Evaporative Water Loss Research Articles

Cuticular hydrocarbons promote desiccation resistance by preventing transpiration in D. melanogaster.

Desiccation is a fundamental challenge confronted by all terrestrial organisms, particularly insects. With a relatively small body size and large surface-to-volume ratio insects are susceptible to rapid evaporative water loss and dehydration. To counter these physical constraints, insects have acquired specialized adaptations, including a hydrophobic cuticle that acts as a physical barrier to transpiration. We previously reported that genetic ablation of the oenocytes - specialized cells required to produce cuticular hydrocarbons (HCs) - significantly reduced survivorship under desiccative conditions in the fruit fly, Drosophila melanogaster. Although increased transpiration - resulting from the loss of the oenocytes and HCs - was hypothesized to be responsible for the decrease in desiccation survival, this possibility was not directly tested. Here we investigate the underlying physiological mechanisms contributing to the reduced survival of oenocyte-less (oe-) flies. Using flow-through respirometry we show that oe- flies, regardless of sex, exhibited an increased rate of transpiration relative to wild-type controls, and that coating oe- flies with fly-derived HC extract restored the rate to near wild-type levels. Importantly, total body water stores, including metabolic water reserves, as well as dehydration tolerance, measured as the percent of total body water lost at time of death, were largely unchanged in oe- flies. Together, our results directly demonstrate the critically important role played by the oenocytes and cuticular HCs to promote desiccation resistance.

Physiological costs of warning: Defensive hissing increases metabolic rate and evaporative water loss in a venomous snake

To minimize predation risk and the cost of confronting predators, prey have developed a range of defensive strategies and warning signals. Although advantageous, defensive warnings may also induce physiological and energy costs to the emitter. Ventilatory sounds (hissing) are the most distributed warning sound in vertebrates. Because they involve the respiratory apparatus, defensive hissing may substantially increase evaporative water loss. Herein, we examined the determinants of hissing as well as its physiological costs in a medium-sized venomous snake, the long-nosed viper (Vipera ammodytes). We first used a neutral arena and applied standardized stimulation to measure the occurrence and acoustic characteristics of warning hissing. Then, we used open-flow respirometry to quantify changes in respiratory gas exchanges (oxygen consumption and evaporative water loss) during defensive responses. We demonstrated that males are more likely to engage in sound warnings when stimulated. Expirations generated the strongest signals compared to inspiration but did not differ between sexes. We found that defensive hissing dramatically increased average metabolic rate and evaporative water loss during the 10-minute stimulation period, and this effect was more pronounced in males. Metabolic rates and evaporative water loss were closely related to the duration of hissing. Overall, our results indicate that respiratory-based warning sounds induce significant physiological costs and may alter water balance. The higher responsiveness in males than females likely reflects sexually selective pressure (higher mobility for mate acquisition) and enhanced risk exposure.

The Pivotal Role of Evaporation in Lake Water Isotopic Variability Across Space and Time in a High Arctic Periglacial Landscape

AbstractRapidly changing climate is disrupting the High Arctic's water systems. As tracers of hydrological processes, stable water isotopes can be used for high quality monitoring of Arctic waters to better reconstruct past changes and assess future environmental threats. However, logistical challenges typically limit the length and scope of isotopic monitoring in High Arctic landscapes. Here, we present a comprehensive isotopic survey of 535 water samples taken in 2018 and 2019 of the lakes and other surface waters of the periglacial Pituffik Peninsula in far northwest Greenland. The δ18O, δ2H, and deuterium‐excess values of these samples, representing 196 unique sites, grant unprecedented insight into the environmental drivers of the regional hydrology and water isotopic variability. We find that the spatial variability of lake water isotopes can best be explained through evaporation and the hydrological ability of a lake to replace evaporative water losses with precipitation and snowmelt. Temporally, summer‐long evaporation can drive lake water isotopes beyond the isotopic range observed in precipitation, and wide interannual changes in lake water isotopes reflect annual weather differences that influenced evaporation. Following this, water isotope samples taken at individual times or sites in similar periglacial landscapes may have limited regional representativeness, and increasing the spatiotemporal extent of isotopic sampling is critical to producing accurate and informative High Arctic paleoclimate reconstructions. Overall, our survey highlights the diversity of isotopic compositions in Pituffik surface waters, and our complete isotopic and geospatial database provides a strong foundation for future researchers to study hydrological changes at Pituffik and across the Arctic.

Seasonal and between-population variation in heat tolerance and cooling efficiency in a Mediterranean songbird

Discrete populations of widely distributed species may inhabit areas with marked differences in climatic conditions across geographic and seasonal scales, which could result in intraspecific variation in thermal physiology reflecting genetic adaptation, phenotypic plasticity, or both. However, few studies have evaluated inter-population variation in physiological responses to heat. We evaluated within- and inter-population seasonal variation in heat tolerance, cooling efficiency and other key thermoregulatory traits in two Mediterranean populations of Great tit Parus major experiencing contrasting thermal environments: a lowland population subject to hotter summers and a higher annual thermal amplitude than a montane population. Specifically, we measured heat tolerance limits (HTL), body temperature, resting metabolic rate, evaporative water loss, and evaporative cooling efficiency (the ratio between evaporative heat loss to metabolic heat production) within and above the thermoneutral zone during winter and summer. Heat tolerance during summer was greater in lowland than in montane birds; indeed, lowland birds seasonally increased this trait to a significant level, while montane ones did to a lesser extent. Besides, lowland birds showed greater evaporative cooling efficiency during summer (possibly due in part to reductions in total endogenous heat load), while surprisingly montane ones showed the opposite trend. Thus, lowland birds displayed greater seasonal flexibility in HTL, body temperature and resting metabolic rate above thermoneutrality, thus giving some support to the climatic variability hypothesis — that flexibility in thermoregulatory traits should increase with climatic variability. Our results partially support the idea that songbirds’ adaptive thermoregulation in the heat is flexible, highlighting the importance of considering intraspecific variation in thermoregulatory traits when modelling the future distribution and persistence of species under different climate change scenarios.

Quantifying water evaporation from large reservoirs: Implications for water management in water-stressed regions

Dam reservoirs are at the core of local water storage and supply, especially in water-stressed regions of the world with acute water shortage problems. However, evaporative losses from these reservoirs and their storage efficiency are often overlooked in water budgeting. We offer a mechanistic approach that combines physically-based modeling with remote sensing information of reservoir characteristics to reliably predict evaporative losses from dam reservoirs. The developed framework is used to predict evaporative water losses from potential dam reservoirs in different basins worldwide. We apply this framework to 10 of the largest dam reservoirs in the world's water-stressed regions to quantify evaporative water losses. Our analysis, spanning from 2000 to 2020, reveals considerable variations in annual evaporation rates in the reservoirs located in water-deprived regions exceeding 3200 mm/year during the study period with the total evaporative loss reaching 26.5 km3/year. The evaporative water loss accounts up to 15.8% of the storage capacity in one of the dam reservoirs, posing significant challenges for water allocation and conservation strategies, with notable economic and environmental consequences in regions already suffering from water scarcity.

Sexual color ornamentation, microhabitat choice, and thermal physiology in the common wall lizard (Podarcis muralis).

Common wall lizards (Podarcis muralis) in Italy show a striking variation in body coloration across the landscape, with highly exaggerated black and green colors in hot and dry climates and brown and white colors in cool and wet climates. Males are more intensely colored than females, and previous work has suggested that the maintenance of variation in coloration across the landscape reflects climatic effects on the strength of male-male competition, and through this sexual selection. However climatic effects on the intensity of male-male competition would need to be exceptionally strong to fully explain the geographic patterns of color variation. Thus, additional processes may contribute to the maintenance of color variation. Here we test the hypothesis that selection for green and black ornamentation in the context of male-male competition is opposed by selection against ornamentation because the genes involved in the regulation of coloration have pleiotropic effects on thermal physiology, such that ornamentation is selected against in cool climates. Field observations revealed no association between body coloration and microhabitat use or field active body temperatures. Consistent with these field data, lizards at the extreme ends of the phenotypic distribution for body coloration did not show any differences in critical minimum temperature, preferred body temperature, temperature-dependent metabolic rate, or evaporative water loss when tested in the laboratory. Combined, these results provide no evidence that genes that underlie sexual ornamentation are selected against in cool climate because of pleiotropic effects on thermal biology.

Effects of lead on avian thermoregulation in the heat: An experimental test with pied crows (Corvus albus)

Many of the negative physiological effects of lead involve the hypothalamus, but the possibility that thermoregulation is affected has received little attention. We tested the hypothesis that lead exposure reduces avian thermoregulatory performance under hot conditions in pied crows (Corvus albus) experimentally exposed to lead in their diet. Crows in our high lead treatment (blood [Pb] = 87.3 ± 44.7 μg dL−1) showed significantly higher air temperature (Tair) inflections for evaporative water loss (EWL) and resting metabolic rate (RMR) compared to control (6.4 ± 1.8 μg dL−1) or intermediate (53.9 ± 23.7 μg dL−1) lead groups, which did not differ. EWL, RMR and body temperature (Tb) all increased more rapidly at Tair >Tb in the high lead treatment. In contrast, neither maximum Tair tolerated by the crows nor maximum Tb varied with treatment. Our data reveal that water and energy balance during hot weather is affected by lead exposure.

Bat thermoregulation in the heat: seasonal variation in evaporative cooling capacities in four species of European bats

Phenotypic flexibility is an important source of physiological variation in endotherms and plays an integral role in species’ response to rapid environmental changes. Studies of phenotypic flexibility have focused on winter acclimatization and cold endurance, and there are fewer data on summer acclimatization and adjustments in heat dissipation capacity, especially in Temperate-Zone species. We used indirect calorimetry and thermometry to test if thermoregulation at high air temperatures (Ta) varies between spring and summer in four species of European vespertilionid bats: Nyctalus noctula, Pipistrellus nathusii, P. pygmaeus, and P. pipistrellus. We measured subcutaneous body temperature (Tsub), evaporative water loss, and resting metabolic rate while exposing bats to a stepped profile of increasing Ta, from 28 °C–48 °C. We predicted that during summer, bats increase heat tolerance and evaporative cooling capacity, to better tolerate hotter Tas. In contrast, we found lower maximum ratios of evaporative heat loss (EHL) to metabolic heat production (MHP) during summer, but no seasonal differences in maximum Ta tolerated or Tsub. The main cause of this seasonal difference in maximum EHL/MHP seems to be from bats increasing EWL more gradually with increasing Ta in summer than spring, particularly in the smaller Pipistrellus species. Therefore, this seasonal variation in heat-dissipation strategies may reflect enhanced water conservation during summer to avoid dehydration, as bats are confined to roosts for longer and hotter days compared to spring.

The heat is on: Thermoregulatory and evaporative cooling patterns of desert-dwelling bats

For small endotherms inhabiting desert ecosystems, defending body temperatures (Tb) is challenging as they contend with extremely high ambient temperatures (Ta) and limited standing water. In the arid zone, bats may thermoconform whereby Tb varies with Ta, or may evaporatively cool themselves to maintain Tb < Ta. We used an integrative approach that combined both temperature telemetry and flow through respirometry to investigate the ecological and physiological strategies of lesser long-eared bats (Nyctophilus geoffroyi) in Australia's arid zone. We predicted individuals would exhibit desert-adapted thermoregulatory patterns (i.e., thermoconform to prioritise water conservation), and that females would be more conservative with their water reserves for evaporative cooling compared to males. Temperature telemetry data indicated that free-ranging N. geoffroyi were heterothermic (Tskin = 18.9–44.9 °C) during summer and thermoconformed over a wide range of temperatures, likely to conserve water and energy during the day. Experimentally, at high Tas, females maintained significantly lower Tb and resting metabolic rates, despite lower evaporative water loss (EWL) rates compared to males. Females only increased EWL at experimental Ta = 42.5 °C, significantly higher than males (40.7 °C), and higher than any bat species yet recorded. During the hottest day of this study, our estimates suggest the water required for evaporative cooling ranged from 18.3% (females) and 25.5% (males) of body mass. However, if we extrapolate these results to a recent heatwave these values increase to 36.5% and 47.3%, which are likely beyond lethal limits. It appears this population is under selective pressures to conserve water reserves and that these pressures are more pronounced in females than males. Bats in arid ecosystems are threatened by both current and future heatwaves and we recommend future conservation efforts focus on protecting current roost trees and creating artificial standing water sites near vulnerable populations.

The effects of humidity on thermoregulatory physiology of a small songbird.

Scholander-Irving curves describe the relationship between ambient temperature and metabolic rate and are fundamental to understanding the energetic demands of homeothermy. However, Scholander-Irving curves are typically measured in dry air, which is not representative of the humidity many organisms experience in nature. Consequently, it is unclear (1) whether Scholander-Irving curves (especially below thermoneutrality) are altered by humidity, given the effects of humidity on thermal properties of air, and (2) whether physiological responses associated with Scholander-Irving curves in the lab reflect organismal performance in humid field conditions. We used laboratory experiments and biophysical models to test the effects of humidity on the thermoregulatory physiology of tree swallows (Tachycineta bicolor). We also tested whether physiological responses measured under lab conditions were correlated with field body temperatures and nestling provisioning rates. We found that humidity reduced rates of evaporative water loss but did not have large effects on body temperature or metabolic rate, suggesting that swallows can decouple evaporative cooling, body temperature and metabolic rate. Although the effect of humidity on metabolic rate in the lab was small, our biophysical models indicated that energetic costs of thermoregulation were ∼8% greater in simulations that used metabolic rates from birds in humid compared with dry conditions. Finally, we found mixed evidence that physiological responses measured in the lab under humid or dry conditions were associated with body temperature and nest provisioning rates in the field. Our results help clarify the effect of humidity on endotherm thermoregulation, which may help forecast organismal responses to environmental change.

Reporting guidelines for terrestrial respirometry: Building openness, transparency of metabolic rate and evaporative water loss data

Respirometry is an important tool for understanding whole-animal energy and water balance in relation to the environment. Consequently, the growing number of studies using respirometry over the last decade warrants reliable reporting and data sharing for effective dissemination and research synthesis. We provide a checklist guideline on five key sections to facilitate the transparency, reproducibility, and replicability of respirometry studies: 1) materials, set up, plumbing, 2) subject conditions/maintenance, 3) measurement conditions, 4) data processing, and 5) data reporting and statistics, each with explanations and example studies. Transparency in reporting and data availability has benefits on multiple fronts. Authors can use this checklist to design and report on their study, and reviewers and editors can use the checklist to assess the reporting quality of the manuscripts they review. Improved standards for reporting will enhance the value of primary studies and will greatly facilitate the ability to carry out higher quality research syntheses to address ecological and evolutionary theories.

No evidence for the melanin desiccation hypothesis in a larval Lepidopteran

Water regulation is an important physiological challenge for insects due to their small body sizes and large surface area to volume ratios. Adaptations for decreasing cuticular water loss, the largest avenue of loss, are especially important. The melanin desiccation hypothesis states that melanin molecules in the cuticle may help prevent water loss, thus offering protection from desiccation. This hypothesis has much empirical support in Drosophila species, but remains mostly untested in other taxa, including Lepidoptera. Because melanin has many other important functions in insects, its potential role in desiccation prevention is not always clear. In this study we investigated the role of melanin in desiccation prevention in the white-lined Sphinx moth, Hyles lineata (Lepidoptera, Sphingidae), which shows high plasticity in the degree of melanin pigmentation during the late larval instars. We took advantage of this plasticity and used density treatments to induce a wide range of cuticular melanization; solitary conditions induced low melanin pigmentation while crowded conditions induced high melanin pigmentation. We tested whether more melanic larvae from the crowded treatment were better protected from desiccation in three relevant responses: i) total water loss over a desiccation period, ii) change in hemolymph osmolality over a desiccation period, and iii) evaporation rate of water through the cuticle. We did not find support for the melanin desiccation hypothesis in this species. Although treatment influenced total water loss, this effect did not occur via degree of melanization. Interestingly, this implies that crowding, which was used to induce high melanin phenotypes, may have other physiological effects that influence water regulation. There were no differences between treatments in cuticular evaporative water loss or change in hemolymph osmolality. However, we conclude that osmolality may not sufficiently reflect water loss in this case. This study emphasizes the context dependency of melanin’s role in desiccation prevention and the importance of considering how it may vary across taxa. In lepidopteran larvae that are constantly feeding phytophagous insects with soft cuticles, melanin may not be necessary for preventing cuticular water loss.

Complexity in the timing of the first postnatal ecdysis in snakes.

Lepidosaurian reptiles, particularly snakes, periodically shed the outer epidermal layers of their skin (ecdysis) to restore or enhance vital functions such as regulating water and gaseous exchange, growth, and protection against insult, infection or physical injury. Although many studies have focused on the nature and mechanisms of skin shedding, little attention has been paid to the timing of the first ecdysis in neonates following birth or hatching. A recent study investigated patterns of the time to first postnatal ecdysis in snakes based on a large dataset taken from the literature. The analysis demonstrated patterns in the time to first postnatal ecdysis related to phylogeny as well as several life history traits. While this assessment provides important advances in our knowledge of this topic, data on known biophysical drivers of ecdysis - temperature and humidity - were largely unavailable and were not evaluated. The first postnatal ecdysis of neonatal snakes can be viewed as an adaptive adjustment to the transition from the aqueous environment of the embryo to the aerial environment of the newborn. Hence, the timing of the first postnatal ecdysis is logically influenced by the aerial environment into which a newborn snake or hatchling finds itself. Therefore, in this Commentary, we first emphasize the putative plasticity of ecdysis with respect to epidermal lipids that structure the water permeability barrier and are established or renewed during ecdysis to reduce transepidermal evaporative water loss. We then discuss the likely importance of biophysical variables as influential covariates that need future investigation as potential co-determinants of the timing of first postnatal ecdysis.

Heat tolerance and evaporative cooling capacity in an arid-zone elephant shrew

Endotherms vary widely in their capacity to defend sublethal body temperature (Tb) during acute heat exposure. Interspecific variation in the upper thermal limits of small mammals remains poorly studied, particularly in taxa other than bats. We hypothesised that rupicolous elephant shrews (Macroscelidae), on account of their occupancy of above-ground rock crevices in hot habitats and rapid cursorial locomotion, have evolved pronounced heat tolerance capacities. To test this hypothesis, we quantified relationships between Tb, resting metabolic rate (RMR) and evaporative water loss (EWL) in western rock elephant shrews (Elephantulus rupestris) exposed to air temperature (Tair) approaching or exceeding Tb under conditions of low humidity representative of the study population’s arid, winter-rainfall habitat in western South Africa. Our data revealed a pronounced capacity to tolerate Tair > Tb, with E. rupestris tolerating Tair up to 48.0 ± 0.1 °C while defending Tb > 7 °C below Tair (maximum Tb = 41.64 ± 0.16 °C). Three behavioural pathways were employed to dissipate heat: open mouth panting, flattening their body posture, and nose-licking. At the highest experimental Tair values achieved, EWL increased 15.05-fold relative to normothermic levels and heat was dissipated evaporatively at rates equivalent to 174%—240% of metabolic heat production. The heat tolerance limit of Tair = 48 °C in E. rupestris is one of the highest yet recorded in a non-volant small mammal, and our data support the notion that elephant shrews possess a pronounced capacity to avoid lethal hyperthermia during acute heat exposure.

Water Availability and Temperature as Modifiers of Evaporative Water Loss in Tropical Frogs.

Water plays a notable role in the ecology of most terrestrial organisms due to the risks associated with water loss. Specifically, water loss in terrestrial animals happens through evaporation across respiratory tissues or the epidermis. Amphibians are ideal systems for studying how abiotic factors impact water loss since their bodies often respond quickly to environmental changes. While the effect of temperature on water loss is well known across many taxa, we are still learning how temperature in combination with humidity or water availability affects water loss. Here, we tested how standing water sources (availability) and temperature (26 and 36°C) together affect water loss in anuran amphibians using a Bayesian framework. We also present a conceptual model for considering how water availability and temperature may interact, resulting in body mass changes. After accounting for phylogenetic and time autocorrelation, we determined how different variables (water loss and uptake rates, temperature, and body size) affect body mass in three species of tropical frogs (Rhinella marina, Phyllobates terribilis, and Xenopus tropicalis). We found that all variables impacted body mass changes, with greater similarities between P. terribilis and X. tropicalis, but temperature only showed a notable effect in P. terribilis. Furthermore, we describe how the behavior of P. terribilis might affect its water budget. This study shows how organisms might manage water budgets across different environments and is important for developing models of evaporative water loss and species distributions.

Integrating spatial analyses of genomic and physiological data to understand avian responses to environmental change.

Projected rates of climate change over the next century are expected to force species to shift ranges, adapt or acclimate to evade extinction. Predicting which of these scenarios may be most likely is a central challenge for conserving biodiversity in the immediate future. Modeling frameworks that take advantage of intraspecific variation across environmental gradients can be particularly important for meeting this challenge. While these space-for-time approaches are essential for climatic and genomic modeling approaches, mechanistic models that incorporate ecological physiology data into assessing species vulnerabilities rarely include intraspecific variation. A major reason for this gap is the general lack of empirical data on intraspecific geographic variation in avian physiological traits. In this review, we outline the evidence for and processes shaping geographic variation in avian traits. We use the example of evaporative water loss to underscore the lack of research on geographic variation even in traits central to cooling costs in birds. We next demonstrate how shifting the focus of avian physiological research to intraspecific variation can facilitate greater integration with emerging genomics approaches. Finally, we outline important next steps for an integrative approach to advance understanding of avian physiological adaptation within species. Addressing the knowledge gaps outlined in this review will contribute to an improved predictive framework that synthesizes environmental, morphological, physiological, and genomic data to assess species specific vulnerabilities to a warming planet.

New insights into the parameterization of the dry surface layer and its hydrogeochemical mechanism: An experimental study

Knowledge of the parameterization of the dry surface layer (DSL) is essential for evaluating near-surface water flow and water balance in arid and semi-arid areas. Existing studies have parameterized DSL thickness and vapor flow as functions of the soil moisture content (SMC) in the surface layer to predict soil evaporation. However, hydrochemical processes related to DSL development have been ignored, including changes in hydrochemistry, the underlying hydrochemical mechanism, and the role of dissolved substances in the DSL development. Herein, we performed a series of soil evaporation experiments for 260 days and explored the factors influencing DSL development (e.g., soil texture, atmospheric temperature, SMC, solutes). Evaporation experiments were performed using silty loess, sandy loess, and fine sand with a 60-cm water table. Results showed that the cumulative evaporation of silty loess, sandy loess, and fine sand over the experimental period were 1,391.52, 460.10, and 185.53 mm, respectively, which determined by the maximum height of liquid flow continuity. The content of total dissolved solids (TDS) and major ions at the surface soil were significantly higher than the values at deep depths of 5‒55 cm, which largely depend on evaporative water loss. Evolutionary trends of chemical facies in sand media along the liquid water migration were from HCO3-Ca type to SO4·Cl-Na type. This was attributed to mineral dissolution at a depth of 5–55 cm and their transport with liquid water, resulting in the precipitation of salt crystals at the surface soil. Furthermore, a consolidated DSL with a thickness of 3.0–3.5 cm in the sandy loess and a loose DSL with a thickness of 1.5–2.0 cm in fine sand were observed at the end of the experiments. The accumulation of solutes at the surface leads to a reduction in effective porosity and the aggregation of soil particles during continuous drying, which facilitates the consolidation of DSL in sandy loess. This overestimated the DSL thickness, resulting in a difference between the experimental and predicted evaporation rates by Fick's law. Overall, these results highlight the limitations of considering DSL thickness as a function of SMC only, providing new insights into hydrochemical processes and dissolved solutes involving DSL parameterization during continuous soil drying.

Assessing responses to heat in a range-shifting, nocturnal, flying squirrel.

Over the last few decades North American flying squirrels (Glaucomys spp.) have experienced dramatic northward range shifts. Previous studies have focused on the potential effects of warming winter temperatures, yet the hypothesis that rising summer temperature had a role in these range shifts remained unexplored. We therefore sought to determine the effect of high environmental temperatures on the thermoregulation and energetics of flying squirrels in an area of the Northeast of North America with a recent species turnover. Unable to find a logistically feasible population of the northern species (Glaucomys sabrinus), we focused on Southern Flying Squirrels (G. volans). Using flow-through respirometry, we measured the relationship between metabolic rate, evaporative water loss, and body temperature at high ambient temperatures. We also measured core body temperature in free-ranging flying squirrels using temperature-sensitive data loggers. We detected no significant increase in metabolic rate up to ambient temperatures as high as 40 °C. However, evaporative water loss increased at temperatures above 36.2 °C. Free-ranging body temperature of flying squirrels followed a circadian pattern with a ~2 °C difference between active and resting phase modal body temperatures. Rest-phase body temperatures were influenced by environmental temperatures with higher resting temperatures observed on days with higher daily maximum ambient temperatures but not to an extent that energy or water costs were significantly increased during rest. We found that, due to a relatively high level of thermal tolerance, high ambient temperatures are unlikely to cause an energetic strain on Southern Flying Squirrels. However, these findings do not preclude negative impacts of high ambient temperatures on the northern species, and these may still play a role in the changing distributions of Glaucomys in North America.

Dual-threshold control architecture for improved thermal and power management in open-cathode direct methanol fuel cell

A new dual-threshold cascade control scheme is proposed for water management in direct methanol fuel cells. The control architecture is constructed with the goal of adjusting the liquid-level of a methanol solution in an internal storage tank, where this control structure is based on a set point constrained by upper and lower thresholds. The logic of the algorithm is driven by a noninverting and an inverting Schmitt triggers comprising a switching function that sets the master controller on manual mode for periods of time during which the operating temperature of the electricity-generating fuel cell’s stack can be fixed at an optimal value. Automatic control is implemented when either threshold is reached, triggering a level recovery action where the temperature is manipulated to promote or inhibit cathodic evaporative water losses, depending on the existing perturbations. Manual mode is reinstated when a chosen mid liquid-level target is reached. The strategy seeks to avoid water flooding and dry-out states in the device, while ensuring constant power and current production during periods of manual operation. Simulation studies show that the new control method is immune to severe perturbations, which stands in contrast to a standard cascade control scheme that causes significant degradation and unstable electrical performance in the cell under analogous conditions. Unlike the standard cascade control where the operating temperature is seldom close to the optimal operating point, the improved dual-threshold logic is superior such that the desired temperature of operation can be realized during the entire manual mode, enhancing the performance of the variables that critically depend on temperature. The new method realizes a successful power transition from 30 to 40W, demonstrating its ability to meet changes in load demands, where this transfer is implemented by altering the operational temperature from 45°C to 60°C at a constant voltage. The new strategy can be incorporated into a higher-level optimization control layer, where different temperature operating points can be chosen based on user-defined performance criteria. The study focuses on a case of two different points of the polarization curve corresponding to a power of 40W. While the operating point at a current of 2.51A and temperature of 57.5°C has a higher voltage efficiency of 10%, the second point operating at 2.79A and 50°C shows a boost by 7.6% in fuel efficiency over the first point. Hence, a user can choose the optimal operating condition based on specific performance metrics while simultaneously meeting the load demands. Few recommendations are presented to advance this work in the future.

Comparison of Hydric and Thermal Physiology in an Environmentally Diverse Clade of Caribbean Anoles.

As the world becomes warmer and precipitation patterns less predictable, organisms will experience greater heat and water stress. It is crucial to understand the factors that predict variation in thermal and hydric physiology among species. This study focuses on investigating the relationships between thermal and hydric diversity and their environmental predictors in a clade of Hispaniolan anole lizards, which are part of a broader Caribbean adaptive radiation. This clade, the "cybotoid" anoles, occupies a wide range of thermal habitats (from sea level to several kilometers above it) and hydric habitats (such as xeric scrub, broadleaf forest, and pine forest), setting up the possibility for ecophysiological specialization among species. Among the thermal traits, only cold tolerance is correlated with environmental temperature, and none of our climate variables are correlated with hydric physiology. Nevertheless, we found a negative relationship between heat tolerance (critical thermal maximum) and evaporative water loss at higher temperatures, such that more heat-tolerant lizards are also more desiccation-tolerant at higher temperatures. This finding hints at shared thermal and hydric specialization at higher temperatures, underscoring the importance of considering the interactive effects of temperature and water balance in ecophysiological studies. While ecophysiological differentiation is a core feature of the anole adaptive radiation, our results suggest that close relatives in this lineage do not diverge in hydric physiology and only diverge partially in thermal physiology.