Geographic variation in incubation temperatures promoting viable offspring production in broadly co-distributed turtles

Organisms whose early life stages are environmentally sensitive produce offspring within a relatively narrow range of suitable abiotic conditions. In reptiles, development rate and survival are often maximized if incubation temperatures remain under 31°C, though this upper bound may vary within and among species. We addressed this expectation by comparing responses to egg incubation at 30°C versus 33°C in congeneric turtle species pairs with broad syntopic geographic distributions. In the two softshell turtles (Apalone spp.), the greatest changes in development rate and phenotypic variance were observed in the northernmost population, which had a low survival rate (40%) at 33°C. The presumably suboptimal temperature (33°C) for northern populations otherwise yielded 76%-93% survival rates and fast swimming speeds in more southern populations. Still, in one species, northern hatchlings incubated at 33°C matched the elevated speeds of their southern counterparts, revealing a countergradient response. In northern populations of the two map turtles (Graptemys spp.), survival was also reduced (28%-60%) at 33°C and the development rate (relative to 30°C) increased by up to 75%. Our experiments on divergent taxa with similar nesting ecologies substantiate that the optimal thermal range for offspring production is variable. These findings encourage further work on how population- and species-level differences relate to local adaptation in widely distributed oviparous species.

Temperature fluctuations and estrone sulfate affect gene expression via different mechanisms to promote female development in a species with temperature-dependent sex determination

Variation in developmental conditions can affect a variety of embryonic processes and shape a number of phenotypic characteristics that can affect offspring throughout their lives. This is particularly true of oviparous species where development typically occurs outside of the female, and studies have shown that traits such as survival and behavior can be altered by both temperature and exposure to steroid hormones during development. In species with temperature-dependent sex determination (TSD), the fate of gonadal development can be affected by temperature and by maternal estrogens present in the egg at oviposition and there is evidence that these factors can affect gene expression patterns. Here, we explore how thermal fluctuations and exposure to an estrogen metabolite, estrone sulfate, affect the expression of several genes known to be involved in sexual differentiation; Kdm6b, Dmrt1, Sox9, FoxL2, and Cyp19A1. We found that most of the genes responded to both temperature and estrone sulfate exposure, but that the responses to these factors was not identical in that estrone sulfate effects occur downstream of temperature effects. Our findings demonstrate that conjugated hormones such as estrone sulfate are capable of influencing temperature dependent pathways to potentially alter how embryos respond to temperature and highlight the importance of studying the interaction of maternal hormone and temperature effects.

Impact of fluctuating developmental temperatures on phenotypic traits in reptiles: a meta-analysis

During the vulnerable stages of early life, most ectothermic animals experience hourly and diel fluctuations in temperature as air temperatures change. While we know a great deal about how different constant temperatures impact the phenotypes of developing ectotherms, we know remarkably little about the impacts of temperature fluctuations on the development of ectotherms. In this study, we used a meta-analytic approach to compare the mean and variance of phenotypic outcomes from constant and fluctuating incubation temperatures across reptile species. We found that fluctuating temperatures provided a small benefit (higher hatching success and shorter incubation durations) at cool mean temperatures compared with constant temperatures, but had a negative effect at warm mean temperatures. In addition, more extreme temperature fluctuations led to greater reductions in embryonic survival compared with moderate temperature fluctuations. Within the limited data available from species with temperature-dependent sex determination, embryos had a higher chance of developing as female when developing in fluctuating temperatures compared with those developing in constant temperatures. With our meta-analytic approach, we identified average mean nest temperatures across all taxa where reptiles switch from receiving benefits to incurring costs when incubation temperatures fluctuate. More broadly, our study indicates that the impact of fluctuating developmental temperature on some phenotypes in ectothermic taxa are likely to be predictable via integration of developmental temperature profiles with thermal performance curves.

Incubation Temperature and Maternal Resource Provisioning, but Not Contaminant Exposure, Shape Hatchling Phenotypes in a Species with Temperature-Dependent Sex Determination

AbstractThe environment experienced during embryonic development is a rich source of phenotypic variation, as environmental signals have the potential to both inform adaptive plastic responses and disrupt normal developmental programs. Environment-by-embryo interactions are particularly consequential for species with temperature-dependent sex determination, a mode of sex determination common in non-avian reptiles and fish, in which thermal cues during a discrete period of development drive the formation of either an ovary or a testis. Here we examine the impact of thermal variation during incubation in combination with developmental exposure to a common endocrine-disrupting contaminant on fitness-related hatchling traits in the American alligator (Alligator mississippiensis), a species with temperature-dependent sex determination. Using a factorial design, we exposed field-collected eggs to five thermal profiles (three constant temperatures, two fluctuating temperatures) and two environmentally relevant doses of the pesticide metabolite dichlorodiphenyldichloroethylene; and we quantified incubation duration, sex ratios, hatchling morphometric traits, and growth (9-10 days post-hatch). Whereas dichlorodiphenyldichloroethylene exposure did not generally affect hatchling traits, constant and fluctuating temperatures produced diverse phenotypic effects. Thermal fluctuations led to subtle changes in incubation duration and produced shorter hatchlings with smaller heads when compared to the constant temperature control. Warmer, male-promoting incubation temperatures resulted in larger hatchlings with more residual yolk reserves when compared to cooler, female-promoting temperatures. Together, these findings advance our understanding of how complex environmental factors interact with developing organisms to generate phenotypic variation and raise questions regarding the mechanisms connecting variable thermal conditions to responses in hatchling traits and their evolutionary implications for temperature-dependent sex determination.

A simple model for the evolution of temperature-dependent sex determination explains the temperature sensitivity of embryonic mortality in imperiled reptiles

A common reptile conservation strategy involves artificial incubation of embryos and release of hatchlings or juveniles into wild populations. Temperature-dependent sex determination (TSD) occurs in most chelonians, permitting conservation managers to bias sex ratios towards females by incubating embryos at high temperatures, ultimately allowing the introduction of more egg-bearing individuals into populations. Here, we revisit classic sex allocation theory and hypothesize that TSD evolved in some reptile groups (specifically, chelonians and crocodilians) because male fitness is more sensitive to condition (general health, vigor) than female fitness. It follows that males benefit more than females from incubation environments that confer high-quality phenotypes, and hence high-condition individuals. We predict that female-producing temperatures, which comprise relatively high incubation temperatures in chelonians and crocodilians, are relatively stressful for embryos and subsequent life stages. We synthesize data from 28 studies to investigate how constant temperature incubation affects embryonic mortality in chelonians with TSD. We find several lines of evidence suggesting that warm, female-producing temperatures are more stressful than cool, male-producing temperatures. Further, we find some evidence that pivotal temperatures (TPiv, the temperature that produces a 1:1 sex ratio) may exhibit a correlated evolution with embryonic thermal tolerance. If patterns of temperature-sensitive embryonic mortality are also indicative of chronic thermal stress that occurs post-hatching, then conservation programs may benefit from incubating eggs close to species-specific TPivs, thus avoiding high-temperature incubation. Indeed, our models predict that, on average, a sex ratio of >75% females can generally be achieved by incubating eggs only 1°C above TPiv. Of equal importance, we provide insight into the enigmatic evolution of TSD in chelonians, by providing support to the hypothesis that TSD evolution is related to the quality of the phenotype conferred by incubation temperature, with males produced in high-quality incubation environments.

Good alimentation can overcome the negative effects of climate change on growth in reptiles

Climate change may lead to higher nest temperatures, which may increase embryo development rate but reduce hatchling size and growth. Larger body size permits better performance, making growth an important fitness trait. In ectotherms, growth is affected by temperature and food quality. To segregate the effects of incubation temperature vs. alimentation on the growth of the Mexican black spiny-tailed iguana, Ctenosaura pectinata, we incubated eggs at 29 or 32 °C, and hatchlings were kept at 30 °C and fed either high- or low-quality food for 1 year, with body size and mass being recorded every 2 weeks. Iguanas incubated at 29 °C grew faster than those incubated at 32 °C. However, food quality had a larger effect on growth than incubation temperature; iguanas fed with high-quality food reached larger body sizes. Growth models suggested that differences in growth between incubation temperatures and food types remain throughout their lives. We found that incubation temperature had long-lasting effects on an ectotherm, and higher incubation temperatures might lead to reduced growth and maturation at a later age. However, food might transcend the effect of increased incubation temperature; therefore, good alimentation might mitigate effects of climate change on growth.

Maternal provisioning and fluctuating thermal regimes enhance immune response in a reptile with temperature-dependent sex determination

The Charnov-Bull model of differential fitness is often used to explain the evolution and maintenance of temperature-dependent sex determination (TSD). Most tests of the model focus on morphological proxies of fitness, such as size traits, whereas early life physiological traits that are closely related to lifetime fitness might provide a framework for generalizing the Charnov-Bull model across taxa. One such trait is the strength of the early-life immune response, which is strongly linked to early-life survival and fitness. Here, we manipulated temperature, variance in temperature, and sex to test the Charnov-Bull model using a physiological trait, immune system strength, in the snapping turtle (Chelydra serpentina). We found no evidence of sex-specific differences in bactericidal capacity of hatchling blood, and no evidence that mean temperature influences bactericidal capacity. However, we did find that fluctuating incubation temperature (i.e. a more naturalized incubation regime) is associated with a greater bactericidal capacity compared with constant temperature incubation. We also found that egg mass, a proxy for maternal provisioning, is positively associated with bactericidal capacity. Our findings suggest that the evolution of temperature-dependent sex determination in reptiles is unrelated to our measure of early-life innate immunity. Our study also underlines how immune response is condition dependent in early life, and questions the biological relevance of constant temperature incubation in experimental studies on ectotherm development.

Ecologically relevant thermal fluctuations enhance offspring fitness: biological and methodological implications for studies of thermal developmental plasticity

Natural thermal environments are notably complex and challenging to mimic in controlled studies. Consequently, our understanding of the ecological relevance and underlying mechanisms of organismal responses to thermal environments is often limited. For example, studies of thermal developmental plasticity have provided key insights into the ecological consequences of temperature variation, but most laboratory studies use treatments that do not reflect natural thermal regimes. While controlling other important factors, we compared the effects of naturally fluctuating temperatures with those of commonly used laboratory regimes on development of lizard embryos and offspring phenotypes and survival. We incubated eggs in four treatments: three that followed procedures commonly used in the literature, and one that precisely mimicked naturally fluctuating nest temperatures. To explore context-dependent effects, we replicated these treatments across two seasonal regimes: relatively cool temperatures from nests constructed early in the season and warm temperatures from late-season nests. We show that natural thermal fluctuations have a relatively small effect on developmental variables but enhance hatchling performance and survival at cooler temperatures. Thus, natural thermal fluctuations are important for successful development and simpler approximations (e.g. repeated sine waves, constant temperatures) may poorly reflect natural systems under some conditions. Thus, the benefits of precisely replicating real-world temperatures in controlled studies may outweigh logistical costs. Although patterns might vary according to study system and research goals, our methodological approach demonstrates the importance of incorporating natural variation into controlled studies and provides biologists interested in thermal ecology with a framework for validating the effectiveness of commonly used methods.

Thermal variability during ectotherm egg incubation: A synthesis and framework

Natural populations of ectothermic oviparous vertebrates typically experience thermal variability in their incubation environment. Yet an overwhelming number of laboratory studies incubate animals under constant thermal conditions that cannot capture natural thermal variability. Here, we systematically searched for studies that incubated eggs of ectothermic vertebrates, including both fishes and herpetofauna, under thermally variable regimes. We ultimately developed a compendium of 66 studies that used thermally variable conditions for egg incubation. In this review, we qualitatively discuss key findings from literature in the compendium, including the phenotypic effects resulting from different patterns of thermally variable incubation, as well as the ontogenetic persistence of these effects. We also describe a physiological framework for contextualizing some of these effects, based on thermal performance theory. Lastly, we highlight key gaps in our understanding of thermally variable incubation and offer suggestions for future studies.

Sex-specific effects of developmental temperature on morphology, growth and survival of offspring in a lizard with temperature-dependent sex determination

The developmental environment plays a pivotal role in shaping fitness-relevant phenotypes of all organisms. Phenotypes are highly labile during embryogenesis, and environmental factors experienced early in development can have profound effects on fitness-relevant traits throughout life. Many reptiles exhibit temperature-dependent sex determination (TSD), whereby temperature during embryonic development permanently determines offspring sex. The leading hypothesis for the adaptive significance of TSD posits that egg incubation temperature differentially affects the fitness of males vs. females so that each sex is produced at its optimal temperature. The goal of this research is to address this hypothesis by quantifying the sex-specific effects of incubation temperature on phenotypes and survival in a lizard (Agama picticauda) with TSD. By incubating eggs under constant and fluctuating temperatures, we demonstrated that incubation temperature affects fitness-relevant phenotypes in A. picticauda; but males and females had similar reaction norms. However, females produced from female-biased incubation temperatures had greater survival than those from male-biased temperatures, and male survival was lowest for individuals produced from a female-biased temperature. In addition, eggs incubated at male-biased temperatures hatched earlier than those incubated at female-biased temperatures, which may have sex-specific consequences later in life as predicted by models for the adaptive significance of TSD.

Effects of constant and fluctuating incubation temperatures on hatching success and hatchling traits in the diamondback terrapin (Malaclemys terrapin) in the context of the warming climate

As global temperatures continue to rise, so too will the nest temperatures of many species of turtles. Yet for most turtle species, including the estuarine diamondback terrapin (Malaclemys terrapin), there is limited information on embryonic sensitivity to elevated temperature. We incubated eggs of M. terrapin at three, mean temperatures (31, 34, 37 °C) under two thermal exposure regimes (constant or semi-naturally fluctuating temperature) and measured hatching success, developmental rate, and hatchling size. Hatching success was 100% at 31 °C and 67% at 34 °C, respectively; at 37 °C, all eggs failed early in the incubation period. These values were unaffected by exposure regime. The modeled LT₅₀ (temperature that was lethal to 50% of the test population) was 34.0 °C in the constant and 34.2 °C in the fluctuating thermal regime, reflecting a steep decline in survival between 33 and 35 °C. Hatchlings having been incubated at a constant 34 °C hatched sooner than those incubated at 31 °C under either constant or fluctuating temperature. Hatchlings were smaller in straight carapace length (CL) and width after having been incubated at 34 °C compared to 31 °C. Larger (CL) hatchlings resulted from fluctuating temperature conditions relative to constant temperature conditions, regardless of mean temperature. Based upon recent temperatures in natural nests, the M. terrapin population studied here appears to possess resiliency to several degrees of elevated mean nest temperatures, beyond which, embryonic mortality will likely sharply increase. When considered within the mosaic of challenges that Maryland's M. terrapin face as the climate warms, including ongoing habitat losses due to sea level rise and impending thermal impacts on bioenergetics and offspring sex ratios, a future increase in embryonic mortality could be a critical factor for a population already experiencing ecological and physiological challenges due to climate change.

Cyclical and stochastic thermal variability affects survival and growth in brook trout

Directional changes in temperature have well-documented effects on ectotherms, yet few studies have explored how increased thermal variability (a concomitant of climate change) might affect individual fitness. Using a common-garden experimental protocol, we investigated how bidirectional temperature change can affect survival and growth of brook trout (Salvelinus fontinalis) and whether the survival and growth responses differ between two populations, using four thermal-variability treatments (mean: 10 °C; range: 7-13 °C): (i) constancy; (ii) cyclical fluctuations every two days; (iii) low stochasticity (random changes every 2 days); (iv) high stochasticity (random changes daily). Recently hatched individuals were monitored under thermal variability (6 weeks) and a subsequent one-month period of thermal constancy. We found that variability can positively influence survival, relative to thermal constancy, but negatively affect growth. The observations reported here can be interpreted within the context of Jensen's Inequality (performance at average conditions is unequal to average performance across a range of conditions). Projections of future population viability in the context of climate change would be strengthened by increased experimental attention to the fitness consequences of stochastic and non-stochastic thermal variability.

Influence of temperature variation on incubation period, hatching success, sex ratio, and phenotypes in Caiman latirostris

Temperature is crucial for reptiles, also during embryonic development, particularly for species with temperature-dependent sex determination. Under natural conditions, Broad-snouted caiman (Caiman latirostris) eggs are influenced by thermal changes in the interior of the nest related to the external environmental temperature. As nests are subject to variations in temperature and most lab studies on crocodilian incubation have been carried out at constant temperatures, we were interested in determining how temperature fluctuations may affect the development of caiman embryos. We investigated the effects of incubation at constant temperatures (31°C, 32°C, and 33°C) and fluctuating temperatures (31 ± 2, 32 ± 1, and 32 ± 2°C) on the following aspects: incubation period duration, hatching success, sex ratio, total length, and body mass of C. latirostris hatchlings. Eggs incubated at 31°C produced 100% females, those at 32°C produced 71.6% females (however, the sex ratio was nest related), and at 33°C produced 100% males. We found a masculinizing effect when incubation was at 31 ± 2°C compared with a constant 31°C; and temperature fluctuations at 32°C (32 ± 1 and 32 ± 2°C) had a negative effect on hatchlings size and mass, and hatching success compared with constant incubation temperatures of 32°C and 33°C. Finally, the effect of temperature variation during the incubation period on sex ratio, hatching success, and phenotype depends on the mean temperature, as the fluctuation around 31°C affected the sex ratios and incubation period, and the fluctuation around 32°C affected hatchling success and size.

Extreme thermal fluctuations from climate change unexpectedly accelerate demographic collapse of vertebrates with temperature-dependent sex determination

Global climate is warming rapidly, threatening vertebrates with temperature-dependent sex determination (TSD) by disrupting sex ratios and other traits. Less understood are the effects of increased thermal fluctuations predicted to accompany climate change. Greater fluctuations could accelerate feminization of species that produce females under warmer conditions (further endangering TSD animals), or counter it (reducing extinction risk). Here we use novel experiments exposing eggs of Painted Turtles (Chrysemys picta) to replicated profiles recorded in field nests plus mathematically-modified profiles of similar shape but wider oscillations, and develop a new mathematical model for analysis. We show that broadening fluctuations around naturally male-producing (cooler) profiles feminizes developing embryos, whereas embryos from warmer profiles remain female or die. This occurs presumably because wider oscillations around cooler profiles expose embryos to very low temperatures that inhibit development, and to feminizing temperatures where most embryogenesis accrues. Likewise, embryos incubated under broader fluctuations around warmer profiles experience mostly feminizing temperatures, some dangerously high (which increase mortality), and fewer colder values that are insufficient to induce male development. Therefore, as thermal fluctuations escalate with global warming, the feminization of TSD turtle populations could accelerate, facilitating extinction by demographic collapse. Aggressive global CO₂ mitigation scenarios (RCP2.6) could prevent these risks, while intermediate actions (RCP4.5 and RCP6.0 scenarios) yield moderate feminization, highlighting the peril that insufficient reductions of greenhouse gas emissions pose for TSD taxa. If our findings are generalizable, TSD squamates, tuatara, and crocodilians that produce males at warmer temperatures could suffer accelerated masculinization, underscoring the broad taxonomic threats of climate change.

Thermal spikes from the urban heat island increase mortality and alter physiology of lizard embryos

Effects of global change (i.e. urbanization, climate change) on adult organisms are readily used to predict the persistence of populations. However, effects on embryo survival and patterns of development are less studied, even though embryos are particularly sensitive to abiotic conditions that are altered by global change (e.g. temperature). In reptiles, relatively warm incubation temperatures increase developmental rate and often enhance fitness-relevant phenotypes, but extremely high temperatures cause death. Due to the urban heat island effect, human-altered habitats (i.e. cities) potentially create unusually warm nest conditions that differ from adjacent natural areas in both mean and extreme temperatures. Such variation may exert selection pressures on embryos. To address this, we measured soil temperatures in places where the Puerto Rican crested anole lizard (Anolis cristatellus) nests in both city and forest habitats. We bred anoles in the laboratory and subjected their eggs to five incubation treatments that mimicked temperature regimes from the field, three of which included brief exposure to extremely high temperatures (i.e. thermal spikes) measured in the city. We monitored growth and survival of hatchlings in the laboratory for 3 months and found that warmer, city temperatures increase developmental rate, but brief, thermal spikes reduce survival. Hatchling growth and survival were unaffected by incubation treatment. The urban landscape can potentially create selection pressures that influence organisms at early (e.g. embryo) and late life stages. Thus, research aimed at quantifying the impacts of urbanization on wildlife populations must include multiple life stages to gain a comprehensive understanding of this important aspect of global change.

Comparative transcriptional profiling analysis of the effect of heat waves during embryo incubation on the hatchlings of the Chinese soft-shelled turtle (Pelodiscus sinensis)

Temperature is one of most the important environmental factors that affect the ontogenesis of organisms. In this study, we incubated Chinese soft‐shelled turtle eggs at 28°C (control temperature, C treatment), a temperature with a 16°C cold shock and a 36°C heat shock twice per week (S treatment) or a ramp‐programmed temperature of 29 ± 9°C (with 12 hr (+) and 12 hr (−) every day) (F treatment). The incubation period, hatching success, hatchling weight, and locomotor performance were significantly different between the controls and the different heat treatment groups. The pathogen challenge results illustrated that hatchlings from the S treatment group were more resistant to bacterial infection, whereas hatchlings from the F treatment group were more vulnerable. We used RNA‐seq quantification analysis to identify differentially expressed genes (DEGs) of hatchlings in the S treatment group. Based on the functional annotation results for the DEGs, 9 genes were chosen to verify the RNA‐seq results. The background expression of DEGs was also analyzed for the three treatments, as was the regulation of the pathogen challenge. The results showed that 8 DEGs were related to the immune response after pathogen challenge and that temperature was an important factor in differential regulation of the immunity pathways.

Temperature fluctuations and maternal estrogens as critical factors for understanding temperature-dependent sex determination in nature

Vertebrates with temperature-dependent sex determination (TSD) have justifiably received a lot of attention when it comes to the potential effects of climate change. Freshwater turtles have long been used to characterize the physiological and genetic mechanisms underlying TSD and provide a great system to investigate how changing climatic conditions will affect vertebrates with TSD. Unfortunately, most of what we know about the mechanisms underlying TSD comes from laboratory conditions that do not accurately mimic natural conditions (i.e., constant incubation temperatures and supraphysiological steroid manipulations). In this paper, we review recent advances in our understanding of how TSD operates in nature that arose from studies using more natural fluctuating incubation temperatures and natural variation in maternal estrogens within the yolk. By incorporating more natural conditions into laboratory studies, we are better able to use these studies to predict how changing climatic conditions will affect species with TSD.

Characterizing the distribution of steroid sulfatase during embryonic development: when and where might metabolites of maternal steroids be reactivated?

All vertebrate embryos are exposed to maternally derived steroids during development. In placental vertebrates, metabolism of maternal steroids by the placenta modulates embryonic exposure, but how exposure is regulated in oviparous vertebrates is less clear. Recent work in oviparous vertebrates has demonstrated that steroids are not static molecules, as they can be converted to more polar steroid sulfates by sulfotransferase enzymes. Importantly, these steroid sulfates can be converted back to the parent compound by the enzyme steroid sulfatase (STS). We investigated when and where STS was present during embryonic development in the red-eared slider turtle, Trachemys scripta We report that STS is present during all stages of development and in all tissues we examined. We conclude that STS activity may be particularly important for regulating maternal steroid exposure in oviparous vertebrates.

Repeatability and sources of variation of the bacteria-killing assay in the common snapping turtle

Research on reptile ecoimmunology lags behind that on other vertebrates, despite the importance of such studies for conservation and evolution. Because the innate immune system is highly conserved across vertebrate lineages, assessments of its performance may be particularly useful in reptiles. The bacteria-killing assay requires a single, small blood sample and quantifies an individual's ability to kill microorganisms. The assay's construct validity and interpretability make it an attractive measure of innate immunity, but it requires proper optimization and sample storage. We optimized this assay for the common snapping turtle (Chelydra serpentina) to assess the repeatability of the assay and the effects of freezing and thawing on bactericidal capacity. We determined whether age (adult female and hatchlings) or incubation temperature influenced bactericidal capacity. We found that the assay was repeatable and that freezing plasma samples for 6 weeks at -80°C did not decrease bactericidal capacity nor did a single 30-min thaw and subsequent refreezing. However, we detected subtle interassay variation and results from one assay were 5-6% greater than those from the other two. Adult females had significantly greater bactericidal ability than hatchlings and we found no relationship between incubation temperature and bactericidal capacity. This assay is a useful tool in snapping turtles and may have applicability in other reptiles. However, species-specific optimization is required to ensure that variation among individuals exceeds interassay variation. Consideration should be given to optimization conditions that facilitate comparisons between or within groups, particularly groups that differ considerably in bactericidal capacity.

The effect of environmental temperature on reptilian peripheral blood B cell functions

Recent studies have identified phagocytic B cells in a variety of species, yet little is understood about their function and how it is influenced by natural environmental variation, such as temperature. Phagocytic B-cells are present in red-eared slider turtles, Trachemys scripta, and the wide range of temperatures experienced by these ectotherms may have an effect on immunity, including B cell antibody secretion and phagocytosis. We examined the impact of environmental temperature on B cell function in vitro using phagocytic and ELISpot assays conducted at biologically relevant temperatures. We found a significant effect of temperature on antibody secretion, with maximal antibody secretion occurring at intermediate temperatures (estimated maximum of 28.8°C). There was no effect of temperature on phagocytosis. We also noted a difference in the efficiency of phagocytosis in this assay between B cells and non-B cells. Interestingly, in our in vitro assay, phagocytic B cells engulfed more foreign fluorescent beads per cell than phagocytes lacking surface immunoglobulin. This work sheds light on our understanding of phagocytic B cells and the importance of environmental temperature on the behavior of reptilian immune cells, which may have relevance for organismal fitness.

Temperature Effects on Development and Phenotype in a Free-Living Population of Western Pond Turtles (Emys marmorata)

Changes in temperature regimes are occurring globally due to climate change as well as habitat alterations. Temperatures are expected to continue to rise in the future, along with a greater degree of climatic instability. Such changes could have potentially serious consequences for oviparous ectotherms, especially those with temperature-dependent sex determination. To investigate the effects of temperature on a range of developmental phenomena in a population of western pond turtles (Emys marmorata), we placed temperature sensors on top of each layer of eggs within nests and recorded temperatures hourly through the first 2-3 mo of incubation. These methods allowed us to look at in situ nest temperatures with high resolution. We found that mean incubation temperatures were similar between different nests and at different levels within nests but that incubation temperature fluctuations and maximum incubation temperatures differed greatly in both cases. The hatchling turtles were more likely to be female if they spent 30% or more of their sex-determining period of incubation above 29°C. Hatching success was best predicted by the maximum incubation temperature. We also found that incubation duration tended to be shorter as the mean temperature increased. However, exposure to either extremely high or low temperatures extended incubation times.

Temperature experienced during incubation affects antioxidant capacity but not oxidative damage in hatchling red-eared slider turtles (Trachemys scripta elegans)

Our understanding of how oxidative stress resistance phenotypes are affected by the developmental environment is limited. One component of the developmental environment, which is likely central to early life oxidative stress among ectothermic and oviparous species, is that of temperature. We investigated how incubation temperature manipulations affect oxidative damage and total antioxidant capacity (TAC) in red-eared slider turtle (Trachemys scripta elegans) hatchlings. First, to determine if temperature fluctuations elicit oxidative stress, eggs from clutches were randomly assigned to either a constant (29.5°C) or daily fluctuating temperature incubation (28.7±3°C) treatment. Second, to assess the effect of temperature fluctuation frequency on oxidative stress, eggs were incubated in one of three fluctuating incubation regimes; 28.7±3°C fluctuations every 12 (Hyper), 24 (Normal), or 48 hours (Hypo). Third, we tested the influence of average incubation temperature by incubating eggs in a daily fluctuating incubation temperature regime with a mean temperature of 26.5°C (Low), 27.1°C (Medium), or 27.7°C (High). Although the accumulation of oxidative damage in hatchlings was unaffected by any thermal manipulation, TAC was affected by both temperature fluctuation frequency and average incubation temperature. Individuals incubated with a low frequency of temperature fluctuations had reduced TAC, while incubation at a lower average temperature was associated with enhanced TAC. These results indicate that while sufficient to prevent oxidative damage, TAC is influenced by developmental thermal environments, potentially due to temperature mediated changes in metabolic rate. The observed differences in TAC may have important future consequences for hatchling fitness and overwinter survival.

Reproducing on time when temperature varies: shifts in the timing of courtship by fiddler crabs

Many species reproduce when conditions are most favorable for the survival of young. Numerous intertidal fish and invertebrates release eggs or larvae during semilunar, large amplitude, nocturnal tides when these early life stages are best able to escape predation by fish that feed near the shore during the day. Remarkably, some species, including the fiddler crabs Uca terpsichores and Uca deichmanni, maintain this timing throughout the year as temperature, and thus the rate of embryonic development, vary. The mechanisms that allow such precision in the timing of the production of young are poorly known. A preliminary study suggested that when temperature decreases, U. terpsichores mate earlier in the tidal amplitude cycle such that larvae are released at the appropriate time. We tested this idea by studying the timing of courtship in U. terpsichores and U. deichmanni as temperature varied annually during two years, at 5 locations that differed in the temperature of the sediment where females incubate their eggs. Uca terpsichores courted earlier at locations where sediment temperature declined seasonally but not where sediment temperature remained elevated throughout the year. In contrast, clear shifts in courtship timing were not observed for U. deichmanni despite variation in sediment temperature. We discuss other mechanisms by which this species may maintain reproductive timing. These two species are likely to be affected differently by changes in the frequency and intensity of cold periods that are expected to accompany climate change.

Constancy in an inconstant world: moving beyond constant temperatures in the study of reptilian incubation

Variable environmental conditions can alter the phenotype of offspring, particularly in ectothermic species such as reptiles. Despite this, the majority of studies on development in reptiles have been carried out under constant conditions in the laboratory, raising the question of just how applicable those investigations are to natural conditions? Here, we first review what we have learned from these constant-temperature studies. Second, we examine the importance of temperature fluctuations for development in reptiles and highlight the outcomes of studies conducted under fluctuating conditions. Next, we report our findings from a new study that examines how the frequency of fluctuations in temperature experienced during development affects phenotype. Finally, we suggest some areas in need of additional research so that we can better understand the complex interactions of temperature and physiology, particularly in species with temperature-dependent sex determination. For questions aimed at understanding the complex effects of the environment on phenotype, we must move toward studies that better capture environmental variation. By taking such an approach, it may be possible to predict more accurately how these thermally sensitive organisms will respond to environmental perturbations, including climatic change.

Daily temperature fluctuations unpredictably influence developmental rate and morphology at a critical early larval stage in a frog

Background

Environmental temperature has profound consequences for early amphibian development and many field and laboratory studies have examined this. Most laboratory studies that have characterized the influence of temperature on development in amphibians have failed to incorporate the realities of diel temperature fluctuations (DTF), which can be considerable for pond-breeding amphibians.

Results

We evaluated the effects of different ecologically relevant ranges of DTF compared with effects of constant temperatures on development of embryos and larvae of the Korean fire-bellied toad (Bombina orientalis). We constructed thermal reaction norms for developmental stage, snout- vent length, and tail length by fitting a Gompertz-Gaussian function to measurements taken from embryos after 66 hours of development in 12 different constant temperature environments between 14°C and 36°C. We used these reaction norms as null models to test the hypothesis that developmental effects of DTF are more than the sum of average constant temperature effects over the distribution of temperatures experienced. We predicted from these models that growth and differentiation would be positively correlated with average temperature at low levels of DTF but not at higher levels of DTF. We tested our prediction in the laboratory by rearing B. orientalis embryos at three average temperatures (20°C, 24°C, and 28°C) and four levels of thermal variation (0°C, 6°C, 13°C, and 20°C). Several of the observed responses to DTF were significantly different from both predictions of the model and from responses in constant temperature treatments at the same average temperatures. At an average temperature of 24°C, only the highest level of DTF affected differentiation and growth rates, but at both cooler and warmer average temperatures, moderate DTF was enough to slow developmental and tail growth rates.

Conclusions

These results demonstrate that both the magnitude of DTF range and thermal averages need to be considered simultaneously when parsing the effects of changing thermal environments on complex developmental responses, particularly when they have potential functional and adaptive significance.

No evidence that estrogens affect the development of the immune system in the red-eared slider turtle, Trachemys scripta

Exposure to maternally derived substances during development can affect offspring phenotype. In ovo exposure to maternally derived steroids has been shown to influence traits such as growth and behavior in the offspring. The development of the immune system also can be altered by exposure to both androgens and glucocorticoids in a variety of species, but much less is known about the potential for estrogens to influence the development of this system. We examined the effect of estradiol on the development of both innate and adaptive immune components in the red-eared slider turtle (Trachemys scripta). A bacterial killing assay was used to assess innate immunity, a delayed-type hypersensitivity test for cellular immunity, and total immunoglobulin levels to measure the humoral immune response. We found no effect of in ovo estradiol treatment on any of our immune measures despite using doses that are known to influence other phenotypic parameters during development and varying the timing of dosing across development. Our results suggest that maternally derived estradiol does not affect the development of the immune system in T. scripta.

Effects of night-time warming on temperate ectotherm reproduction: potential fitness benefits of climate change for side-blotched lizards

Temperate ectotherms, especially those at higher latitudes, are expected to benefit from climate warming, but few data yet exist to verify this prediction. Furthermore, most previous studies on the effects of climate change utilized a model of uniform annual change, which assumes that temperature increases are symmetric on diurnal or seasonal time scales. In this study, we simulated observed trends in the asymmetric alteration of diurnal temperature range by increasing night-time temperatures experienced by female lizards during their ovarian cycle as well as by the resulting eggs during their incubation. We found that higher night-time temperatures during the ovarian cycle increased the probability of reproductive success and decreased the duration of the reproductive cycle, but did not affect embryo stage or size at oviposition, clutch size, egg mass or relative clutch mass. Furthermore, higher incubation temperatures increased hatchling size and decreased incubation period but had no effect on incubation success. Subsequent hatchlings were more likely to survive winter if they hatched earlier, though our sample size of hatchlings was relatively small. These findings indicate that higher night-time temperatures mainly affect rate processes and that certain aspects of life history are less directly temperature dependent. As our findings confirm that climate warming is likely to increase the rate of development as well as advance reproductive phenology, we predict that warmer nights during the breeding season will increase reproductive output as well as subsequent survival in many temperate ectotherms, both of which should have positive fitness effects.

The lesser known challenge of climate change: thermal variance and sex-reversal in vertebrates with temperature-dependent sex determination

Climate change is expected to disrupt biological systems. Particularly susceptible are species with temperature-dependent sex determination (TSD), as in many reptiles. While the potentially devastating effect of rising mean temperatures on sex ratios in TSD species is appreciated, the consequences of increased thermal variance predicted to accompany climate change remain obscure. Surprisingly, no study has tested if the effect of thermal variance around high-temperatures (which are particularly relevant given climate change predictions) has the same or opposite effects as around lower temperatures. Here we show that sex ratios of the painted turtle (Chrysemys picta) were reversed as fluctuations increased around low and high unisexual mean-temperatures. Unexpectedly, the developmental and sexual responses around female-producing temperatures were decoupled in a more complex manner than around male-producing values. Our novel observations are not fully explained by existing ecological models of development and sex determination, and provide strong evidence that thermal fluctuations are critical for shaping the biological outcomes of climate change.

State-dependent physiological maintenance in a long-lived ectotherm, the painted turtle (Chrysemys picta)

Energy allocation among somatic maintenance, reproduction and growth varies not only among species, but among individuals according to states such as age, sex and season. Little research has been conducted on the somatic (physiological) maintenance of long-lived organisms, particularly ectotherms such as reptiles. In this study, we examined sex differences and age- and season-related variation in immune function and DNA repair efficiency in a long-lived reptile, the painted turtle (Chrysemys picta). Immune components tended to be depressed during hibernation, in winter, compared with autumn or spring. Increased heterophil count during hibernation provided the only support for winter immunoenhancement. In juvenile and adult turtles, we found little evidence for senescence in physiological maintenance, consistent with predictions for long-lived organisms. Among immune components, swelling in response to phytohemagglutinin (PHA) and control injection increased with age, whereas basophil count decreased with age. Hatchling turtles had reduced basophil counts and natural antibodies, indicative of an immature immune system, but demonstrated higher DNA repair efficiency than older turtles. Reproductively mature turtles had reduced lymphocytes compared with juvenile turtles in the spring, presumably driven by a trade-off between maintenance and reproduction. Sex had little influence on physiological maintenance. These results suggest that components of physiological maintenance are modulated differentially according to individual state and highlight the need for more research on the multiple components of physiological maintenance in animals of variable states.

Interactions among thermal parameters determine offspring sex under temperature-dependent sex determination

In many animals, temperatures experienced by developing embryos determine offspring sex (e.g. temperature-dependent sex determination, TSD), but most studies focus strictly on the effects of mean temperature, with little emphasis on the importance of thermal fluctuations. In the jacky dragon (Amphibolurus muricatus), an Australian lizard with TSD, data from nests in the field demonstrate that offspring sex ratios are predictable from thermal fluctuations but not from mean nest temperatures. To clarify this paradox, we incubated eggs in a factorial experiment with two levels of mean temperature and three levels of diel fluctuation. We show that offspring sex is determined by an interaction between these critical thermal parameters. Intriguingly, because these two thermal descriptors shift in opposing directions throughout the incubation season, this interactive effect inhibits seasonal shifts in sex ratio. Hence, our results suggest that TSD can yield offspring sex ratios that resemble those produced under genotypic sex-determining systems. These findings raise important considerations for understanding the diversity of TSD reaction norms, for designing experiments that evaluate the evolutionary significance of TSD, and for predicting sex ratios under past and future climate change scenarios.

Temperature fluctuations affect offspring sex but not morphological, behavioral, or immunological traits in the Northern Painted Turtle (Chrysemys picta)

Evolutionary theory predicts that when phenotypic variation arises during development that differently influences the fitness of each sex, selection should favor the maternal ability to match offspring phenotype to the sex that incurs a fitness benefit from that phenotype. In reptiles with temperature-dependent sex determination, the temperatures experienced during incubation can influence numerous phenotypic parameters, including sex. To mimic more naturalistic conditions, this experiment examined how variation in temperature fluctuations affects offspring sex as well as a suite of phenotypic parameters having putative fitness consequences in the Northern Painted Turtle ( Chrysemys picta (Schneider, 1783)). We also characterized variation in natural nest temperatures, including the daily temperature range, related to the vegetation cover surrounding the nest. We found that temperature fluctuations did not affect hatchling morphology, immune response, or behavior, but did significantly affect offspring sex ratios. Thermal profiles of natural nests were related to the amount of surrounding vegetation. Results suggest that nest-site choice by females could influence the sex of their offspring, but we found no evidence that variation in temperature fluctuations adaptively matches offspring sex and phenotype.

Protein expression and genetic structure of the coral Porites lobata in an environmentally extreme Samoan back reef: does host genotype limit phenotypic plasticity?

The degree to which coral reef ecosystems will be impacted by global climate change depends on regional and local differences in corals' susceptibility and resilience to environmental stressors. Here, we present data from a reciprocal transplant experiment using the common reef building coral Porites lobata between a highly fluctuating back reef environment that reaches stressful daily extremes, and a more stable, neighbouring forereef. Protein biomarker analyses assessing physiological contributions to stress resistance showed evidence for both fixed and environmental influence on biomarker response. Fixed influences were strongest for ubiquitin-conjugated proteins with consistently higher levels found in back reef source colonies both pre and post-transplant when compared with their forereef conspecifics. Additionally, genetic comparisons of back reef and forereef populations revealed significant population structure of both the nuclear ribosomal and mitochondrial genomes of the coral host (F(ST) = 0.146 P < 0.0001, F(ST) = 0.335 P < 0.0001 for rDNA and mtDNA, respectively), whereas algal endosymbiont populations were genetically indistinguishable between the two sites. We propose that the genotype of the coral host may drive limitations to the physiological responses of these corals when faced with new environmental conditions. This result is important in understanding genotypic and environmental interactions in the coral algal symbiosis and how corals may respond to future environmental changes.