Hatchling Size in the Turtle Chrysemys picta bellii from Western Nebraska: Relationships to Egg and Maternal Body Size
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Eggs of the turtle Chrysemys picta bellii from four western Nebraska populations were incubated under laboratory conditions to compare egg size and hatchling size as measures of offspring size among populations and in the context of maternal body size. Generally, hatchling size (mass and carapace length) was linearly related to egg size (mass, width, and length) and did not vary between the two years of the study or among populations after adjusting for egg size. However, hatchling carapace length adjusted for egg width was greatest in a population with the most elongate eggs indicating that linear egg measurements may not be useful for interpopulation comparisons. The smaller range of variation, but greater variability in hatchling size relative to egg size and a narrow range of maternal body sizes (carapace length and mass), seemed to preclude a correlation between maternal body size and hatchling size within two populations. In two other populations, hatchling carapace length, egg width, and egg length increased with maternal carapace length by a common slope, although egg wet mass had a steeper slope than did hatchling wet mass. It is probable that under natural incubation conditions (e.g., warmer, drier, and more variable), hatchling mass may increase only slightly or not at all with maternal body size. My results suggest that both hatchling and egg size should be considered in turtle life history studies, particularly for models that predict delayed sexual maturation when offspring size and survival increase with maternal body size and age.Keywords:
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Most North American freshwater turtle neonates typically hatch in late summer or early fall and move from their subterranean nests to nearby lakes and ponds, avoiding winter's subzero temperatures in the water's depths. However, painted turtle (Chrysemys picta) hatchlings remain entombed in their shallow subterranean nest throughout their first winter, frequently experiencing ice and body temperatures as low as -10°C. Nevertheless, the hatchlings emerge unscathed when the ground thaws in the spring.Although two mechanisms could explain the painted turtle hatchling's remarkable cold tolerance (a capacity for supercooling and an ability to tolerate freezing), supercooling is thought to play a significant role in the neonate's ability to overwinter. However, the importance of freeze tolerance in survival continues to be debated. Gary and Mary Packard of Colorado State wondered why hatchling painted turtles can only recover from a few days of freezing under relatively mild conditions (-2 to -2.5°C) and decided to investigate whether lactate accumulation in the hatchling's tissues affects the youngster's freeze tolerance.The Packards explain that when hatchlings freeze, much of the extracellular water forms ice, preventing the circulatory system from functioning and the delivery of oxygen for mitochondrial ATP production. Cells are instead forced to meet their ATP demands through anaerobic glycolysis, which causes an increase in tissue lactic acid levels, known as anoxic lactic acidosis. Normally, unfrozen turtles counteract the dangerous drop in tissue pH that accompanies lactate accumulation by the release of carbonate buffers into the blood from the shell, and lactate is also transported to and sequestered in the shell itself. However, these buffering processes may not be available to frozen hatchlings due to their arrested circulation.Assessing lactate's role in freeze tolerance, the Packards exposed recently hatched painted turtles to freezing conditions at -2°C for periods between zero and eight days. Every second day, the Packards thawed a group of animals and recorded the mortality percentage. They also measured whole-body lactate levels from another group that was not allowed to thaw. The team found that the frozen hatchlings' mortality levels were correlated with the amount of whole-body lactate. They also found that the frozen turtles' whole-body lactate levels were greater than those previously measured from comparable supercooled turtles, which maintain a functional circulatory system. Thus,freezing compromises the hatchling's ability to effectively deal with the anoxic lactic acidosis that accompanies anaerobic metabolism, a phenomenon the Packards suspect is due to the cessation of the circulation that occurs with freezing.The Packards hypothesize that without a functional circulatory system,frozen turtles cannot shuttle lactate from individual tissues to the shell to be sequestered or mobilize the shell's buffers. As a result, individual organs may accumulate lactate to lethal levels sooner than if the circulatory system remained operational. As follows, the Packards suggest that a cold tolerance strategy based on freezing is more stressful than a strategy based on supercooling and argue that the inability to deal with lactic acidosis may be one of the reasons why hatchling painted turtles are unable to tolerate freezing for prolonged time periods.
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Painted turtle
Supercooling
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Eggs of the turtle Chrysemys picta bellii from four western Nebraska populations were incubated under laboratory conditions to compare egg size and hatchling size as measures of offspring size among populations and in the context of maternal body size. Generally, hatchling size (mass and carapace length) was linearly related to egg size (mass, width, and length) and did not vary between the two years of the study or among populations after adjusting for egg size. However, hatchling carapace length adjusted for egg width was greatest in a population with the most elongate eggs indicating that linear egg measurements may not be useful for interpopulation comparisons. The smaller range of variation, but greater variability in hatchling size relative to egg size and a narrow range of maternal body sizes (carapace length and mass), seemed to preclude a correlation between maternal body size and hatchling size within two populations. In two other populations, hatchling carapace length, egg width, and egg length increased with maternal carapace length by a common slope, although egg wet mass had a steeper slope than did hatchling wet mass. It is probable that under natural incubation conditions (e.g., warmer, drier, and more variable), hatchling mass may increase only slightly or not at all with maternal body size. My results suggest that both hatchling and egg size should be considered in turtle life history studies, particularly for models that predict delayed sexual maturation when offspring size and survival increase with maternal body size and age.
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Painted turtle
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We submerged hatchling western painted turtles Chrysemys picta Schneider, snapping turtles Chelydra serpentina L. and map turtles Graptemys geographica Le Sueur in normoxic and anoxic water at 3 degrees C. Periodically, turtles were removed and whole-body [lactate] and [glycogen] were measured along with relative shell mass, shell water, and shell ash. We analyzed the shell for [Na+], [K+], total calcium, total magnesium, Pi and total CO2. All three species were able to tolerate long-term submergence in normoxic water without accumulating any lactate, indicating sufficient extrapulmonary O2 extraction to remain aerobic even after 150 days. Survival in anoxic water was 15 days in map turtles, 30 days in snapping turtles, and 40 days in painted turtles. Survival of hatchlings was only about one third the life of their adult conspecifics in anoxic water. Much of the decrease in survival was attributable to a dramatically lower shell-bone content (44% ash in adult painted turtles vs. 3% ash in hatchlings of all three species) and a smaller buffer content of bone (1.3 mmol g(-1) CO2 in adult painted turtles vs. 0.13-0.23 mmol g(-1) CO2 in hatchlings of the three species). The reduced survivability of turtle hatchlings in anoxic water requires that hatchlings either avoid aquatic hibernacula that may become severely hypoxic or anoxic (snapping turtles), or overwinter terrestrially (painted turtles and map turtles).
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Painted turtle
Chelydra
Hibernation
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Hatchling
Chelydra
Painted turtle
Animal ecology
Altricial
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Abstract Neonatal painted turtles ( Chrysemys picta ) do not emerge from their nest immediately upon hatching but instead remain inside the nest cavity until the following spring. Because the nest chamber is only 7–12 cm beneath the surface of the ground, hatchlings in northerly populations may be exposed to subzero temperatures during their first winter of life. We studied the pattern of survival by hatchling painted turtles exposed to subzero temperatures in a controlled laboratory experiment and discovered that 50% of the animals in our samples could withstand exposure to approximately −8.6°C for 18 hr. In contrast, hatchling snapping turtles ( Chelydra serpentina ) died at −2°C. This extraordinary ability for hatchling painted turtles to withstand exposure to subzero temperatures is a key feature in the unusual life history of the species and distinguishes painted turtles from other species occurring at high latitudes.
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Painted turtle
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Painted turtles (Chrysemys picta) have a natural history unlike that of other chelonians from the northern United States and southern Canada. Although neonates of other freshwater turtles usually emerge from their subterranean nests in late summer or autumn and move to nearby marshes, lakes, or streams to spend their first winter, hatchling painted turtles typically remain inside their shallow (8-14 cm) nests throughout their first winter and do not emerge above ground until the following spring (Ernst et al., 1994). This behavior commonly causes neonatal painted turtles from Nebraska (Packard, 1997; Packard et al., 1997a), northern Illinois (Weisrock and Janzen, 1999), and New Jersey (DePari, 1996) northward to the limit of distribution in southern Canada (Storey et al., 1988)
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We monitored temperatures during the winter of 1995–1996 inside 18 nests containing hatchling painted turtles (Chrysemys picta). The study was performed at the Valentine National Wildlife Refuge in north-central Nebraska to assess survival of neonatal turtles in relation to the thermal environment inside their hibernacula. Minimum temperatures in the nests varied from −3 to −21 °C, and were better predictors of survival of hatchlings than other measures of the thermal environment. All hatchlings survived in nests where the temperature never went below −7 °C, some animals survived in nests where the minimum was between −7 and −13 °C, but no turtle survived in a nest where the minimum was below −14 °C. Hatchlings probably survived the cold by sustaining a state of supercooling, because the duration of exposure to low temperatures was far too long for animals in most nests to have survived in a frozen state.
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Painted turtle
Overwintering
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Critical thermal maximum
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1. Laboratory experiments have documented substantial temperature effects on the physiological ecology of reptilian eggs, embryos and offspring. However, functional links between important habitat characteristics, nest microenvironments and fitness‐related traits of neonates in natural nests have rarely been studied. 2. A field study of 11 Painted Turtle ( Chrysemys picta ) nests was conducted to quantify the relationships between a habitat characteristic (i.e. vegetational cover around nests at oviposition) and (1) developmental temperature and its effect on offspring sex ratio and (2) hibernation temperature and its effect on offspring survivorship. 3. Vegetational cover was negatively correlated with nest temperatures in July, the period when offspring sex is determined. However, neither vegetational cover nor mean nest temperature predicted nest sex ratios, although correlations among these variables were consistent with causal relationships derived from laboratory studies. 4. Summer vegetational cover was also negatively correlated with measures of winter nest temperatures. Of the three nests exhibiting overwinter mortality, two were surrounded by thick vegetation and all experienced temperatures below – 8 °C. The remaining nests reached temperatures as low as – 6 °C without mortality, indicating that hatchlings in these nests exhibited remarkable supercooling ability. 5. The results suggest that habitat characteristics and nest microenvironments are functionally linked and have fitness consequences for both embryos and offspring, implying that nest‐site choice by female turtles could have considerable utility.
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Painted turtle
Avian clutch size
Chelydra
Hibernation
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Temporal patterns of embryonic metabolism integrate ontogenetic changes in the energetic costs of growth and maintenance. Unlike most other chelonians, which show a peaked pattern of embryonic metabolic rate (MR) over time, turtles of the family Emydidae have been depicted with unimodally increasing patterns. We incubated eggs of the emydid Chrysemys picta picta under standard conditions at 30°C and serially measured rates of oxygen consumption. Five eggs showed clear decreases in MR before hatching; two others that hatched during MR measurements showed increases in MR associated with muscular activity. MRs of hatchling turtles were lower than those of prehatching embryos for at least 60 days, suggesting persistent costs of biosynthesis in yolk-dependent late-stage embryos and early hatchlings.
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Painted turtle
Yolk
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