Adaptation of sea turtles to climate warming: Will phenological responses be sufficient to counteract changes in reproductive output?

The Opposite Effects of Atrazine and Warming on the Reproductive Processes in Female Lizards (Eremias argus): Potential Roles of Hypothalamic-Pituitary-Gonadal Axis Regulation and Energy Metabolism

Declines in reptile populations due to climate warming and environmental pollution have been documented. Although recent ecotoxicological studies of reptiles have increased, little is known about how these two stressors interact to affect reptile reproductive processes. This study investigated the single and combined effects of atrazine and warming on reproduction in female lizards (Eremias argus) following 10 weeks of exposure to environmentally relevant concentrations of atrazine (0-10 mg·kg-1) at two temperature treatments (control or warming). Reproductive traits, clutch characteristics, and endpoints related to endocrine disruption (HPG axis gene expression) and energy metabolism (enzyme activity, hepatic metabolomics) were assessed. Atrazine inhibits female reproduction by disrupting HPG axis-related gene expression and energy metabolism, resulting in delayed spawning time and reduced fecundity. In contrast, warming promoted female reproduction and partly alleviated the inhibitory effects of atrazine, which is related to the upregulation of HPG axis-related gene expression and an additional energy metabolism compensatory response. Additionally, atrazine and/or warming altered the direction and intensity of the trade-off between egg number and size and affected maternal nutritional investment in eggs. These findings highlight the complex interplay of environmental stressors on lizard reproduction and add to a better understanding of reptile reproductive strategies and ecological consequences under environmental stress.

The effects of warming on loggerhead turtle nesting counts

Global trends in marine turtle nesting numbers vary by region, influenced by environmental or anthropogenic factors. Our study investigates the potential role of past temperature fluctuations on these trends, particularly whether warmer beaches are linked to increased nesting due to higher female production (since sea turtles have temperature-dependent sex determination). We selected the loggerhead turtle (Caretta caretta) due to its wide distribution, strong philopatry and vulnerability to environmental changes. We compiled nest counts per year on 35 globally significant rookeries, analysing trends at regional and individual beach levels. We compiled air (CHELSA) and land surface (MODIS) temperature data sets spanning the last four decades (1979-2023) for each location. To analyse temporal trends in nest counts and temperatures, we used generalised additive models and Mann-Kendall trend tests. Additionally, we correlated nest counts with lagged air temperature variables. We found significant warming at 33 nesting locations, 23 of which also showed significant increases in nest counts. Our results suggest that rising temperatures may be boosting nest numbers in regions of the Caribbean, Atlantic Ocean and Mediterranean (sites in Cayman, Mexico, Brazil, Cyprus and Turkey). Furthermore, while some regions temporarily benefit, continued warming could precipitate long-term population declines. This regional variability helps predict species responses to climate change, with the general global increase in nest counts already indicating short-term warming effects. Nesting count trends might reflect a combination of natural ecological phenomena, conservation efforts, and warming effects. Long-term studies are needed to assess global trends in the sex ratio of hatchlings and the extent to which feminisation is driving nest numbers.

A phenological shift to save the boys? Current and projected trends of hatchling sex ratio of the loggerhead turtle Caretta caretta at Dalyan Beach, Türkiye

Dalyan beach, Mugla, Türkiye hosts one of the largest loggerhead turtle rookeries in the Mediterranean. The sex of marine turtles, like many reptile species, is influenced by incubation temperature, with the threat of climate change looming increasing temperatures across the world may lead to an imbalance in the sex ratio of turtle populations. Over a ten-year study period (2012-2021), temperature dataloggers (n = 497) were placed during or the morning after ovipositioning. Using middle third of incubation as a proxy for the thermosensitive period and subsequent application of the Hill Equation, this study estimates the current and projected cohort sex ratios for Dalyan beach. The estimated overall male ratio over the ten-year study period was 25.7 % ± 23.3, with considerable interannual variation when data were compared from overlapping dates. Using the observed data, a GAM was built to predict nesting temperature using archived climate data, which explained 66.7 % of the variance. This model was applied to future projections of temperature using IPCC climate change scenario SSP 3-7.0, which resulted in a significant decrease in male ratio compared in the near term (2021-2040) 17.2 % ± 0.6 s d, mid-term (2041-2060) 14 % ± 0.5 s d and far term (2081-2100) 10.7 %. A hypothetical 10-day shift of nesting phenology would quell the effects of warming and maintain or increase current male ratio in the near term 25.6 % ± 0.8 s d. A 20-day shift would have the same effect in the near term (37.3 % ± 0.9 s d) and mid-term (31.4 % ± 0.9 s d) projections. These nesting grounds are important to the sustained survival of the species and while this study indicates restorative potential to the sex imbalance, a reliance on the development of such a phenological shift is less than favourable. While climate change projections vary between models, a situation that gives enough buffering time is unlikely, and feminisation of the population seems inevitable without further action.

Individual plasticity in response to rising sea temperatures contributes to an advancement in green turtle nesting phenology

Phenological changes (i.e. shifts in the timing of biological events) are among the most frequently reported population-level responses to climate change and are often assumed to be adaptive and increase population viability. These may be driven by both individual-level phenotypic plasticity and population-level evolutionary and demographic changes. However, few studies have explored how individual-level versus population-level processes drive phenological trends. Using a 31-year dataset of over 600 individually marked nesting green turtles (Chelonia mydas), we quantify the population- and individual-level temporal trend in their first nest date. Of the latter, approximately 30% is attributable to individual phenological plasticity in response to sea surface temperature, with females advancing their nesting by 6.47 days for every degree (Celsius) increase. The remaining change is almost entirely explained by individual- and population-level changes in size and breeding experience (correlates of age), as well as the number of clutches laid per season. This is the first study of individual-level phenological change in a marine ectotherm, furthering our understanding of how this and similar species may respond to rising temperatures.

High thermal tolerance of egg clutches and potential adaptive capacity in green turtles

Climate warming threatens sea turtles, among other effects, because high temperatures increase embryo mortality. However, not all species and populations are expected to respond the same way because they could have different thermal tolerances and capacities to adapt. We tested the effect of incubation temperature on egg mortality in a population of green turtles (Chelonia mydas) previously suggested to be less affected by extreme climatic events than others. We (1) assessed the relationship between temperature and hatching success, (2) defined an optimal range of temperatures that maximized hatching success and (3) assessed the variability in the response to temperature among clutches laid by different mothers, which could allow adaptation. Hatching success was consistently high in green turtle clutches with a skew toward high values, with 50 % of clutches having a success above 94 %. Yet, it was mildly affected by temperature, declining at both low and high temperatures. The optimal range of mean incubation temperatures was between ~30.5 °C and 32.5 °C. Current mean temperatures (31.3 °C) fall within the middle of the optimal range, indicating a potential resilience to further rises in mean nest temperature. Hatching success was best described by nest temperature and the interaction between female identity and temperature. This last predictor indicated a variability in thermal tolerance among clutches laid by different mothers and therefore, a capacity to adapt. The studied population of green turtles seems to be less vulnerable than others to climate warming. Understanding how different populations could respond to increasing temperatures could help complete the picture on the potential effects of climate change on sea turtles.

Re-Equilibrating Sex Ratios: Adjustment of Reaction Norms in Species With Temperature-Dependent Sex Determination

Fisher's general principle for sex allocation holds that population sex ratios are typically balanced because parents producing the rare sex are benefited and the rare sex alternates over time. In species that have temperature-dependent sex determination (TSD), thermal reaction norms need to be adjusted at the population level to avoid extremely biased sex ratios and extinction. Extant species with TSD experienced drastic climatic changes in the geological past and must necessarily have mechanisms of adaptation. I propose here a conceptual framework to explain how TSD curves could be adjusted by means of natural selection, based on Fisher's equilibrium sex-ratio principle. Through a process that alternatively favors mothers that tend to produce the rare sex under new temperatures, sex ratios eventually return toward a theoretical equilibrium. Prerequisites for this model are variability among mothers in the tendency to produce a particular sex at a given temperature (i.e., variability in the thermal reaction norm), inheritance of this trend, and higher fitness of the rare sex. This straightforward mechanism could facilitate thermal adaptation in species with TSD over multiple generations.

Cropland Microclimate and Leaf-nesting Behavior Shape the Growth of Caterpillar under Future Warming

Predicting performance responses of insects to climate change is crucial for biodiversity conservation and pest management. While most projections on insects' performance under climate change have used macro-scale weather station data, few incorporated the microclimates within vegetation that insects inhabit and their feeding behaviors (e.g., leaf-nesting: building leaf nests or feeding inside). Here, taking advantage of relatively homogenous vegetation structures in agricultural fields, we built microclimate models to examine fine-scale air temperatures within two important crop systems (maize and rice) and compared microclimate air temperatures to temperatures from weather stations. We deployed physical models of caterpillars and quantified effects of leaf-nesting behavior on operative temperatures of two Lepidoptera pests: Ostrinia furnacalis (Pyralidae) and Cnaphalocrocis medinalis (Crambidae). We built temperature-growth rate curves and predicted the growth rate of caterpillars with and without leaf-nesting behavior based on downscaled microclimate changes under different climate change scenarios. We identified widespread differences between microclimates in our crop systems and air temperatures reported by local weather stations. Leaf-nesting individuals in general had much lower body temperatures compared to non-leaf-nesting individuals. When considering microclimates, we predicted leaf-nesting individuals grow slower compared to non-leaf-nesting individuals with rising temperature. Our findings highlight the importance of considering microclimate and habitat-modifying behavior in predicting performance responses to climate change. Understanding the thermal biology of pests and other insects would allow us to make more accurate projections on crop yields and biodiversity responses to environmental changes.

Potential drivers and implications of a balanced breeding sex ratio in a small population of an imperiled species with environmental sex determination

Small populations of imperiled species are susceptible to the negative consequences of skewed sex-ratios. In imperiled species with environmental sex determination such as sea turtles, examining sex ratios across a range of environments and population abundance levels can provide insight into factors that influence population resilience, which can then be the foci of management plans for these species. Breeding sex ratios (the ratio of actively breeding males to females during a reproductive season; BSRs) extrapolated from genetic parentage analyses are a common approach for enumerating sex ratios in sea turtles. Such analyses also allow for the characterization of multiple paternity within sea turtle clutches, which should reflect BSRs and breeding behaviors. We characterized the first BSR for a breeding assemblage of loggerhead sea turtles (Caretta caretta) belonging to the temperate, low-abundance Northern Gulf of Mexico Recovery Unit using genotypes of 16 microsatellite loci from nesting females and hatchlings. Unlike prior studies at both more-tropical and more-temperate, and higher-abundance, Recovery Units in this region, we found a balanced BSR of 1.3:1 males:female and a low incidence (~17%) of multiple paternity. This suggests that there are relatively few males breeding at this assemblage and within this Recovery Unit. Beaches in this region are expected to produce substantial numbers of male hatchlings based on sand temperature data. The relative dearth of mature males may then be due to hydrologic disturbances that disproportionately affect the fitness and survival of male hatchlings, or due to demographic stochasticity. More work is needed to study the factors that might influence male hatchling production and fitness in this region, particularly as climate change is predicted to lead to feminization in global sea turtle populations. Our work demonstrates the broad utility of characterizing BSRs and other sex ratios across a range of populations in imperiled, environmentally sensitive species.

Variability in thermal tolerance of clutches from different mothers indicates adaptation potential to climate warming in sea turtles

The current climate warming is a challenge to biodiversity that could surpass the adaptation capacity of some species. Hence, understanding the means by which populations undergo an increase in their thermal tolerance is critical to assess how they could adapt to climate warming. Specifically, sea turtle populations could respond to increasing temperatures by (1) colonizing new nesting areas, (2) nesting during cooler times of the year, and/or (3) by increasing their thermal tolerance. Differences in thermal tolerance of clutches laid by different females would indicate that populations have the potential to adapt by natural selection. Here, we used exhaustive information on nest temperatures and hatching success of leatherback turtle (Dermochelys coriacea) clutches over 14 years to assess the occurrence of individual variability in thermal tolerance among females. We found an effect of temperature, year, and the interaction between female identity and nest temperature on hatching success, indicating that clutches laid by different females exhibited different levels of vulnerability to high temperatures. If thermal tolerance is a heritable trait, individuals with higher thermal tolerances could have greater chances of passing their genes to following generations, increasing their frequency in the population. However, the high rate of failure of clutches at temperatures above 32°C suggests that leatherback turtles are already experiencing extreme heat stress. A proper understanding of mechanisms of adaptation in populations to counteract changes in climate could greatly contribute to future conservation of endangered populations in a rapidly changing world.

Spatial and life history variation in a trait-based species vulnerability and impact model

Anthropogenic pressures threaten biodiversity, necessitating conservation actions founded on robust ecological models. However, prevailing models inadequately capture the spatiotemporal variation in environmental pressures faced by species with high mobility or complex life histories, as data are often aggregated across species' life histories or spatial distributions. We highlight the limitations of static models for dynamic species and incorporate life history variation and spatial distributions for species and stressors into a trait-based vulnerability and impact model. We use green sea turtles in the Greater Caribbean Region to demonstrate how vulnerability and anthropogenic impact for a dynamic species change across four life stages. By incorporating life stages into a trait-based vulnerability model, we observed life stage-specific vulnerabilities that were otherwise unnoticed when using an aggregated trait value set. Early life stages were more vulnerable to some stressors, such as inorganic pollution or marine heat waves, and less vulnerable to others, such as bycatch. Incorporating spatial distributions of stressors and life stages revealed impacts differ for each life stage across spatial areas, emphasizing the importance of stage-specific conservation measures. Our approach showcases the importance of incorporating dynamic processes into ecological models and will enable better and more targeted conservation actions for species with complex life histories and high mobility.

Influence of incubation temperature, maternal effects, and paternity on quality of olive ridley hatchlings (Lepidochelys olivacea) from a mass-nesting beach in the Mexican Pacific

Future climate change scenarios project that the increase in surface temperatures will affect ocean temperatures, inducing shifts in marine biodiversity. Sea turtles are species that are particularly vulnerable to the effects of climate change because temperature is a factor that influences embryonic development. We collected clutches of olive ridley turtles from a mass-nesting beach in the Mexican Pacific, which were incubated in ex situ conditions. When the hatchlings emerged, we measured the body condition index-which evaluates the weight-length relationship-and swim thrust, both were considered traits associated with fitness, termed "fitness proxies," and evaluated the effects of incubation temperature, maternal effects, and paternity on these fitness proxies. The body condition index was correlated positively and significantly with the arribada month and temperature during the last third of the incubation period but showed an inverse relationship with the maternal effect. While swim thrust was positively correlated with the maternal effect and the arribada month, there was an inverse relationship with incubation temperature during the first third of the period. Paternity, whether single or multiple, did not have a significant effect on either fitness proxies; however, it may have effects on the average fitness of a population of hatchlings. These results underscore the need to expand research on the sublethal effects of high incubation temperatures on the adaptation and survival of sea turtles, particularly in scenarios of rapid climate change.