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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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DOE OSTI2020

Spatial and temporal variation in nest temperatures forecasts sex ratio skews in a crocodilian with environmental sex determination

Species displaying temperature-dependent sex determination (TSD) are especially vulnerable to the effects of a rapidly changing global climate due to their profound sensitivity to thermal cues during development. Predicting the consequences of climate change for these species, including skewed offspring sex ratios, depends on understanding how climatic factors interface with features of maternal nesting behaviour to shape the developmental environment. Here, we measure thermal profiles in 86 nests at two geographically distinct sites in the northern and southern regions of the American alligator's ( Alligator mississippiensis ) geographical range, and examine the influence of both climatic factors and maternally driven nest characteristics on nest temperature variation. Changes in daily maximum air temperatures drive annual trends in nest temperatures, while variation in individual nest temperatures is also related to local habitat factors and microclimate characteristics. Without any compensatory nesting behaviours, nest temperatures are projected to increase by 1.6–3.7°C by the year 2100, and these changes are predicted to have dramatic consequences for offspring sex ratios. Exact sex ratio outcomes vary widely depending on site and emission scenario as a function of the unique temperature-by-sex reaction norm exhibited by all crocodilians. By revealing the ecological drivers of nest temperature variation in the American alligator, this study provides important insights into the potential consequences of climate change for crocodilian species, many of which are already threatened by extinction.

Environmental Sciences & Ecology↗

DOE OSTI2023

Relationships between maternally-transferred mercury and hatchling development, behavior, and survival in the American alligator ( Alligator mississippiensis )

Mercury is a toxic and pervasive environmental contaminant that can be transferred from mother to offspring during development. Consequences of maternally-transferred mercury have been observed in vertebrate taxa, including reduced clutch viability, reduced offspring size, and behavioral alterations. These sublethal effects have been assumed to decrease survivorship, though this is seldom assessed. Here, we examined how maternally-transferred mercury interacts with incubation temperature to influence reproductive success, offspring behavior, and subsequent survival in the American alligator (Alligator mississippiensis). We collected nine clutches of eggs from a mercury contaminated reservoir on the Savannah River Site, South Carolina, and incubated eggs at either female- or male-promoting temperatures. Clutch-averaged mercury in egg yolk was high relative to other studies in crocodilians and ranged from 0.248 to 0.554 ppm compared to 0.018–0.052 ppm at a site with low levels of mercury contamination; mercury levels in hatchling blood ranged from 0.090 to 0.490 ppm ($\overline{x}$= 0.240 ppm, n = 158). We found few, mostly negligible correlations between life history traits and mercury but noted a positive relationship with egg mass, possibly mediated by correlated maternal effects such as resource provisioning. Incubation temperature exerted strong effects on hatchling phenotypes, with warmer, male-promoting temperatures producing larger and bolder hatchlings. Presumptive females, produced from cooler incubation temperatures, spent more time in warm areas during behavior trials. Hatchlings were released 10–15 days post-hatch and surveyed over eight months to assess survival. Survivorship was positively correlated with hatchling size and negatively correlated with proportional time spent in warm areas. Presumptive females had much lower survival, and overall survivorship for the eight-month period was 0.185–0.208, depending on the modelling approach. Finally, our study suggests that, within the range of concentrations we observed, incubation temperature has a stronger effect on offspring behavior and survival than maternally-transferred mercury pollution in American alligators.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2021

Evolution of eggshell structure in relation to nesting ecology in non-avian reptiles

Amniotic eggs are multifunctional structures that enabled early tetrapods to colonize the land millions of years ago, and are now the reproductive mode of over 70% of all terrestrial amniotes. Eggshell morphology is at the core of animal survival, mediating the interactions between embryos and their environment, and has evolved into a massive diversity of forms and functions in modern reptiles. These functions are critical to embryonic survival and may serve as models for new antimicrobial and/or breathable membranes. However, we still lack critical data on the basic structural and functional properties of eggs, particularly of reptiles. Here, we first characterized egg shape, shell thickness, porosity, and mineralization of eggs from 91 reptile species using optical images, scanning electron microscopy, and micro computed tomography, and collected data on nesting ecology from the literature. We then used comparative analyses to test hypotheses on the selective pressures driving their evolution. We hypothesized that eggshell morphology has evolved to protect shells from physical damage and desiccation, and, in support, found a positive relationship between thickness and precipitation, and a negative relationship between porosity and temperature. Although mineralization varied extensively, it was not correlated with nesting ecology variables. Ancestral state reconstructions show thinning and increased porosity over evolutionary time in squamates, but the opposite in turtles and crocodilians. Egg shape, size, porosity and calcification were correlated, suggesting potential structural or developmental tradeoffs. This study provides new data and insights into the morphology and evolution of reptile eggs, and raises numerous questions for additional research.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2023

Differential early–life survival underlies the adaptive significance of temperature–dependent sex determination in a long–lived reptile

1. Many ectotherms rely on temperature cues experienced during development to determine offspring sex. The first descriptions of temperature-dependent sex determination (TSD) were made over 50 years ago, yet an understanding of its adaptive significance remains elusive, especially in long-lived taxa. 2. One novel hypothesis predicts that TSD should be evolutionarily favoured when two criteria are met—(a) incubation temperature influences annual juvenile survival and (b) sexes mature at different ages. Under these conditions, a sex-dependent effect of incubation temperature on offspring fitness arises through differences in age at sexual maturity, with the sex that matures later benefiting disproportionately from temperatures that promote juvenile survival. 3. The American alligator (Alligator mississippiensis) serves as an insightful model in which to test this hypothesis, as males begin reproducing nearly a decade after females. Here, through a combination of artificial incubation experiments and mark-recapture approaches, we test the specific predictions of the survival-to-maturity hypothesis for the adaptive value of TSD by disentangling the effects of incubation temperature and sex on annual survival of alligator hatchlings across two geographically distinct sites. 4. Hatchlings incubated at male-promoting temperatures (MPTs) consistently exhibited higher survival compared to those incubated at female-promoting temperatures. This pattern appears independent of hatchling sex, as females produced from hormone manipulation at MPT exhibit similar survival to their male counterparts. 5. Additional experiments show that incubation temperature may affect early-life survival primarily by affecting the efficiency with which maternally transferred energy resources are used during development. 6. Results from this study provide the first explicit empirical support for the adaptive value of TSD in a crocodilian and point to developmental energetics as a potential unifying mechanism underlying persistent survival consequences of incubation temperature.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2022

Tissue Distribution of Mercury in the Bodies of Wild American Alligators ( Alligator mississippiensis ) from a Coastal Marsh in Louisiana (USA)

Total mercury (THg) concentrations were measured in wild alligators inhabiting a coastal marsh in southern Louisiana, to determine the tissue distribution of THg among various body organs and tissue compartments. Concentrations of THg in claws and dermal tail scutes were compared to those in blood, brain, gonad, heart, kidney, liver, and skeletal muscle to determine if the former tissues, commonly available by non-lethal sampling, could be used as measures of body burdens in various internal organs. Mercury was found in all body organs and tissue compartments. However, overall, THg concentrations measured in alligators were below the FDA action level for fish consumption and were comparable to previous data reported from southwestern Louisiana. Here, our results suggest consumption of meat from alligators found in this region may be of little public health concern. However, the extended period of time between sampling (in this study) and the present-day highlight the need for continuous, additional, and more recent sampling to ensure consumer safety. Total mercury concentrations were highest in the kidney (3.18 ± 0.69 mg/kg dw) and liver (3.12 ± 0.76 mg/kg dw). THg levels in non-lethal samples (blood, claws, and dermal tail scutes) were positively correlated with all tissue THg concentrations (blood: R 2 = 0.513 – 0.988; claw: R 2 = 0.347 – 0.637, scutes: R 2 = 0.333 – 0.649). Because THg concentrations from blood, claws, and scutes were correlated with those of the internal organs, non-lethal sampling methods may be a viable method of estimating levels of THg in other body tissues.

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗

DOE OSTI2023

Maternal provisioning interacts with incubation temperature to affect hatchling mercury exposure in an oviparous reptile

The thermal environment experienced by developing embryos can influence the utilization of maternally provisioned resources. Despite being particularly consequential for oviparous ectotherms, these dynamics are largely unexplored within ecotoxicological frameworks. Here, we test if incubation temperature interacts with maternally transferred mercury to affect subsequent body burdens and tissue distributions of mercury in hatchling American alligators (Alligator mississippiensis). Nine clutches of alligator eggs were collected from a mercury-contaminated reservoir and incubated at either female- or male-promoting temperatures. Total mercury (THg) concentration was measured in egg yolk collected during incubation and in a suite of tissues collected from hatchlings. THg concentrations in residual yolk and blood were higher in hatchlings incubated at cooler, female-promoting temperatures compared to the warmer, male-promoting temperatures. THg concentrations in most tissues were positively correlated with THg concentrations in blood and dermis, and egg yolk THg concentration was the best predictor of THg concentration in many resultant tissues. Furthermore, our results highlight a hereto unknown role of the developmental environment in mediating tissue specific uptake of contaminants in an oviparous reptile.

59 BASIC BIOLOGICAL SCIENCES↗