soil to which water-binding agents (clay or peat) had been added, despite the fact that all substrata contained the same amount of moisture (7.5 % moisture, w/w). The temperature of crystallization (Tc) of turtles exposed to frozen native soil was -1.6 0.4 C (N=5), whereas turtles exposed to frozen soil/clay and soil/peat mixtures supercooled extensively (mean Tc values approximately -13 C). Hatchlings cooled in contact with drier (less than or equal to 4 % moisture) 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 and late winter contained variable amounts of moisture (4-15 % w/w) and organic matter (1-3 % w/w). In addition to ice, the soil at turtle nesting locales may harbor inorganic and organic ice nuclei that may also seed the freezing of hatchlings. Bulk samples of native soil, which were autoclaved to destroy any organic nuclei, nucleated aqueous solutions at approximately -7 C (Tc range -6.1 to -8.2 C). Non-autoclaved samples contained water-extractable, presumably organic, ice nuclei (Tc range -4.4 to -5.3 C). Ice nuclei of both classes varied in potency among turtle nesting locales. Interaction with ice nuclei in the winter microenvironment determines whether hatchling C. picta remain supercooled or freeze and may ultimately account for differential mortality in nests at a given locale and for variation in winter survival rates among populations.
This article has been cited by other articles:
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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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J. P. Costanzo, P. J. Baker, S. A. Dinkelacker, and R. E. Lee Jr Endogenous and exogenous ice-nucleating agents constrain supercooling in the hatchling painted turtle J. Exp. Biol., February 1, 2003; 206(3): 477 - 485. [Abstract] [Full Text] [PDF]
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G. Packard, M. Packard, and L. McDaniel Seasonal change in the capacity for supercooling by neonatal painted turtles J. Exp. Biol., May 1, 2001; 204(9): 1667 - 1672. [Abstract] [PDF]
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J. Costanzo, J. Litzgus, J. Iverson, and R. Lee Seasonal changes in physiology and development of cold hardiness in the hatchling painted turtle Chrysemys picta J. Exp. Biol., January 11, 2000; 203(22): 3459 - 3470. [Abstract] [PDF]
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