PubMed Health⌕ Search

Biomedical subjects

G C Packard

Publications and source records attributed to G C Packard.

At least 19 recordsLinked to original sources
PubMed2001

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.

Acclimatization↗
PubMed2000

The role of the integument as a barrier to penetration of ice into overwintering hatchlings of the painted turtle (Chrysemys picta).

Hatchlings of the North American painted turtle (Chrysemys picta) spend their first winter of life inside a shallow, subterranean hibernaculum (the natal nest) where they may be exposed for extended periods to ice and cold. Hatchlings seemingly survive exposure to such conditions by becoming supercooled (i.e., by remaining unfrozen at temperatures below the equilibrium freezing point for body fluids), so we investigated the role of their integument in preventing ice from penetrating into body compartments from surrounding soil. We first showed that hatchlings whose epidermis has been damaged are more likely to be penetrated by growing crystals of ice than are turtles whose cutaneous barrier is intact. We next studied integument from a forelimb by light microscopy and discovered that the basal part of the alpha-keratin layer of the epidermis contains a dense layer of lipid. Skin from the forelimb of other neonatal turtles lacks such a layer of lipid in the epidermis, and these other turtles also are highly susceptible to inoculative freezing. Moreover, epidermis from the neck of hatchling painted turtles lacks the lipid layer, and this region of the skin is readily penetrated by growing crystals of ice. We therefore conclude that the resistance to inoculation imposed by skin on the limbs of hatchling painted turtles results from the presence of lipids in the alpha-keratin layer of the epidermis. Neonates apparently are able to avoid freezing during winter by drawing much of the body inside the shell, leaving only the ice-resistant integument of the limbs exposed to ice in the environment. The combination of behavior and skin morphology enables overwintering hatchlings to exploit an adaptive strategy based on supercooling.

Adaptation, Physiological↗
PubMed2000

Accumulation of lactate by supercooled hatchlings of the painted turtle (Chrysemys picta): implications for overwinter survival.

Hatchlings of the North American painted turtle (Chrysemys picta) typically spend their first winter of life inside the shallow, subterranean nest where they completed embryogenesis the preceding summer. Neonates at northern localities consequently may be exposed during winter to subzero temperatures and frozen soil. Hatchlings apparently survive exposure to such conditions by supercooling, but the physiological consequences of this adaptive strategy have not been examined. We measured lactate in hatchling painted turtles after exposure to each of three temperatures (0 degree C, -4 degrees C, and -8 degrees C) for three time periods (5 days, 15 days, and 25 days) to determine the extent to which overwintering hatchlings might rely on anaerobic metabolism to regenerate ATP. Whole-body lactate increased with increasing duration of exposure and decreasing temperature, and the highest levels were associated with the group that experienced the highest mortality. These results indicate that animals may develop a considerable lactic acidosis during a winter in which temperatures fall below 0 degree C for weeks or months and that accumulation of lactate may contribute to mortality of overwintering animals.

Acidosis, Lactic↗
PubMed2000

Availability of water affects organ growth in prenatal and neonatal snapping turtles (Chelydra serpentina).

We manipulated the amount of water that was available to prenatal and neonatal snapping turtles (Chelydra serpentina) in order to assess the impact of water on growth by different organs in these animals. Three treatments were used: (1) turtles that completed their incubation on a wet substrate, (2) turtles that completed their incubation on a dry substrate, and (3) turtles that spent a few days in water after completing incubation on a dry substrate. Turtles hatching on a dry substrate (treatment 2) were smaller than animals in the other two treatments (which did not differ in size), so data for mass of different organs were adjusted by ANCOVA to remove effects of body size. Scaled masses of liver, stomach, lungs, kidneys, and small intestine did not differ between turtles emerging in wet environments and those hatching in dry environments, but hearts of turtles hatching in dry settings were substantially larger than those of animals hatching in wet ones. Thus, the mass of most organs in turtles developing in wet and dry environments scaled to body size, whereas the heart was hypertrophied in embryos developing in dry environments. Turtles that spent a few days in water after hatching from eggs in dry environments grew rapidly in size, and the increase in body size was accompanied by disproportionately rapid growth in the liver, stomach, lungs, kidneys, and small intestine. The heart did not increase in size during this period, despite the substantial increase in body mass over that at hatching. The enlarged heart of turtles hatching on dry substrates may have been caused by a circulatory hypovolemia late in incubation; the rapid growth of organs other than the heart when these animals were placed in water may reflect a release from constraints on growth once circulatory volume was restored.

Animals↗
PubMed1998

Hatchling painted turtles (Chrysemys picta) survive only brief freezing of their bodily fluids.

Neonatal painted turtles (Chrysemys picta) spend their first winter inside the shallow, subterranean nest cavity where they completed embryogenesis. Consequently, hatchlings at high latitudes may be exposed to ice and cold during the winter. This study was undertaken to determine how long hatchlings withstand freezing at temperatures slightly below 0 degree C because tolerance for freezing has been proposed to be the key to survival by overwintering animals. A thermocouple was glued to the carapace of each hatchling. The animal was dipped in water to provide a site of nucleation of ice and was then placed into a glass jar that was partially immersed in a circulating bath at -2 degrees C. Carapace temperature was monitored throughout the procedure. When a freezing exotherm was detected, timing of the freezing event began. Animals were maintained in a frozen state for 12-48 h prior to being warmed to room temperature. Of the 39 hatchlings, 22 did not survive, and mortality increased as the duration of freezing increased. Logistic regression indicates that no turtle would have survived in a frozen state for more than 54 h. These results indicate that hatchlings can survive only brief exposure to freezing of the body fluids. Thus, hatchlings cannot tolerate freezing during prolonged periods of cold.

Acclimatization↗
PubMed1993

Water loss from eggs of domestic fowl and calcium status of hatchlings.

Water balance in eggs of domestic fowl was manipulated by drilling holes (each 3 mm in diameter) through the calcareous layer into the air cell on day 10 of incubation. Water loss between days 0 and 18 averaged 6 g for eggs in the control group (no hole) but increased to 8, 12, and 15 g for eggs with 1, 2, or 3 holes, respectively. Hatching success was 79-87% for eggs with 0-2 holes through the eggshell but only 43% for eggs with three holes. Live mass of hatchlings declined as the number of holes drilled in the eggshell increased, but dry mass of carcasses was unaffected by the treatments. The quantity of Ca2+, Mg2+, and phosphorus in residual yolks and yolk-free carcasses of hatchlings was not influenced by the amount of water lost from eggs. Plasma Ca2+ and Mg2+ were elevated in hatchlings from eggs with high rates of water loss. The inability to regulate plasma Ca2+ may be a negative consequence of excessive water loss and could contribute to increased mortality of embryos.

Animals↗
PubMed1989

Mobilization of calcium, phosphorus, and magnesium by embryonic alligators (Alligator mississippiensis).

The yolk of an alligator (Alligator mississippiensis) egg contains approximately 115 mg of Ca, 180 mg of P, and 17 mg of Mg at oviposition. This compartment is the primary (or sole) source of P and Mg for the forming embryo. Both of these elements are depleted rapidly from the yolk once the embryo enters the growth phase of development. The yolk also is an important source of Ca for the embryo, particularly during the first two-thirds of incubation. During the last trimester of development, however, the embryo supplements Ca from the yolk with Ca mobilized from the eggshell. Indeed, more Ca is withdrawn from the eggshell during the last 2 wk of incubation than can be used by the embryo in skeletogenesis. The excess Ca is stored in the yolk, thereby causing net transfer of Ca to shift from withdrawal from the yolk to deposition in the yolk. Consequently, the residual yolk in the hatchling alligator contains a substantial reserve of Ca to support skeletal growth during the neonatal period. The pattern of mobilization and deposition of Ca during embryogenesis in alligators is similar to that characterizing avian embryos, but is distinct from that characterizing embryos of all other oviparous reptiles.

Alligators and Crocodiles↗
PubMed1989

Control of metabolism and growth in embryonic turtles: a test of the urea hypothesis.

We performed two experiments to determine (1) whether the metabolism and growth of embryonic snapping turtles (Chelydra serpentina) incubating in wet and dry environments are correlated inversely with the concentration of urea inside their eggs, and (2) whether urea accumulating inside eggs might be the cause of reductions in metabolism and growth by embryos. Eggs in the first experiment were incubated in different hydric environments to induce different patterns of net water exchange between the eggs and their surroundings. Turtles hatching from eggs that were in positive water balance had larger carcasses, smaller residual yolks and lower concentrations of urea in their blood than animals emerging from eggs that were in negative water balance. Thus, we confirmed the existence of correlations among water exchange by eggs, concentrations of urea in fluid compartments inside eggs, and metabolism and growth of embryos. In the second experiment, eggs were injected with solutions of urea at the mid-point of incubation to induce different levels of uremia in developing embryos. The injection protocol induced variation in the concentration of urea in blood of hatchlings similar to that observed in the first experiment for turtles hatching in wet and dry environments. However, the injection protocol did not induce variation in size of hatchlings or in mass of their residual yolk. Thus, the reduction in metabolism and growth of chelonian embryos developing in dry environments does not result from an inhibition of intermediary metabolism caused by urea, and the 'urea hypothesis' for control of metabolism cannot be accepted in its present form.

Animals↗
PubMed1985

Calcium mobilization, water balance, and growth in embryos of the agamid lizard Amphibolurus barbatus.

Embryos of the agamid lizard Amphibolurus barbatus are at developmental stage 29 of Dufaure and Hubert at the time of oviposition. Mobilization of calcium and other nutrients from the yolk proceeds slowly for the first half of incubation, during which time embryonic growth also proceeds slowly. During the second half of incubation, however, embryos withdraw calcium and other nutrients from the yolk very rapidly, and growth rates are correspondingly high. Approximately 60% of the calcium used by developing embryos is obtained from the yolk, but fully 40% of their requirement is met by calcium mobilized from the eggshell. Very little calcium remains in residual yolk of hatchlings, so this yolk must be used in maintenance metabolism rather than in growth of neonates. No dichotomy exists among oviparous, amniotic vertebrates with respect to sources of calcium used by developing embryos, but one does exist with respect to patterns of mobilization of this element. Whereas calcium is extracted from yolk of embryonic reptiles throughout incubation, it actually is deposited in yolk of embryonic birds after the midpoint in development.

Animals↗
PubMed1984

Calcium metabolism in embryos of the oviparous snake Coluber constrictor.

Total calcium in embryos of an oviparous, colubrid snake (Coluber constrictor L.) rises rapidly during the last half of incubation as the embryos increase in size. Although most of this calcium is drawn from stores in the yolk, hatchlings contain more calcium than was present in yolk of eggs at oviposition. Because shells from eggs incubated to hatching contain less calcium than do shells from freshly-laid eggs, the extra calcium appears to be drawn from the eggshell. Indeed, approximately 20% of the calcium required for development in this snake is obtained from the eggshell, with the remainder coming from the yolk. Thus, embryos of oviparous snakes, like embryonic chelonians, crocodilians and birds, withdraw calcium from their eggshells and do not rely exclusively on calcium supplied in their yolk for support of growth and development.

Animals↗
PubMed1979

Water-vapor conductance of testudinian and crocodilian eggs (class reptilia).

Flexible-shelled eggs of snapping turtles (Chelydra serpentina) have conductances to water vapor that are 55 times higher than predicted for avian eggs of similar size, whereas rigid-shelled eggs of softshell turtles (Trionyx spiniferus) and American alligators (Alligator mississippiensis) have conductances that are only five times higher than expected for comparable eggs of birds. The differences between empirical and predicted values result from the much higher effective pore areas in reptilian eggshells than in those of birds. The relatively high porosities of these reptilian eggs presumably facilitate the transport of oxygen and carbon dioxide eggshells in later stages of incubation when air trapped inside nest chambers may become hypoxic and hypercapnic, yet seem not to lead to excessive transpiration of water vapor owing to the high humidities in nests where incubation occurs.

Alligators and Crocodiles↗
PubMed1975

The influence of thyroxine and acclimation temperature on glycogen reserves of the frog Rana pipiens.

Leopard frogs were given daily injections of Na-l-thyroxine (2.0 mug/10 g body weight) for seven days, and control animals were given daily injections of the vehicle. During the period of treatment, half of the frogs were acclimated, in darkness and without food, at a constant temperature of 15 degrees C, whereas the remainder of the animals were acclimated at 25 degrees C. At the end of the seventh day, the frogs were killed by decapitation, and the glycogen concentration in samples of liver tissue and skeletal muscle was determined spectrophotometrically. Total hepatic glycogen of each frog subsequently was calculated from data on glycogen concentration and liver weight. Treatment of leopard frogs with thyroxine had no apparent effect on hepatic glycogen reserves of animals acclimated at 15 degrees C, but there was a striking reduction in glycogen content of livers from hyperthyroid frogs held at 25 degrees C. Mobilization of hepatic glycogen in thyroxine-treated animals held at 25 degrees C presumably was secondary to a general stimulation of oxidative metabolism. Treatment with thyroxine had no effect on the concentration of glycogen in skeletal muscle of leopard frogs held at either of the acclimation temperatures used in this study.

Acclimatization↗