Seasonal changes in physiology and development of cold hardiness in the hatchling painted turtle Chrysemys picta

Evolution of GCGR family ligand-receptor extensive cross-interaction systems suggests a therapeutic direction for hyperglycemia in mammals

Glucose is essential to the physiological processes of vertebrates. Mammalian physiological stability requires a relatively stable blood glucose level (~5 mM), whereas other vertebrates have greater flexibility in regulating blood glucose (0.5-25 mM). GCGR family receptors play an important role in vertebrate glucose regulation. Here, we examine the evolution of the GCGR family ligand-receptor systems in different species. Comparatively, we discover that the conserved sequences among GCG family ligands lead to the non-specific activation of ligands across species. In particular, we observe that glucagon-like peptide 1 receptor (GLP1R), glucagon-like peptide 2 receptor (GLP2R), and glucagon-like receptor (GCGLR, also called GCRPR) are arbitrarily activated by other members of the ligand family in birds. Moreover, we reveal that Gallus gallus GLP2 (gGLP2) effectively activates mammalian GLP1R and improves glucose tolerance in diabetic mice. Our study has important implications for understanding blood glucose stabilization in vertebrates and demonstrates that gGLP2 may be a potential drug for treating type 2 diabetes.

Designing a Seasonal Acclimation Study Presents Challenges and Opportunities

Organisms living in seasonal environments often adjust physiological capacities and sensitivities in response to (or in anticipation of) environment shifts. Such physiological and morphological adjustments (“acclimation” and related terms) inspire opportunities to explore the mechanistic bases underlying these adjustments, to detect cues inducing adjustments, and to elucidate their ecological and evolutionary consequences. Seasonal adjustments (“seasonal acclimation”) can be detected either by measuring physiological capacities and sensitivities of organisms retrieved directly from nature (or outdoor enclosures) in different seasons or less directly by rearing and measuring organisms maintained in the laboratory under conditions that attempt to mimic or track natural ones. But mimicking natural conditions in the laboratory is challenging—doing so requires prior natural-history knowledge of ecologically relevant body temperature cycles, photoperiods, food rations, social environments, among other variables. We argue that traditional laboratory-based conditions usually fail to approximate natural seasonal conditions (temperature, photoperiod, food, “lockdown”). Consequently, whether the resulting acclimation shifts correctly approximate those in nature is uncertain, and sometimes is dubious. We argue that background natural history information provides opportunities to design acclimation protocols that are not only more ecologically relevant, but also serve as templates for testing the validity of traditional protocols. Finally, we suggest several best practices to help enhance ecological realism.

The lizard abides: cold hardiness and winter refuges of Liolaemus pictus argentinus in Patagonia, Argentina

In environments where the temperature periodically drops below zero, it is remarkable that some lizards can survive. Behaviorally, lizards can find microsites for overwintering where temperatures do not drop as much as the air temperature. Physiologically, they can alter their biochemical balance to tolerate freezing or avoid it by supercooling. We evaluated the cold hardiness of a population of Liolaemus pictus argentinus Müller and Hellmich, 1939 in the mountains of Esquel (Patagonia, Argentina) during autumn. Additionally, we assessed the thermal quality (in degree-days) of potential refuges in a mid-elevation forest (1100 m above sea level (asl)) and in the high Andean steppe (1400 m asl). We analyzed the role of urea, glucose, total proteins, and albumin as possible cryoprotectants, comparing a group of lizards gradually exposed to temperatures lower than 0 °C with a control group maintained at room temperature. However, we found no evidence to support the presence of freeze tolerance or supercooling mechanisms in this species as related to the analyzed metabolites. Instead, the low frequency of degree-days below 0 °C and temperatures never lower than −3 °C in potential refuges suggest that L. p. argentinus might avoid physiological investments (such as supercooling and freeze tolerance) by behaviorally selecting appropriate refuges to overcome cold environmental temperatures.

Daily thermal fluctuations to a range of subzero temperatures enhance cold hardiness of winter-acclimated turtles

Although seasonal increases in cold hardiness are well documented for temperate and polar ectotherms, relatively little is known about supplemental increases in cold hardiness during winter. Because many animals are exposed to considerable thermal variation in winter, they may benefit from a quick enhancement of cold tolerance prior to extreme low temperature. Hatchling painted turtles (Chrysemys picta) overwintering in their natal nests experience substantial thermal variation in winter, and recently, it was found that brief subzero chilling of winter-acclimated hatchlings decreases subsequent chilling-induced mortality, increases blood concentrations of glucose and lactate, and protects the brain from cryoinjury. Here, we further characterize that phenomenon, termed 'cold conditioning', by exposing winter-acclimated hatchling turtles to -3.5, -7.0, or -10.5 °C gradually or repeatedly via daily thermal fluctuations over the course of 5 days and assessing their survival of a subsequent cold shock to a discriminating temperature of -12.7 °C. To better understand the physiological response to cold conditioning, we measured changes in glucose and lactate concentrations in the liver, blood, and brain. Cold conditioning significantly increased cold-shock survival, from 9% in reference turtles up to 74% in cold-conditioned turtles, and ecologically relevant daily thermal fluctuations were at least as effective at conferring cryoprotection as was gradual cold conditioning. Cold conditioning increased glucose concentrations, up to 25 μmol g^-1, and lactate concentrations, up to 30 μmol g^-1, in the liver, blood, and brain. Turtles that were cold conditioned with daily thermal fluctuations accumulated more glucose in the liver, blood, and brain, and had lower brain lactate, than those gradually cold conditioned. Given the thermal variation to which hatchling painted turtles are exposed in winter, we suggest that the supplemental protection conferred by cold conditioning, especially that induced by daily thermal fluctuations, may be important for their overwinter survival. Investigation into the duration of the cold-conditioning induced protection and its occurrence in natural field conditions is needed to better understand its ecological significance. We suggest that future work exploring the underlying mechanisms of cold conditioning should focus on non-colligative effects of glucose, expression of small Hsps, changes in membrane structure, and ion homeostasis.

First record of hatchling overwintering inside the natal nest of a chelid turtle

Hatchling overwintering inside the natal nest is a strategy used by several Northern Hemisphere species of freshwater turtles. We recorded hatchling overwintering in the nest by Chelodina longicollis (Chelidae) in south-eastern Australia, during three reproductive seasons. Hatchlings spent, on average, 320 days inside the nest from the date eggs were laid until emergence. Some nests were carefully opened adjacent to the nest plug, one during winter and one in spring, to confirm that eggs had hatched and were not in diapause, although we could not precisely confirm hatching dates. Despite our small sample size, we observed a dichotomous overwintering strategy, with hatchlings from one nest emerging in autumn and spending their first winter in the aquatic environment, and hatchlings from three nests overwintering in the nest and emerging in spring. These findings expand the phylogenetic range of turtles exhibiting hatchling overwintering behaviour. Future research should evaluate whether this strategy is widespread among other long-necked turtles in temperate regions and examine physiological mechanisms involved in coping with winter temperatures.

More sires may enhance offspring fitness in Northern Map Turtles (Graptemys geographica)

Sexual selection theory predicts that males should be promiscuous to maximize their reproductive success, while females should be choosy. Yet females of many taxa often produce progeny sired by multiple males, indicating that promiscuity can be important for the reproductive success of females. Promiscuity may enhance the fitness of females if it increases the genetic quality, or the genetic variety, and thus the viability of their offspring. We quantified the number of sires per clutch in a population of Northern Map Turtles (Graptemys geographica (LeSueur, 1817)) in Lake Opinicon, Ontario, Canada, and tested whether the number of sires affects several metrics of viability in hatchlings. Based on the most conservative estimate, at least 71% of clutches in this population are sired by multiple males, but there was no evidence that larger clutches are sired by more males. Clutches sired by more males had higher hatching success and survival, but the differences were not statistically significant. We did not find any effect of the number of sires on hatchling morphology or locomotor performance. Collectively, our results partially support the hypothesis that promiscuity can increase the reproductive success of female Northern Map Turtles.

Isolation technique and proteomic analysis of the erythrocyte ghosts of red-eared turtle (Trachemys scripta)

To proceed proteomic analysis of erythrocyte of the red-eared turtle Trachemys scripta, a method for obtaining turtle erythrocyte ghosts (TEG) was first developed. After hypotonic lysis using a special buffer, forcing the erythrocyte through the syringe with an "N"-shaped needle, applying low speed homogenizing and differential centrifugation, highly purified TEG fractions were obtained. The isolated TEG proteins were treated with in-gel digestion separated by SDS-PAGE or in-solution digestion followed by HPLC predissociation, and then identified by nano-ESI-LC MS/MS techniques. A total of 169 TEG proteins was identified, validated, and categorized. Among these proteins, tubulins, and cell-surface-located F-type ATP synthase revealed important information into the super tolerance of Trachemys scripta in anoxia and low temperature exposure. Altogether, this study not only provided a method to isolate high quality TEG and a dataset of comprehensive characterization of TEG proteins, but also provides a tool for proteomic research of all nucleated red blood cells, and thus opened a new research field for exploring the mechanisms of super tolerance of turtles in harsh environment.

Avoidance and tolerance of freezing in ectothermic vertebrates

Ectothermic vertebrates have colonized regions that are seasonally or perpetually cold, and some species, particularly terrestrial hibernators, must cope with temperatures that fall substantially below 0°C. Survival of such excursions depends on either freeze avoidance through supercooling or freeze tolerance. Supercooling, a metastable state in which body fluids remain liquid below the equilibrium freezing/melting point, is promoted by physiological responses that protect against chilling injury and by anatomical and behavioral traits that limit risk of inoculative freezing by environmental ice and ice-nucleating agents. Freeze tolerance evolved from responses to fundamental stresses to permit survival of the freezing of a substantial amount of body water under thermal and temporal conditions of ecological relevance. Survival of freezing is promoted by a complex suite of molecular, biochemical and physiological responses that limit cell death from excessive shrinkage, damage to macromolecules and membranes, metabolic perturbation and oxidative stress. Although freeze avoidance and freeze tolerance generally are mutually exclusive strategies, a few species can switch between them, the mode used in a particular instance of chilling depending on prevailing physiological and environmental conditions.

Localization and regulation of a facilitative urea transporter in the kidney of the red-eared slider turtle (Trachemys scripta elegans)

Urea is the major excretory end product of nitrogen metabolism in most chelonian reptiles. In the present study, we report the isolation of a 1632 base pair cDNA from turtle kidney with one open reading frame putatively encoding a 403-residue protein, the turtle urea transporter (turtle UT). The first cloned reptilian UT has high homology with UTs (facilitated urea transporters) cloned from vertebrates, and most closely resembles the UT-A subfamily. Injection of turtle UT cRNA into Xenopus oocytes induced a 6-fold increase in [(14)C]urea uptake that was inhibited by phloretin. The turtle UT mRNA expression and tissue distribution were examined by RT-PCR with total RNA from various tissues. Expression of turtle UT mRNA was restricted to the kidney, and no signal was detected in the other tissues, such as brain, heart, alimentary tract and urinary bladder. An approximately 58 kDa protein band was detected in membrane fractions of the kidney by western blot using an affinity-purified antibody that recognized turtle UT expressed in Xenopus oocytes. In an immunohistochemical study using the anti-turtle UT antibody, UT-immunopositive cells were observed along the distal tubule but not in the collecting duct. In turtles under dry conditions, plasma osmolality and urea concentration increased, and using semi-quantitative RT-PCR the UT mRNA expression level in the kidney was found to increase 2-fold compared with control. The present results, taken together, suggest that the turtle UT probably contributes to urea transport in the distal tubule segments of the kidney in response to hyperosmotic stress under dry conditions.

Physiological ecology of overwintering in hatchling turtles

Temperate species of turtles hatch from eggs in late summer. The hatchlings of some species leave their natal nest to hibernate elsewhere on land or under water, whereas others usually remain inside the nest until spring; thus, post-hatching behavior strongly influences the hibernation ecology and physiology of this age class. Little is known about the habitats of and environmental conditions affecting aquatic hibernators, although laboratory studies suggest that chronically hypoxic sites are inhospitable to hatchlings. Field biologists have long been intrigued by the environmental conditions survived by hatchlings using terrestrial hibernacula, especially nests that ultimately serve as winter refugia. Hatchlings are unable to feed, although as metabolism is greatly reduced in hibernation, they are not at risk of starvation. Dehydration and injury from cold are more formidable challenges. Differential tolerances to these stressors may explain variation in hatchling overwintering habits among turtle taxa. Much study has been devoted to the cold-hardiness adaptations exhibited by terrestrial hibernators. All tolerate a degree of chilling, but survival of frost exposure depends on either freeze avoidance through supercooling or freeze tolerance. Freeze avoidance is promoted by behavioral, anatomical, and physiological features that minimize risk of inoculation by ice and ice-nucleating agents. Freeze tolerance is promoted by a complex suite of molecular, biochemical, and physiological responses enabling certain organisms to survive the freezing and thawing of extracellular fluids. Some species apparently can switch between freeze avoidance or freeze tolerance, the mode utilized in a particular instance of chilling depending on prevailing physiological and environmental conditions.

Oxidative stress and antioxidant capacity of a terrestrially hibernating hatchling turtle

Hatchlings of the painted turtle, Chrysemys picta, hibernate terrestrially and can survive subfreezing temperatures by supercooling or by tolerating the freezing of their tissues. Whether supercooled or frozen, an ischemic hypoxia develops because tissue perfusion is limited by low temperature and/or freezing. Oxidative stress can occur if hatchlings lack sufficient antioxidant defenses to minimize or prevent damage by reactive oxygen species. We examined the antioxidant capacity and indices of oxidative damage in hatchling C. picta following survivable, 48 h bouts of supercooling (-6 degrees C), freezing (-2.5 degrees C), or hypoxia (4 degrees C). Samples of plasma, brain, and liver were collected after a 24 h period of recovery (4 degrees C) and assayed for Trolox-equivalent antioxidant capacity (TEAC), thiobarbituric acid reactive substances (TBARS), and carbonyl proteins. Antioxidant capacity did not vary among treatments in any of the tissues studied. We found a significant increase in TBARS in plasma, but not in the brain or liver, of frozen/thawed hatchlings as compared to untreated controls. No changes were found in the concentration of TBARS or carbonyl proteins in supercooled or hypoxia-exposed hatchlings. Our results suggest that hatchling C. picta have a well-developed antioxidant defense system that minimizes oxidative damage during hibernation.

Experimental test of the effects of fluctuating incubation temperatures on hatchling phenotype

In the painted turtle (Chrysemys picta) and red-eared slider turtle (Trachemys scripta), the temperature that eggs are exposed to during incubation determines the sex of the developing embryo. Constant temperature incubation experiments have shown that for each of these species there is a pivotal temperature that produces a 1:1 sex ratio; higher temperatures bias sex ratios toward females, and lower temperatures toward males. Few studies have examined how fluctuating temperatures, as would be experienced in natural nests, affect hatchling phenotype. Models predict that under fluctuating temperatures sex determination depends on the proportion of development that occurs above or below the pivotal temperature. We tested the effect of fluctuating versus constant temperature incubation regimes on sex ratios and other hatchling traits for both painted and red-eared slider turtles. Eggs were divided into two treatments with half of the eggs from each species incubated at a constant intermediate temperature, 28.5 degrees C, and half incubated under temperatures that fluctuated 3 degrees C above and below 28.5 degrees C. We converted the fluctuating temperature data into a constant temperature equivalent (CTE) so that we could directly compare constant and fluctuating incubation regimes. The CTE for the fluctuating regime for both species was higher than the constant temperature, which would predict an increase in the production of females. The fluctuating regime did produce a higher proportion of females, but also resulted in increased developmental time and increased hatchling mass, indicating that fluctuating temperatures produce complex effects on hatchling phenotype.

Physiological responses to freezing in hatchlings of freeze-tolerant and -intolerant turtles

Freeze tolerance is a complex cold-hardiness adaptation that has independently evolved in a diverse group of organisms, including several ectothermic vertebrates. Because little is known about the mechanistic basis for freeze tolerance in reptiles, we compared responses to experimental freezing in winter-acclimatized hatchlings representing nine taxa of temperate North American turtles, including ones that tolerated freezing and others that did not. Viability rates of hatchlings frozen to -3 degrees C for 72 h ranged from 0 to 100%. Tolerance to freezing was poor in Sternotherus odoratus, Graptemys geographica and Trachemys scripta, intermediate in Chelydra serpentina, and high in Emydoidea blandingii, Chrysemys picta bellii, C. p. marginata, Malaclemys terrapin, and Terrapene ornata, and generally reflected the winter thermal ecology of each taxon. Plasma activity of lactate dehydrogenase (LDH), a novel in vivo index of freeze/thaw damage, corroborated viability assessments and demonstrated that cryoinjury occurred even in surviving turtles. Irrespective of taxon, cryoinjury tended to be higher in smaller individuals and in those having relatively low water contents; however, bases for these associations were not apparent. Screening for certain organic osmolytes that might promote freezing survival by colligatively reducing ice content and limiting cell dehydration showed that the plasma of unfrozen (control) turtles contained small quantities of glucose (1.3-5.8 mmol l(-1)) and lactate (0.6-3.2 mmol l(-1)) and modest amounts of urea (range of mean values for all taxa 8.2-52.3 mmol l(-1)). Frozen/thawed turtles of all taxa accumulated modest amounts of glucose and lactate that jointly raised the plasma solute concentration by 30-100 mmol l(-1). We conclude that organic osmolytes accumulated both before and during freezing may promote survival in species that have evolved a tolerance to freezing, but are not necessarily accumulated for that purpose.

The relationship between gut contents and supercooling capacity in hatchling painted turtles (Chrysemys picta)

Painted turtles (Chrysemys picta) typically spend their first winter of life in a shallow, subterranean hibernaculum (the natal nest) where they seemingly withstand exposure to ice and cold by resisting freezing and becoming supercooled. However, turtles ingest soil and fragments of eggshell as they are hatching from their eggs, and the ingestate usually contains efficient nucleating agents that cause water to freeze at high subzero temperatures. Consequently, neonatal painted turtles have only a modest ability to undergo supercooling in the period immediately after hatching. We studied the limit for supercooling (SCP) in hatchlings that were acclimating to different thermal regimes and then related SCPs of the turtles to the amount of particulate matter in their gastrointestinal (GI) tract. Turtles that were transferred directly from 26 degrees C (the incubation temperature) to 2 degrees C did not purge soil from their gut, and SCPs for these animals remained near -4 degrees C for the 60 days of the study. Animals that were held at 26 degrees C for the duration of the experiment usually cleared soil from their GI tract within 24 days, but SCPs for these turtles were only slightly lower after 60 days than they were at the outset of the experiment. Hatchlings that were acclimating slowly to 2 degrees C cleared soil from their gut within 24 days and realized a modest reduction in their SCP. However, the limit of supercooling in the slowly acclimating animals continued to decline even after all particulate material had been removed from their GI tract, thereby indicating that factors intrinsic to the nucleating agents themselves also may have been involved in the acclimation of hatchlings to low temperature. The lowest SCPs for turtles that were acclimating slowly to 2 degrees C were similar to SCPs recorded in an earlier study of animals taken from natural nests in late autumn, so the current findings affirm the importance of seasonally declining temperatures in preparing animals in the field to withstand conditions that they will encounter during winter.

Reptile freeze tolerance: Metabolism and gene expression

Terrestrially hibernating reptiles that live in seasonally cold climates need effective strategies of cold hardiness to survive the winter. Use of thermally buffered hibernacula is very important but when exposure to temperatures below 0 degrees C cannot be avoided, either freeze avoidance (supercooling) or freeze tolerance strategies can be employed, sometimes by the same species depending on environmental conditions. Several reptile species display ecologically relevant freeze tolerance, surviving for extended times with 50% or more of their total body water frozen. The use of colligative cryoprotectants by reptiles is poorly developed but metabolic and enzymatic adaptations providing anoxia tolerance and antioxidant defense are important aids to freezing survival. New studies using DNA array screening are examining the role of freeze-responsive gene expression. Three categories of freeze responsive genes have been identified from recent screenings of liver and heart from freeze-exposed (5h post-nucleation at -2.5 degrees C) hatchling painted turtles, Chrysemys picta marginata. These genes encode (a) proteins involved in iron binding, (b) enzymes of antioxidant defense, and (c) serine protease inhibitors. The same genes were up-regulated by anoxia exposure (4 h of N2 gas exposure at 5 degrees C) of the hatchlings which suggests that these defenses for freeze tolerance are aimed at counteracting the injurious effects of the ischemia imposed by plasma freezing.

Survival and Physiological Responses of Hatchling Blanding’s Turtles (Emydoidea blandingii) to Submergence in Normoxic and Hypoxic Water under Simulated Winter Conditions

Overwintering habits of hatchling Blanding's turtles (Emydoidea blandingii) are unknown. To determine whether these turtles are able to survive winter in aquatic habitats, we submerged hatchlings in normoxic (155 mmHg Po2) and hypoxic (6 mmHg Po2) water at 4 degrees C, recording survival times and measuring changes in key physiological variables. For comparison, we simultaneously studied hatchling softshell (Apalone spinifera) and snapping (Chelydra serpentina) turtles, which are known to overwinter in aquatic habitats. In normoxic water, C. serpentina and A. spinifera survived to the termination of the experiment (76 and 77 d, respectively). Approximately one-third of the E. blandingii died during 75 d of normoxic submergence, but the cause of mortality was unclear. In hypoxic water, average survival times were 6 d for A. spinifera, 13 d for E. blandingii, and 19 d for C. serpentina. Mortality during hypoxic submergence was probably caused by metabolic acidosis, which resulted from accumulated lactate. Unlike the case with adult turtles, our hatchlings did not increase plasma calcium and magnesium, nor did they sequester lactate within the shell. Our results suggest that hatchling E. blandingii are not particularly well suited to hibernation in hypoxic aquatic habitats.

Anoxia tolerance and freeze tolerance in hatchling turtles

Freezing survival in hatchling turtles may be limited by ischemic anoxia in frozen tissues and the associated accumulation of lactate and reactive oxygen species (ROS). To determine whether mechanisms for coping with anoxia are also important in freeze tolerance, we examined the association between capacities for freezing survival and anoxia tolerance in hatchlings of seven species of turtles. Tolerance to freezing (-2.5 degrees C) was high in Emydoidea blandingii, Chrysemys picta, Terrapene ornata, and Malaclemys terrapin and low in Graptemys geographica, Chelydra serpentina, and Trachemys scripta. Hatchlings survived in a N(2) atmosphere at 4 degrees C for periods ranging from 17 d (M. terrapin) to 50 d (G. geographica), but survival time was not associated with freeze tolerance. Lactate accumulated during both stresses, but plasma levels in frozen/thawed turtles were well below those found in anoxia-exposed animals. Activity of the antioxidant enzyme catalase in liver increased markedly with anoxia exposure in most species, but increased with freezing/thawing only in species with low freeze tolerance. Our results suggest that whereas oxygen deprivation occurs during somatic freezing, freeze tolerance is not limited by anoxia tolerance in hatchling turtles.

Hypoxia and the regulation of mitogen-activated protein kinases: gene transcription and the assessment of potential pharmacologic therapeutic interventions

Oxygen is an environmental/developmental signal that regulates cellular energetics, growth, and differentiation processes. Despite its central role in nearly all higher life processes, the molecular mechanisms for sensing oxygen levels and the pathways involved in transducing this information are still being elucidated. Altering gene expression is the most fundamental and effective way for a cell to respond to extracellular signals and/or changes in its microenvironment. During development, the expression of specific sets of genes is regulated spatially (by position/morphogenetic gradients) and temporally, presumably via the sensing of molecular oxygen available within the microenvironment. Regulation of signaling responses is governed by transcription factors that bind to control regions (consensus sequences) of target genes and alter their expression in response to specific signals. Complex signal transduction during hypoxia (deficiency of oxygen in inspired gases or in arterial blood and/or in tissues) involves the coupling of ligand-receptor interactions to many intracellular events. These events basically include phosphorylations by tyrosine kinases and/or serine/threonine kinases, such as those of mitogen-activated protein kinases (MAPKs), a superfamily of kinases responsive to stress nonhomeostatic conditions. Protein phosphorylations imposed during hypoxia change enzyme activities and protein conformations, and the eventual outcome is rather complex, comprising of an alteration in cellular activity and changes in the programming of genes expressed within the responding cells. These molecular changes serve as signals that are crucial for cell survival under contingent conditions imposed during hypoxia. This review correlates current concepts of hypoxic sensing pathways with hypoxia-related phosphorylation mechanisms mediated by MAPKs via the genetic and pharmacologic regulation/manipulation of specific transcription factors and related cofactors.

To freeze or not to freeze: adaptations for overwintering by hatchlings of the North American painted turtle

Many physiologists believe that hatchling painted turtles (Chrysemys picta) provide a remarkable, and possibly unique, example of `natural freeze-tolerance' in an amniotic vertebrate. However, the concept of natural freeze-tolerance in neonatal painted turtles is based on results from laboratory studies that were not placed in an appropriate ecological context,so the concept is suspect. Indeed, the weight of current evidence indicates that hatchlings overwintering in the field typically withstand exposure to ice and cold by avoiding freezing altogether and that they do so without benefit of an antifreeze to depress the equilibrium freezing point for bodily fluids. As autumn turns to winter, turtles remove active nucleating agents from bodily fluids (including bladder and gut), and their integument becomes a highly efficient barrier to the penetration of ice into body compartments from frozen soil. In the absence of a nucleating agent or a crystal of ice to `catalyze'the transformation of water from liquid to solid, the bodily fluids remain in a supercooled, liquid state. The supercooled animals nonetheless face physiological challenges, most notably an increased reliance on anaerobic metabolism as the circulatory system first is inhibited and then caused to shut down by declining temperature. Alterations in acid/base status resulting from the accumulation of lactic acid may limit survival by supercooled turtles, and sublethal accumulations of lactate may affect behavior of turtles after the ground thaws in the spring. The interactions among temperature,circulatory function, metabolism (both aerobic and anaerobic), acid/base balance and behavior are fertile areas for future research on hatchlings of this model species.

Accumulation of Lactate by Frozen Painted Turtles (Chrysemys picta) and Its Relationship to Freeze Tolerance

Hatchling painted turtles (Chrysemys picta) survived freezing at -2 degrees C for 4 d, few recovered from freezing lasting 6 d, and none survived being frozen for 8 d. Whole-body glucose and lactate were low in animals that had not been subjected to cold and ice but increased precipitously in animals that were frozen for 2 d. Both metabolites continued to increase, but at a somewhat lower rate, in animals frozen for 4, 6, or 8 d. The increase in whole-body lactate reflects a reliance by frozen hatchlings on anaerobiosis, whereas the increase in glucose presumably results from mobilization of glycogen reserves to support anaerobic metabolism. Mortality of frozen hatchlings is correlated with the increase in whole-body lactate. Factors that may contribute to the observed correlation include a compromised capacity for individual organs to cope with the lactic acidosis that accompanies anaerobic metabolism and organ-specific depletion of energy reserves. Individual organs must rely on buffering and glucose reserves available in situ because blood of frozen hatchlings does not circulate. Thus, buffer from the shell cannot be transported to other organs, lactate cannot be sequestered in the shell, and glucose mobilized from liver glycogen is not available to supplement glucose reserves of other tissues. This integrated suite of physiological disruptions may limit tolerance of freezing to conditions with little or no ecological relevance.

Cold-hardiness and dehydration resistance of hatchling Blanding's turtles (Emydoidea blandingii): implications for overwintering in a terrestrial habitat

The overwintering habits of hatchling Blanding's turtles, Emydoidea blandingii (Holbrook, 1838), are not well understood. To ascertain whether these turtles are well suited to hibernation on land, we examined susceptibility to dehydration, supercooling capacity, resistance to inoculative freezing, capacity for freeze tolerance, and physiological responses to somatic freezing in laboratory-reared, hatchling E. blandingii. Rates of evaporative water loss (mean ± SE = 4.1 ± 0.2 mg·g–1·d–1) were intermediate to rates previously reported for the hatchlings of species known to hibernate on land and in water. Supercooled hatchlings recovered from a 1-h exposure to –8 °C or a 7-d exposure to –4 °C. Additional turtles supercooled to –14.3 ± 1.2 °C (mean ± SE) before spontaneously freezing. However, when immersed in frozen soil, their capacity to supercool was severely limited by an inability to resist inoculative freezing following contact with external ice and ice nuclei. Therefore, hatchlings likely do not use supercooling as a winter survival strategy. Hatchlings tolerated a 72-h period of somatic freezing to –3.5 °C and responded to somatic freezing by increasing plasma concentrations of the putative cryoprotectants lactate and glucose. Our results suggest that hatchling E. blandingii could overwinter in moist, terrestrial hibernacula where risk of dehydration is reduced and freeze tolerance is promoted.

Physiological Ecology of Overwintering in the Hatchling Painted Turtle: Multiple‐Scale Variation in Response to Environmental Stress

We integrated field and laboratory studies in an investigation of water balance, energy use, and mechanisms of cold-hardiness in hatchling painted turtles (Chrysemys picta) indigenous to west-central Nebraska (Chrysemys picta bellii) and northern Indiana (Chrysemys picta marginata) during the winters of 1999-2000 and 2000-2001. We examined 184 nests, 80 of which provided the hatchlings (n=580) and/or samples of soil used in laboratory analyses. Whereas winter 1999-2000 was relatively dry and mild, the following winter was wet and cold; serendipitously, the contrast illuminated a marked plasticity in physiological response to environmental stress. Physiological and cold-hardiness responses of turtles also varied between study locales, largely owing to differences in precipitation and edaphics and the lower prevailing and minimum nest temperatures (to -13.2 degrees C) encountered by Nebraska turtles. In Nebraska, winter mortality occurred within 12.5% (1999-2000) and 42.3% (2000-2001) of the sampled nests; no turtles died in the Indiana nests. Laboratory studies of the mechanisms of cold-hardiness used by hatchling C. picta showed that resistance to inoculative freezing and capacity for freeze tolerance increased as winter approached. However, the level of inoculation resistance strongly depended on the physical characteristics of nest soil, as well as its moisture content, which varied seasonally. Risk of inoculative freezing (and mortality) was greatest in midwinter when nest temperatures were lowest and soil moisture and activity of constituent organic ice nuclei were highest. Water balance in overwintering hatchlings was closely linked to dynamics of precipitation and soil moisture, whereas energy use and the size of the energy reserve available to hatchlings in spring depended on the winter thermal regime. Acute chilling resulted in hyperglycemia and hyperlactemia, which persisted throughout winter; this response may be cryoprotective. Some physiological characteristics and cold-hardiness attributes varied between years, between study sites, among nests at the same site, and among siblings sharing nests. Such variation may reflect adaptive phenotypic plasticity, maternal or paternal influence on an individual's response to environmental challenge, or a combination of these factors. Some evidence suggests that life-history traits, such as clutch size and body size, have been shaped by constraints imposed by the harsh winter environment.

Adaptations to terrestrial overwintering of hatchling northern map turtles, Graptemys geographica

We conducted a 3-year field and laboratory study of winter biology in hatchlings of the northern map turtle ( Graptemys geographica). At our study area in northern Indiana, hatchlings routinely overwintered in their natal nests, emerging after the weather warmed in spring. Winter survival was excellent despite the fact that hatchlings were exposed frequently to subfreezing temperatures (to -5.4 degrees C). In the laboratory, cold-acclimated hatchlings exhibited low rates of evaporative water loss (mean=2.0 mg g(-1) day(-1)), which would enable them to conserve body water during winter. Laboratory-reared hatchlings were intolerant of freezing at -2.5 degrees C for 24 h, conditions that are readily survived by freeze-tolerant species of turtles. Winter survival of hatchling G. geographica probably depended on their extensive capacity for supercooling (to -14.8 degrees C) and their well-developed resistance to inoculative freezing, which may occur when hatchlings contact ice and ice-nucleating agents present in nesting soil. Supercooled hatchlings survived a brief exposure to -8 degrees C. Others, held at -6 degrees C for 5 days, maintained ATP concentrations at control levels, although they did accumulate lactate and glucose, probably in response to tissue hypoxia. Therefore, anoxia tolerance, as evidenced by the viability of hatchlings exposed to N(2) gas for 8 days, may promote survival during exposure to subfreezing temperatures.

Seasonal acclimatisation of muscle metabolic enzymes in a reptile (Alligator mississippiensis)

Reptiles living in heterogeneous thermal environments are often thought to show behavioural thermoregulation or to become inactive when environmental conditions prevent the achievement of preferred body temperatures. By contrast, thermally homogeneous environments preclude behavioural thermoregulation, and ectotherms inhabiting these environments (particularly fish in which branchial respiration requires body temperature to follow water temperature) modify their biochemical capacities in response to long-term seasonal temperature fluctuations. Reptiles may also be active at seasonally varying body temperatures and could, therefore, gain selective advantages from regulating biochemical capacities. Hence, we tested the hypothesis that a reptile (the American alligator Alligator mississippiensis) that experiences pronounced seasonal fluctuations in body temperature will show seasonal acclimatisation in the activity of its metabolic enzymes. We measured body temperatures of alligators in the wild in winter and summer (N=7 alligators in each season), and we collected muscle samples from wild alligators (N=31 in each season) for analysis of metabolic enzyme activity (lactate dehydrogenase, citrate synthase and cytochrome c oxidase). There were significant differences in mean daily body temperatures between winter (15.66+/-0.43 degrees C; mean +/- S.E.M.) and summer (29.34+/-0.21 degrees C), and daily body temperatures fluctuated significantly more in winter compared with summer. Alligators compensated for lower winter temperatures by increasing enzyme activities, and the activities of cytochrome c oxidase and lactate dehydrogenase were significantly greater in winter compared with summer at all assay temperatures. The activity of citrate synthase was significantly greater in the winter samples at the winter body temperature (15 degrees C) but not at the summer body temperature (30 degrees C). The thermal sensitivity (Q(10)) of mitochondrial enzymes decreased significantly in winter compared with in summer. The activity of mitochondrial enzymes was significantly greater in males than in females, but there were no differences between sexes for lactate dehydrogenase activity. The differences between sexes could be the result of the sex-specific seasonal demands for locomotor performance. Our data indicate that biochemical acclimatisation is important in thermoregulation of reptiles and that it is not sufficient to base conclusions about their thermoregulatory ability entirely on behavioural patterns.

Endogenous and exogenous ice-nucleating agents constrain supercooling in the hatchling painted turtle

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 degrees 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 (T(c)) of recently hatched (summer) turtles acclimated to 22 degrees C and reared on a substratum of vermiculite or nesting soil was approximately 5 degrees C higher than the T(c) determined for turtles acclimated to 4 degrees 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 T(c), 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 T(c) of winter turtles reared on nesting soil (mean +/- S.E.M.=-11.6+/-1.4 degrees C) was approximately 6 degrees C higher than the T(c) 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 (T(c) approximately -7.5 degrees C) we found in turtles overwintering in natural nests and may therefore pose a formidable challenge to the winter survival of hatchling C. picta.

Natural freeze-tolerance in hatchling painted turtles?

Hatchlings of the North American painted turtle (Family Emydidae: Chrysemys picta) typically spend their first winter of life inside a shallow, subterranean hibernaculum (the natal nest) where life-threatening conditions of ice and cold commonly occur. Although a popular opinion holds that neonates exploit a tolerance for freezing to survive the rigors of winter, hatchlings are more likely to withstand exposure to ice and cold by avoiding freezing altogether-and to do so without the benefit of an antifreeze. In the interval between hatching by turtles in late summer and the onset of wintery weather in November or December, the integument of the animals becomes highly resistant to the penetration of ice into body compartments from surrounding soil, and the turtles also purge their bodies of catalysts for the formation of ice. These two adjustments, taken together, enable the animals to supercool to temperatures below those that they routinely experience in nature. However, cardiac function in hatchlings is diminished at subzero temperatures, thereby compromising the delivery of oxygen to peripheral tissues and eliciting an increase in reliance by those tissues on anaerobic metabolism for the provision of ATP. The resulting increase in production of lactic acid may disrupt acid/base balance and lead to death even in animals that remain unfrozen. Although an ability to undergo supercooling may be key to survival by overwintering turtles in northerly populations, a similar capacity to resist inoculation and undergo supercooling characterizes animals from a population near the southern limit of distribution, where winters are relatively benign. Thus, the suite of characters enabling hatchlings to withstand exposure to ice and cold may have been acquired prior to the northward dispersal of the species at the end of the Pleistocene, and the characters may not have originated as adaptations specifically to the challenges of winter.

Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.

Cold-hardiness and evaporative water loss in hatchling turtles

North American turtles hatch in late summer and spend their first winter either on land or underwater. Adaptations for terrestrial overwintering of hatchlings in northern regions, where winter thermal and hydric regimes are harsh, have not been systematically investigated in many species. We measured intrinsic supercooling capacity, resistance to inoculative freezing, and desiccation resistance in hatchlings of terrestrial and aquatic turtles collected from northern (Terrapene ornata, Chrysemys picta bellii, Kinosternon flavescens, Chelydra serpentina) and southern (Chrysemys picta dorsalis, Trachemys scripta, Sternotherus odoratus, Sternotherus carinatus) locales. Supercooling capacity was estimated from the crystallization temperature of turtles cooled in the absence of external ice nuclei. Mean values ranged from -8.1 degrees to -15.5 degrees C and tended to be lower in terrestrial hibernators. Inoculation resistance was estimated from the crystallization temperature of turtles cooled in a matrix of frozen soil. These values (range of means: -0.8 degrees to -13.6 degrees C) also tended to be lower in the terrestrial hibernators, especially C. picta bellii. Mean rates of evaporative water loss varied markedly among the species (0.9-11.4 mg g(-1) d(-1)) and were lowest in the terrestrial hibernators. Most species tolerated the loss of a modest amount of body water, although half of the sample of S. carinatus died from desiccation. In general, turtles did not regain lost body water from wet soil, and immersion in free water was required for rehydration. Therefore, desiccation resistance may be an important adaptation to terrestrial hibernation. Resistances to inoculative freezing and desiccation were directly correlated, perhaps because they are governed by the same morphological characteristics.

Seasonal change in the capacity for supercooling by neonatal painted turtles

Hatchlings of the North American painted turtle (Chrysemys picta) typically spend their first winter of life inside the shallow, subterranean nest where they completed incubation the preceding summer. This facet of their natural history commonly causes neonates in northerly populations to be exposed in mid-winter to ice and cold, which many animals survive by remaining unfrozen and supercooled. We measured the limit of supercooling in samples of turtles taken shortly after hatching and in other samples after 2 months of acclimation (or acclimatization) to a reduced temperature in the laboratory or field. Animals initially had only a limited capacity for supercooling, but they acquired an ability to undergo deeper supercooling during the course of acclimation. The gut of most turtles was packed with particles of soil and eggshell shortly after hatching, but not after acclimation. Thus, the relatively high limit of supercooling for turtles in the days immediately after hatching may have resulted from the ingestion of soil (and associated nucleating agents) by the animals as they were freeing themselves from their eggshell, whereas the relatively low limit of supercooling attained by acclimated turtles may have resulted from their purging their gut of its contents. Parallels may, therefore, exist between the natural-history strategy expressed by hatchling painted turtles and that expressed by numerous terrestrial arthropods that withstand the cold of winter by sustaining a state of supercooling.