Temperature Effects During Early Life Stages of the Alligator Snapping Turtle (Macrochelys temminckii)
Abstract
Alligator snapping turtle (Macrochelys temminckii) populations have declined across much of the southeastern United States in recent decades, due at least in part to overcollection. Recently, however, legal protection from large-scale harvesting has been granted to the species in all states where it is native, thereby drastically reducing one of the greatest threats to its survival. There is growing interest in captive propagation of alligator snapping turtles for reintroduction where populations have been decimated. In conjunction with one such effort, we analyzed the physiological effects of temperature on embryonic and posthatching development. Results indicate that extreme high and low incubation temperatures negatively affected embryo survival, and high incubation temperatures corresponded with shorter incubation time but also produced smaller hatchlings. The effects of temperature on gonadal differentiation indicated that the upper pivotal temperature was approximately 27.5°C. Posthatching growth was faster at warmer water temperatures, and there was little to no acclimation of metabolic rate to exposure to either incubation or water temperature. We conclude that intermediate (27.5°–28.5°C) incubation temperatures produce a female-biased mixed sex ratio and maximize hatching success and hatchling size while increasing incubation duration only slightly over that at the higher temperatures. In addition, posthatching growth was positively influenced by hatchling body temperature; therefore, warmer water temperatures (~30°C) decreased the time required to rear turtles to a size suitable for reintroduction.
Turtles are long-lived organisms characterized by delayed maturity (resulting in a long generation time), low fecundity, low nest survival, and high adult survival (Congdon et al. 1993, 1994). Survivorship in turtles is generally proportional to body size (Wilbur 1975; Frazer et al. 1990; Iverson 1991; Haskell et al. 1996; Janzen et al. 2000; but see Congdon et al. 1999) and as a result tends to increase substantially during the juvenile life stages when growth rates are highest (Cagle 1946; Dunham and Gibbons 1990; Bobyn and Brooks 1994).
Presumably, the benefits associated with delayed maturity yield higher average lifetime fitness than for individuals that mature earlier. However, the long generation time exhibited by most turtles makes them susceptible to chronic environmental disturbances that impact egg or juvenile survival (Congdon et al. 1993, 1994). Similarly, adverse effects on population demographics can result from increased adult mortality (Deevey 1947; Congdon et al. 1993, 1994). Commercial collection of turtles for the pet and food trades represents an important anthropogenic source of disturbance that disproportionately impacts larger size classes (review in van Dijk et al. 2000). Collection for consumption, in particular, targets larger turtles, which represent the reproductively mature age classes that otherwise enjoy high annual survivorship. Although still a serious problem in many parts of the world, collection of a number of threatened and endangered turtle species in the United States has been reduced in recent years by improved state and federal protection.
Embryonic and juvenile age classes are more likely to be affected by abiotic environmental disturbances than are adults. Such events include fluctuating water levels that flood nests and long-term temperature fluctuations that can in turn affect a number of variables, ranging from food availability to the incubation conditions experienced by embryos (Janzen 1994). For example, whereas most vertebrates have genetically fixed sex determination, most turtles exhibit temperature-dependent sex determination (TSD; Bull 1980; Bull and Vogt 1981). As a result, rising global temperatures resulting from the emission of greenhouse gases and other factors present a potentially serious threat to turtles by skewing population sex ratios (Fisher 1930; Janzen and Paukstis 1991).
In addition to sex, several other physiological and morphological characteristics have been identified that may be influenced by incubation temperature (Tinc). Among these are size at hatching (Brooks et al. 1991; Rhen and Lang 1995), posthatching growth rate (Etchberger et al. 1990; Ryan et al. 1990; Brooks et al. 1991; Bobyn and Brooks 1994; Etchberger 1993; McKnight and Gutzke 1993; Paez et al. 1995; Roosenburg and Johnson 1995), and temperature acclimation (O'Steen and Janzen 1999; Steyermark and Spotila 2000; Ewert 2008). Because these variables can reasonably be expected to influence survivorship among hatchling and juvenile age classes, it follows that incubation conditions can dramatically affect average lifetime fitness (Miller 1993; Janzen 1993, 1995; Bobyn and Brooks 1994).
Although legal protection has been granted to some turtles, such actions are often taken after a species has already experienced declines sufficient to compromise hatchling and juvenile recruitment. In response to severe population declines, a number of efforts have been made—with some success—to rehabilitate populations of a few species through captive rearing of hatchlings to a size that is expected to substantially reduce mortality following release (Iverson 1991; Haskell et al. 1996; Caillouet 1998). Unfortunately, due to a lack of critical data, such efforts are sometimes made without the benefit of basic knowledge of the factors affecting reproduction and fitness (Morreale et al. 1982; Spotila et al. 1987, 1994; Wibbels et al. 1989; Moll and Moll 2000, 2004). Although such ‘headstart' programs have the potential for improving conservation of threatened and endangered turtles, it will become increasingly important to maximize their efficiency as more turtle species face critical population declines.
Alligator snapping turtles (Macrochelys temminckii) have experienced population declines in recent decades and have likely been extirpated in many parts of the species' historical range (Pritchard 1989; Ernst et al. 1994; Wagner et al. 1996; Heck 1998; Riedle et al. 2005). This species is found in river systems throughout much of the southeastern United States; it is the largest freshwater turtle in North America and ranks among the most highly aquatic species as well (Pritchard 1989; Ernst et al. 1994). Among the many turtle species throughout the region for which population declines have been documented, M. temminckii has been identified by several state and federal agencies as a particularly promising candidate for conservation via captive propagation and release.
A recent 3-year survey in eastern Oklahoma, USA, found M. temminckii in restricted portions of just 4 river systems within the historical range of the species (Riedle et al. 2005), and reports of incidental catch by fishermen at 3 additional locations were reported to one of us (DBL) in 2002, 2007, and 2008. Although river impoundments have likely affected alligator snapping turtle populations in the state by fragmenting populations and creating barriers to migration (Pritchard 1989; Moll and Moll 2000), the most devastating impact has likely stemmed from the harvesting of adult turtles to meet the demands of the turtle meat market (Pritchard 1989; Heck 1998).
The US Fish and Wildlife Service was petitioned in 1983 to list the alligator snapping turtle as threatened, but the petition failed because information available for the species was deemed insufficient (Heck 1998). In recent years, however, M. temminckii has been awarded some level of protection in all states in which it occurs (Roman and Bowen 2000) and in 2006 was listed in Appendix III (controlled trade) of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES 2008). In Oklahoma, M. temminckii is currently listed as a Species of Special Concern (Ramus 1998), a status that prohibits possession and export from the state. It is hoped that this ban will curb further population declines. In response to apparent extirpation of the species across much of the state (Riedle et al. 2005), Tishomingo National Fish Hatchery, in collaboration with Sequoyah National Wildlife Refuge, initiated a pilot captive-breeding study in 1999 to assess the merit of using captive-bred stock to repopulate rivers where alligator snapping turtles were extirpated.
The objective of this study was to determine conditions that optimized embryonic development, hatching success, and postembryonic growth. Here, we report Tinc effects on M. temminckii sex, size at hatching, and tail morphology. Additionally, we measured Tinc and water temperature (Twater) effects on posthatching growth rates, metabolic compensation, and metabolic thermal sensitivity. Finally, we evaluated the potential effects of these variables on survival of captive-reared turtles. These objectives were designed to be relevant to current captive propagation/release programs for the species and to provide insights into the viability of similar programs for other threatened and endangered turtle species.
METHODS
All procedures for this research were approved by the Oklahoma State University Institutional Animal Care and Use Committee (protocol No. AS023), assuring compliance with animal care guidelines (Institute of Laboratory Animal Research 1996).
Eggs
Alligator snapping turtle eggs were obtained in 2002 and 2004 from nests laid by females housed at Tishomingo National Fish Hatchery. An additional clutch of eggs was obtained in 2002 from L. Andrews, a private snapping turtle breeder in north-central Oklahoma. Eggs were removed from nests and numbered, then transported to Oklahoma State University within 24 hours of oviposition. There, they were weighed (± 0.1 g) and distributed among plastic shoeboxes containing damp vermiculite (1:1 water:vermiculite by mass) in a randomized block design with clutch serving as the blocking variable. Eggs were distributed among 6 incubation temperatures ranging from 23° to 31°C in 2002. Due to high egg failure at the upper and lower temperatures used in 2002 (see Results), incubation was restricted to 3 temperatures (26.5°, 28.5°, and 30.5°C) in 2004. Incubator temperatures were monitored daily with calibrated thermocouple wires inserted into 1 egg container in each incubator. Each egg box was weighed weekly and, when necessary, distilled water was added to compensate for evaporation. Additionally, egg boxes were rotated daily and eggs were redistributed within each box weekly to minimize effects of thermal or moisture gradients.
Hatchlings
Eggs were removed from the plastic shoeboxes and placed individually in plastic jars lined with damp paper towel after pipping to retain individual identification. Hatchlings remained in their individual containers until residual yolk was internalized (3–8 days), at which time they were weighed (± 0.1 g) and midline carapace length and postvent tail length were measured (± 0.1 mm). Additionally, metabolic rate (MR) was measured in terms of oxygen consumption for each hatchling in 2004. Following MR measurements, unique combinations of posterior marginal scutes were marked by tying loops of dental floss through needle holes to facilitate long-term identification of individuals (O'Steen 1998), and each turtle was assigned to 1 of 2 Twater treatments maintained at 25° and 30°C. Turtles were fed a commercially produced fish-based pellet diet ad libitum and weighed and measured weekly for 11 weeks.
Metabolic Rate
Oxygen consumption was measured via closed-system respirometry (Vleck 1987) and used to calculate MR (Peterson 1990). Oxygen consumption rate of each turtle was measured at 2 different times at all 3 Tinc (26.5°, 28.5°, and 30.5°C): first, 1–2 days after internalizing yolk and second approximately 6 months posthatching (x̄ = 187 days, range = 185–198 days).
Metabolic chambers were constructed from 169-, 322-, and 959-mL cylindrical plastic jars with screw-on lids. A stopcock was inserted through the lid of each jar, and a thin film of vacuum grease was applied on the inside of each lid to ensure an airtight seal when the stopcock was closed.
Prior to the 6-month MR measurements, turtles were fasted for 4 days to minimize metabolic costs associated with specific dynamic action and growth. Hatchling measurements were conducted as soon as residual yolk was internalized, and therefore digestion and growth likely still contributed to MR during this early measurement. On the day of measurement, each turtle was weighed and placed into a metabolic chamber. Chambers were then placed inside an environmental chamber for 1.5–2 hours to allow body temperatures to stabilize and equilibrate to ambient temperature. With the overhead lights off to minimize disturbance to the turtles, chambers were carefully removed from the environmental chamber. After screwing on lids to create an airtight seal, pretrial air samples were drawn into 20-mL syringes, also equipped with stopcocks. The stopcocks on the syringe and chamber were then closed, the syringes removed, and the time of sampling recorded. Chambers were placed back into the dark environmental chamber and removed after approximately 1 hour, when posttrial samples were drawn from the stopcock after pumping each syringe several times to ensure mixing of the air inside. After the final measurement at a given temperature, turtles were moved to an environmental chamber set at another temperature and the process was repeated. Measurements at all 3 temperatures were conducted on the same day.
Oxygen concentrations of all air samples were analyzed in 10-mL aliquots with a Sable Systems FC-1 oxygen analyzer. Air was drawn from outside the building at a regulated flow rate of 100 mL/min and through serial columns of Drierite and Ascarite to remove water and CO2, respectively. Each aliquot was injected into the air stream, which passed through a small column of Drierite and Ascarite and then through the oxygen analyzer. Oxygen consumption by each turtle was calculated as the difference between the initial and final volumes of oxygen after correcting for chamber volume (Peterson 1990).
Sex Identification
Individuals were sexed 267–278 (x̄ = 273) days after hatching via laparoscopic surgery (Rostal et al. 1994). Food was withheld for 6 days prior to the procedure to reduce the volume of gut contents. The turtles were transferred to the veterinary facilities at the Tulsa Zoo on the mornings that surgeries were performed by K. Backues, DVM. General anesthesia was achieved with a mixture of 10 mg/kg Ketamine and 0.1 mg/kg Medetomidine. After cleaning the site with Chlorhexidine, a 3–4-mm incision was made posterior to the bridge. A laparoscope was inserted into the incision, and the gonads were identified visually. Small tissue biopsies were taken from 4 individuals and examined histologically to validate macroscopic determinations. The gonads of M. temminckii at this age were distinct; ovaries appeared as gray tissue with varying numbers of primordial follicles lying ventral to the oviducts; whereas, testes were cream-colored and highly vascularized. Following gonad identification, incisions were closed with a suture and surgical adhesive. Turtles were maintained under moist conditions but out of water for 48 hours following surgery to ensure recovery from anesthesia.
Statistics
Mass and MR values were log10-transformed prior to statistical analyses to normalize the distribution and homogeneity of variance among treatments. After transformation, assumptions of parametric statistics were met for all subsequent analyses.
Hatchling mass and tail length were analyzed using analysis of covariance (ANCOVAs), with Tinc as a fixed effect and clutch as a random effect. Initial egg mass was included as a covariate for mass analyses, and carapace length was used as a covariate for analyzing tail length.
Growth rate was calculated for each 7-day interval for 77 days after hatching using the equation Growth rate = (massn − massn−1) × (massn−1)−1 × [n − (n − 1)]−1, where n = the last day of each measurement interval. These growth rate values were then analyzed over 77 days that growth was monitored using a repeated-measures analysis in which Tinc and Twater were treated as fixed effects, clutch was treated as a random effect, and hatchling ID was repeated over the 11 measurement periods. Additionally, the effects of Tinc, Twater (fixed effects) and clutch (random effect) on the maximum growth rate over any 7-day interval of each turtle was analyzed using ANCOVA.
Metabolic compensation following acclimation to a constant temperature (either during incubation or prolonged exposure to constant Twater) was measured shortly after hatching and 6 months posthatching by measuring MR at all 3 incubation temperatures: 26.5°, 28.5°, and 30.5°C. Analyses of data from both stages were performed in a repeated-measures ANCOVA, with Tinc and ambient temperature as fixed effects, turtle ID repeated over each temperature, and mass as a covariate. Six-month measurements were analyzed similarly but with Twater included as a third fixed effect.
Metabolic sensitivity to changes in body temperature, expressed as temperature coefficient (Q10) values, were calculated for each turtle after hatching and at 6 months posthatching using MR measurements obtained at 26.5° and 30.5°C. Separate ANCOVAs were used to analyze clutch and Tinc effects on hatchling Q10, and clutch, Tinc and Twater effects on 6-month-old juveniles.
RESULTS
Eggs
Eighty-eight alligator snapping turtle eggs comprising 3 clutches were obtained in 2002. Clutch size ranged from 15 to 37, but eggs in the smallest clutch proved infertile (Table 1). One hundred eighty-six alligator snapping turtle eggs were obtained from 6 nests in 2004. Clutch size was 17–42 eggs (x̄ = 31.2), and hatching success was varied among clutches, again including 1 infertile clutch (Table 1).
Tinc strongly influenced hatching success (Fig. 1). Turtles at 23.0° and 24.5°C in 2002 appeared fully formed but failed to pip; whereas, embryos at 31.0°C initiated development but died in the first 3 weeks of incubation. Hatching success also varied within the narrower Tinc range in 2004; 26.5° and 28.5°C exhibited 85% and 73% hatching success, respectively; whereas, hatch rate fell to 40% at 30.5°C.
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
Incubation duration varied nonlinearly with Tinc. There was substantial overlap in incubation duration among eggs incubated at 28.5° and 30.5°C but no overlap between those incubated at 26.5° and 28.5°C (26.5° range = 90–99 days; 28.5° range = 80–87 days; 30.5° range = 75–85 days). In 2004, embryonic development took an average 11 days longer at 26.5°C compared to 28.5°C but took an average 3 days longer at 28.5°C compared to 30.5°C (Fig. 2).
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
Variation in egg mass was analyzed separately for 2002 and 2004 due to the likelihood that individual females contributed clutches in both years. Clutch strongly influenced egg mass (2002, 2004: p < 0.0001); however, because each clutch was distributed randomly among treatments, mean egg mass did not differ among Tincs (p > 0.05).
Hatchlings
There was a positive correlation between log10 egg mass and log10 hatchling mass (log10 hatchling mass = 0.9151[log10 egg mass] − 0.0716, r2 = 0.495; Fig. 3A). Eggs that incubated at 30.5°C produced smaller hatchlings than those incubated at 26.5°C or 28.5°C (ANOVA: F2, 94 = 4.84, p = 0.010); this relationship remained consistent after correcting for differences in initial egg mass (ANCOVA: F2, 93 = 3.72, p = 0.028; Fig. 3B). However, the relationship between mass and Tinc became progressively weaker and had disappeared by the third week posthatching (7 days: F2, 86 = 4.60, p = 0.011; 14 days: F2, 86 = 4.00, p = 0.022; 21 days: F2, 86 = 1.35, p = 0.264).
![Figure 3. Mass conversion efficiency of yolk to tissue at 3 incubation temperatures. A) Regression line fitted to the scatter plot fits the equation: log10 [hatchling mass] = 0.9151(log10 egg mass) − 0.0716, r2 = 0.4952. Closed circles = 26.5°C, triangles = 28.5°C, and inverted triangles = 30.5°C incubation temperatures. Dashed lines = 95% Confidence Interval. B) Least-squares residuals from log–log plot. Error bars = ± 1 Standard Error.](/view/journals/ccab/8/1/i1071-8443-8-1-74-f03.gif)
![Figure 3. Mass conversion efficiency of yolk to tissue at 3 incubation temperatures. A) Regression line fitted to the scatter plot fits the equation: log10 [hatchling mass] = 0.9151(log10 egg mass) − 0.0716, r2 = 0.4952. Closed circles = 26.5°C, triangles = 28.5°C, and inverted triangles = 30.5°C incubation temperatures. Dashed lines = 95% Confidence Interval. B) Least-squares residuals from log–log plot. Error bars = ± 1 Standard Error.](/view/journals/ccab/8/1/full-i1071-8443-8-1-74-f03.gif)
![Figure 3. Mass conversion efficiency of yolk to tissue at 3 incubation temperatures. A) Regression line fitted to the scatter plot fits the equation: log10 [hatchling mass] = 0.9151(log10 egg mass) − 0.0716, r2 = 0.4952. Closed circles = 26.5°C, triangles = 28.5°C, and inverted triangles = 30.5°C incubation temperatures. Dashed lines = 95% Confidence Interval. B) Least-squares residuals from log–log plot. Error bars = ± 1 Standard Error.](/view/journals/ccab/8/1/inline-i1071-8443-8-1-74-f03.gif)
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
Growth rates during the first 11 weeks after hatching were affected by Tinc, Twater, and age (Repeated Measures ANOVA—Tinc × age interaction: F20, 950 = 2.44, p = 0.0004; Twater × age interaction: F10, 950 = 4.76, p < 0.0001). Tukey's post hoc tests indicated that turtles reared in warm water grew consistently faster than those maintained in cool water in weeks 6–11 (Fig. 4A). In contrast to this consistent pattern, differences among Tinc treatments were variable, with no regular pattern emerging across multiple weeks (Fig. 4A). However, over time, these modest and variable differences in mass-specific growth rates translated into consistent differences in mass among Tinc treatments (Fig. 4B). At Twater = 30°C, turtles incubated at 26.5°C and 30.5°C were larger than those incubated at 28.5°C beginning 8 weeks after hatching; whereas, at Twater = 25°C turtles from the 2 lower incubation temperatures were larger than those from 30.5°C in weeks 7–11 (Fig. 4B). Averaged across incubation temperatures, after 11 weeks turtles raised in 30°C water had gained more than twice the mass as had those maintained at 25°C (30°C: + 31.04 ± 1.47 g; 25°C: + 14.92 ± 0.55 g).
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
Maximum growth rate (independent of age) was strongly influenced by Twater but not affected by Tinc (Twater: F1, 87 = 9.04, p = 0.004; Tinc: F2, 87 = 0.89, p = 0.416; Fig. 5A). The time at which maximum growth occurred also varied with Twater but not Tinc (Twater: F1, 87 = 4.89, p = 0.029; Tinc: F2, 87 = 2.68, p = 0.075; Fig. 5B). Turtles that were maintained at 25°C exhibited average maximum growth rates of 19.23 ± 0.72 mg·g−1·d−1, compared to maximum growth rates of 28.27 ± 0.95 mg·g−1·d−1 among turtles raised in 30°C water.
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
Average postvent tail length was shorter among hatchlings from Tinc = 30.5°C than from lower Tincs, and was not significantly different between 26.5°C and 28.5°C (ANCOVA: F2, 82 = 12.22, p < 0.0001).
Gonadal differentiation followed a pattern consistent with previously published data (Ewert et al. 1994; Fig. 6). A high proportion of males (81.4%) was produced at 26.5°C; whereas, males constituted 3.3% and 0% of hatchlings at 28.5°C and 30.5°C, respectively.
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
Metabolic Rate
Hatchling oxygen consumption rates showed a positive correlation with body temperature (log10 [oxygen consumption] = 0.047 [body temperature] − 1.180; r2 = 0.549; p < 0.0001). Among hatchlings, Tinc induced metabolic compensation (F2, 84 = 11.47, p < 0.0001; Fig. 7A), though the effect was subtle. Prolonged exposure to constant Tinc resulted in slightly higher MR among turtles incubated at 26.5°C compared to those incubated at 28.5°C and 30.5°C (Tukey's post hoc tests: p < 0.01). MR of turtles incubated at 28.5°C and 30.5°C did not differ (Tukey's post hoc test: p = 0.185). Among 6-month-old juveniles, the previously observed effects of Tinc on metabolic compensation had disappeared, and no effect of recent exposure to constant Twater was apparent (Tinc: F2, 85 = 0.76, p = 0.471; Twater: F1, 85 = 0.02, p = 0.884; Fig. 7A, B).
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
MR Q10 values did not differ among hatchlings incubated at different temperatures (F2, 17 = 2.94, p = 0.079; Fig. 8A), and neither Tinc nor Twater affected Q10 among 6-month-old juveniles (Tinc: F2, 86 = 0.15, p = 0.865; Twater: F1, 86 = 0.90, p = 0.346; Fig. 8A, B).
Citation: Chelonian Conservation and Biology 8, 1; 10.2744/CCB-0738.1
DISCUSSION
Incubation Duration
The inverse relationship between the duration of embryonic development and temperature that was evident in the present study is not surprising. However, at any given Tinc, incubation duration was longer than has been reported for many other sympatric species (Ewert 1979; 1985). For example, incubation period at 28.0°C varied inversely with latitude in Chelydra serpentina, ranging from 79 days in Florida to just 59 days in Michigan (Ewert et al. 2005). In comparison, we found that M. temminckii originating from Oklahoma hatched in 82 days at 28.5°C. Our results corroborate the idea that M. temminckii, and other species that produce relatively large eggs, may be range-limited at northern latitudes by long incubation periods at relatively cooler nest temperatures (Ewert 1985). High mortality among M. temminckii overwintering in nests in captivity (Grimpe 1987) suggests that this strategy is likely not utilized to compensate for long incubation at high latitudes. These limitations imposed on the embryic life stage may account for the fact that there are records of adult M. temminckii in Illinois (the northernmost extent of the species' range), including 1 gravid female (Phillips et al. 1999), but no records of nests or hatchlings (Galbreath 1961; Smith 1961; S. Ballard, Illinois Department of Natural Resources, pers. comm.).
Sex
In contrast to many turtle species that exhibit TSD, the pivotal temperature range within which a mixed sex ratio is produced in M. temminckii spans a wide range (Ewert et al. 2004). Our results support previously published data suggesting that no constant temperature results in 100% male production (Ewert et al. 1994). However, temperatures that progressively increase during development can produce higher male ratios than any single constant temperature (Ewert and Jackson 1994). It has been hypothesized that the production of females at all Tincs is due to interactions between TSD and genetic sex determination (GSD) mechanisms (Ewert et al. 1994). This hypothesis has not been explicitly tested in M. temminckii, but more recent data from another turtle with TSD suggest that among-clutch variation in the concentrations of maternally derived sex steroids deposited in the yolks of eggs presents a more parsimonious explanation for the phenomenon (Bowden et al. 2000; Elf 2004).
Hatchling Survival, Size, Growth, and Tail Length
Hatchling survival, mass, and tail length were reduced at the highest incubation temperatures used in this study. Survival has obvious direct effects on average lifetime fitness, but even in the absence of embryo mortality, it is generally assumed that the probability of survival increases with size among age classes that are at risk of predation (Janzen 1995). Therefore, high Tincs may be detrimental to the average fitness of M. temminckii, even when embryo mortality is not affected. However, this effect could be dampened to some degree by the fact that turtles from high Tincs develop and hatch faster, and therefore, would have greater opportunity to acquire resources and grow prior to winter torpor.
Severely curled tails among hatchling M. temminckii have been described, including speculation about the causes and ecological consequences of such malformations (McCallum and Trauth 2000). The consequences of possessing a straight but unusually short tail are not immediately clear, and its impact on overall fitness may in fact be minimal. However, possessing a short tail could affect turtles' capacity to right themselves when flipped onto their carapace (McCallum and Trauth 2000), impact terrestrial locomotion (Finkler and Claussen 1997), or correlate with other less apparent developmental problems.
Temperature Effects on MR
The metabolic rates of hatchling turtles from all 3 Tincs scaled positively with body temperature. Tinc did not affect sensitivity to temperature, but the long-term exposure to a single temperature during development did induce incomplete metabolic compensation that could dampen the effects of incubation at extreme temperatures. However, this effect was short-lived, and M. temminckii exhibited no compensatory response following periods of exposure to different temperatures after hatching. Therefore, it appears likely that this species has only limited capacity to physiologically dampen temperature effects. This leaves the species 2 options: it might simply tolerate a wide range of nonoptimal temperatures, or could restrict activity to areas and seasons in which suitable temperatures prevail. Although the first option is certainly a possibility, one study reported seasonal movements from shallow water in spring to deeper water during the heat of the summer, suggesting that at least some seasonal temperature selection occurs (Riedle et al. 2006).
Conservation
Much remains to be discovered about the thermal ecology of M. temminckii; however, there is sufficient information to make informed recommendations for conservation programs that incorporate captive hatching and rearing into their protocols. Intermediate temperatures (27.5°–28.5°C) will produce a female-biased mixed sex ratio. Although bias is not predicted in natural populations where individual selection dominates (Fisher 1930), it is reasonable to produce more females than males when manipulating population demographics to maximize reproduction. Additionally, female-biased sex ratios have been observed in several natural populations of turtles with TSD (Bull and Charnov 1989). The same temperatures that produce a desirable sex ratio also result in high embryo survivorship, normal development, efficient yolk-to-tissue conversion that results in relatively large hatchlings, and substantially shorter development times than occurred at lower Tincs.
After hatching, turtles grew much more quickly at 30°C than at 25°C. Because fast growth minimizes the time that juvenile turtles need to be maintained in captivity, and/or maximizes the size at which turtles are released, warmer water temperatures should be utilized when it is feasible. Other studies have measured effects of diet on growth and shown that high protein (45%–55%) will also increase the rate at which turtles grow (Harrell 1998). These results, in combination with studies that have addressed causes of population decline (Roman and Bowen 2000), population genetics (Roman et al. 1999; Hackler 2004), and behavior and population demographics (Riedle et al. 2005) should be instrumental in maximizing the success of M. temminckii conservation.
Alligator snapping turtle hatching success by temperature of incubation in A) 2002 and B) 2004. Bar height indicates the number of eggs incubated at each temperature, and the black portion of each bar indicates the percentage that successfully hatched.
Incubation duration among M. temminckii incubated in 2004 at 3 constant temperatures. Error bars: ± 1 Standard Error.
Mass conversion efficiency of yolk to tissue at 3 incubation temperatures. A) Regression line fitted to the scatter plot fits the equation: log10 [hatchling mass] = 0.9151(log10 egg mass) − 0.0716, r2 = 0.4952. Closed circles = 26.5°C, triangles = 28.5°C, and inverted triangles = 30.5°C incubation temperatures. Dashed lines = 95% Confidence Interval. B) Least-squares residuals from log–log plot. Error bars = ± 1 Standard Error.
Changes in A) growth rate and B) mass over time of alligator snapping turtles incubated at 3 temperatures and maintained at 2 different water temperatures. Circles = 26.5°C, triangles = 28.5°C, and inverted triangles = 30.5°C incubation temperatures. Closed symbols = 25°C and open symbols = 30°C water temperatures. Error bars = ± 1 Standard Error.
A) Peak growth rates of alligator snapping turtles from 3 incubation temperatures and reared at 2 water temperatures. Rates calculated over 7-day intervals. Error bars = ± 1 Standard Error. B) Timing of peak growth during the first 11 weeks post-hatching. Black bars = 25°C water, gray bars = 30°C water.
Sex ratio of alligator snapping turtles incubated in 2004 (open circles). Closed symbols are data adapted from Ewert et al. (1994).
Metabolic response to changes in body temperature following prolonged exposure to A) constant incubation temperature and B) constant water temperatures. Symbols in (A) are as in Fig. 4. B) open squares = 25°C, closed squares = 30°C water temperatures.
Metabolic rate Q10 values calculated from O2 consumption measurements at 26.5°C and 30.5°C. A) Hatchlings (black bars) and 6-month-old juveniles (gray bars) from 3 constant incubation temperatures, and B) juveniles after prolonged exposure to either 25°C or 30°C water temperatures. Error bars = ± 1 Standard Error.
