COSTANZO, JP; BAKER, PJ*; LEE, RE; Miami University: Comparative Physiology of Freeze Tolerance in Hatchling Turtles

Freeze Tolerance in Hatchling Turtles: a Comparative Study J.P. Costanzo, P.J. Baker, and R.E. Lee, Jr. Department of Zoology, Miami University, Oxford, OH Hatchlings of some, though not all, species of northern freshwater turtles are tolerant of somatic freezing, making this group a good candidate for comparative study to elucidate the mechanisms underlying reptilian freeze tolerance. We examined the capacity for freeze tolerance in hatchlings representing nine turtle taxa. Turtles were exposed to �3.0&degC over 36 h, passively thawed at 0�C, and tested for neurobehavioral function over the next 7 d. A well-developed freeze tolerance was found in Terrapene ornata, Emydoidea blandingii, , Malaclemys terrapin, , and two subspecies of Chrysemys picta, . In contrast, all Sternotherus odoratus, and Graptemys geographica, succumbed, and Chelydra serpentina, and Trachemys scripta, exhibited an intermediate response. Turtles of the highly tolerant taxa usually recovered neurobehavioral functions within 2-3 d of thawing. Morbidity and mortality coincided with strongly elevated plasma levels of lactate dehydrogenase, which presumably leaked from cells through damaged membranes. Unfrozen and frozen/thawed turtles were tested for the presence of various solutes that might limit freeze/thaw injury. The plasma of unfrozen turtles contained little lactate (≤ 3 mM) and glucose (≤ 6 mM), but a moderate quantity (8-53 mM) of urea. In all taxa, freezing/thawing was associated with an increase in plasma glucose (up to 45 mM) and lactate (up to 55 mM), with a corresponding 6-25% rise in osmolality. Estimated amounts of ice forming in tissues under the conditions of the experiment varied slightly among taxa (62-69% of total body water frozen) but were not associated with viability. Freeze tolerance capacity did not follow phylogenetic lines, but may reflect winter life history and the likelihood of freezing in nature.