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Cold Tolerance in Hatchling Painted Turtles (Chrysemys picta): Supercooling or Tolerance for Freezing?

Gary C. Packard, Leslie D. Lohmiller, Mary J. Packard and Jeffrey W. Lang
Physiological Zoology
Vol. 70, No. 6 (November/December 1997), pp. 670-678
DOI: 10.1086/515875
Stable URL: http://www.jstor.org/stable/10.1086/515875
Page Count: 9
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Cold Tolerance in Hatchling Painted Turtles (Chrysemys picta): Supercooling or Tolerance for Freezing?
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Abstract

ABSTRACT We studied tolerance for cold in hatchling painted turtles (Chrysemys picta) from Lake Metigoshe, Bottineau County, North Dakota, to determine whether neonates in populations near the northern limit of distribution rely on a tolerance for freezing or on a capacity for supercooling to survive their first winter of life. We placed hatchlings individually into artificial hibernacula constructed in jars of damp, loamy sand and then cooled the jars to approximately −0.45°C, which was below the equilibrium freezing point for water held by the sand but above that for body fluids of the neonatal turtles. A piece of ice next was placed on the surface of the sand in each jar to induce freezing of the soil water. After the soil water had frozen to an equilibrium, the temperature in the jars was lowered by 1°C/d to minima averaging −2.5°C, −4.5°C, −6.5°C, and −10.5°C in different treatments. These temperatures were maintained for varying periods, so that animals in each treatment were exposed to temperatures below the equilibrium freezing point for their body fluids for a total of 11 d. Thirty of 32 hatchlings survived exposure to −2.5°C; 24 of 32 survived at −4.5°C; 14 of 32 withstood −6.5°C; and 7 of 32 tolerated −10.5°C. Freezing exotherms were detected in temperature profiles for turtles that succumbed but not in those for hatchlings that survived. Thus, the ability of hatchlings to withstand subzero temperatures for extended periods apparently requires that they avoid freezing. Although other workers contend that tolerance for freezing is the key to survival over winter by hatchling painted turtles from the region of Lake Metigoshe, our findings indicate that neonates rely primarily on their ability to remain unfrozen and supercooled.

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