Journal of Thermal Biology

The intensity and duration of heat waves, as well as average global temperatures, are expected to increase due to climate change. Heat waves can cause physiological stress and reduce fitness in animals. Species can reduce overheating risk through phenotypic plasticity, which allows them to raise their thermal tolerance limits over time. This mechanism could be important for ectotherms whose body temperatures are directly influenced by available environmental temperatures. Geckos are a large, diverse group of ectotherms that vary in their thermal habitats and times of daily activity, which could affect how they physiologically adjust to heat waves. Data on thermal physiology are scarce for reptiles, with only one study in geckos. Understanding thermal tolerance and plasticity, and their relationship, is essential for understanding how some species are able to adjust or adapt to changing temperatures. In this study, we estimated thermal tolerance and plasticity, and their interaction, in the crepuscular gecko, Eublepharis macularius, a species that is emerging as a model for reptile biology. After estimating basal thermal tolerance for 28 geckos, thermal tolerance was measured for each individual a second time at several timepoints (3, 6, or 24 h) to determine thermal tolerance plasticity. We found that thermal tolerance plasticity (1) does not depend on the basal thermal tolerance of the organism, (2) was highest after 6 hours from initial heat shock, and (3) was negatively influenced by individual body mass. Our findings contribute to the increasing body of work focused on understanding the influence of biological and environmental factors on thermal tolerance plasticity in organisms and provide phenotypic data to further investigate the molecular basis of thermal tolerance plasticity in organisms.

Body coloration in ectotherms serves multiple biological functions, including avoiding predators, communicating with conspecific individuals, and involvement in thermoregulation. As ectotherms rely on environmental sources of heat to regulate their internal body temperature, stable melanistic body coloration or color change can be used to increase or decrease heat absorption and heat exchange with the environment. While the function of melanistic coloration for thermoregulation has been found to increase solar radiation absorption for heating in many diurnal ectotherms, research on crepuscular and nocturnal ectotherms is lacking. Since crepuscular and nocturnal ectotherms generally absorb heat from the substrate, coloration is likely under different selective pressures than in diurnal ectotherms. We tested if the proportion of dorsal melanistic body coloration is related to differences in body temperature heating and cooling rates in the crepuscular gecko Eublepharis macularius and whether changes in environmental temperature trigger color changes in this species. Temperature measurements of the geckos and of the environment were taken using infrared thermography and temperature loggers. Color data were obtained using objective photography and a newly developed custom software package. We found that body temperature reflected substrate temperatures, and that the proportion of melanistic coloration has no influence on heating or cooling rates or on color changes. These findings suggest that, in E. macularius, melanistic coloration may not be used for thermoregulation. Future research should further test the function of melanistic coloration in other crepuscular and nocturnal vertebrates to understand the evolution of melanistic pattern in animals active in low light conditions.

The heat sensitivity of reproduction is a critical determinant of population persistence under climate change. However, the heat sensitivity of gametes is poorly known relative to adults. We developed a method to measure the heat tolerance of lizard sperm cells, and used the method to test several aspects of sperm cell thermal biology in the brown anole lizard (Anolis sagrei). We estimated the repeatability of sperm traits by measuring heat tolerance and baseline motility of ejaculated sperm from the same individuals multiple times over 21 days. To investigate co-adaptation of sperm and adult thermal traits, we tested for a correlation between sperm heat tolerance and the heat tolerance of adults that produced them. Furthermore, we tested for effects of episodic heat stress experienced by males on sperm performance. Sperm heat tolerance and motility were both repeatable, consistent with evolutionary potential, though there was clear evidence for environmental effects on these traits as well. Contrary to the expectation of thermal co-adaptation, we found no correlation between sperm and adult heat tolerance. A single, episodic extreme heat event experienced by adult males immediately impaired sperm motility, consistent with detrimental effects of adult heat stress on stored sperm. Our study adds to the mounting evidence that sperm are heat-sensitive and represent a vulnerability to global warming, but also suggest evolutionary potential for thermal adaptation at the gamete level. Summary statementThis study investigates gamete heat sensitivity in lizards, revealing heat tolerance and repeatability in sperm thermal traits. These findings are essential for understanding reproductive responses to climate change.

Measuring behavioural and physiological thermal limits is crucial to understanding how they interact with the environment under a climate change scenario. We experimentally assessed the effects of acclimation on sequentially measured voluntary (VTmax), critical (CTmax), and upper thermal limit (UTL) limits in the Mediterranean silverfish Sceletolepisma guadianicum. Individuals were acclimated for six days at either 25{degrees}C (n=32) or 35{degrees}C (n=29) and heated at [~]0.5{degrees}C min-1, and VTmax, CTmax, and lethal limits were recorded. S. guadianicum exhibited some of the highest thermal limits reported to date among terrestrial arthropods. VTmax showed limited (1.04 {degrees}C) but statistically detectable plasticity, increasing with high acclimation temperature and heating rate, whereas CTmax rate and lethal limits remained unchanged. We provide hypotheses explaining the co-ocurrence of exceptional heat tolerance levels together with their reduced plasticity in this and other extremely heat-tolerant species. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=124 SRC="FIGDIR/small/690525v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@1f01660org.highwire.dtl.DTLVardef@14cfcb2org.highwire.dtl.DTLVardef@1621c39org.highwire.dtl.DTLVardef@2aec20_HPS_FORMAT_FIGEXP M_FIG C_FIG

O_LIAnthropogenic climate change is thought to present a significant threat to biodiversity, in particular to tropical ectotherms, and the effects of long-term developmental heat stress on this group have received relatively little research attention. C_LIO_LIHere we study the effects of experimentally raising developmental temperatures in a tropical butterfly. We measured survival, development time, adult body mass, and wing size of a neotropical butterfly, Heliconius erato demophoon, across three temperature treatments. C_LIO_LIEgg survival was lower in the hotter treatments, with 83%, 73%, and 49% of eggs eclosing in the 20-30{degrees}C, 23-33{degrees}C, and 26-36{degrees}C treatments, respectively. Larval survival was five times lower in the 26-36{degrees}C treatment (4%) compared to the 20-30{degrees}C treatment (22%), and we did not detect differences in pupal survival across treatments due to high mortality in earlier stages. C_LIO_LIAdults in the 20-30{degrees}C treatment had a lower body mass and larvae had a lower growth rate compared to the intermediate 23-33{degrees}C treatment, but were heavier than the few surviving adults in the 26-36{degrees}C treatment. Females were heavier and grew faster as larvae than males in the 23-33{degrees}C treatment, but there was no associated increase in wing size. C_LIO_LIIn summary, high developmental temperatures are particularly lethal for eggs and less so for larvae, and also affect adult morphology. This highlights the importance of understanding the effects of temperature variation across ontogeny in tropical ectotherms. C_LI

Balancing survival and reproduction presents a fundamental evolutionary challenge, especially in extreme and unpredictable environments. Thermoregulatory behavior in particular imposes a costly trade-off, as time spent maintaining optimal body temperature precludes other essential activities and forces individuals to balance competing selective pressures. By combining field observations, behavioral assays, and gene expression profiling of red-sided garter snakes (Thamnophis sirtalis parietalis), we describe how an ectothermic vertebrate navigates this trade-off in an extreme thermal environment through a combination of finely tuned mechanisms of behavioral and physiological plasticity. Snakes demonstrated remarkably consistent critical thermal limits, with minimal individual variation, suggesting a hard physiological constraint. Importantly, behavioral temperature thresholds for voluntary withdrawal and courtship cessation occurred within 5 {degrees}C of lethal, demonstrating that males routinely operate perilously close to lethal body temperatures in order to maximize reproductive opportunities. Context-dependent plasticity allows males to prioritize mating opportunities over thermoregulation until survival becomes an immediate threat. Molecular analyses of multiple tissues revealed rapid, tissue-specific transcriptomic responses activated within one hour of thermal challenge, well before critical limits were reached. Heat shock proteins were universally upregulated across all tissues under both heat and cold stress, suggesting anticipatory protective mechanisms that support risky behavioral decisions rather than simply responding to thermal damage. Tissue-specific patterns of gene expression reflected functional priorities with liver showing extensive metabolic flexibility, heart maintaining conservative stability, while brain and testis appeared to balance critical functions with stress responses. These findings demonstrate precise thermal reaction norms that integrate behavioral and molecular plasticity to maximize reproductive effort without compromising survival. The narrow margin between behavioral thresholds and lethal limits suggests this system may already be approaching evolutionary constraints. While phenotypic plasticity currently buffers populations against extreme thermal variability, it may paradoxically limit long-term adaptive potential as increasing environmental temperatures further narrow safety margins. Precise thermal decision-making, rapid physiological responses, and context-dependent behavioral switching suggest an evolved solution to the problem of balancing competing selective pressures.

Once a year, penguins undergo a catastrophic moult replacing their entire plumage during a fasting period on land or on sea-ice during which time individuals can lose 45% of their body mass. In penguins, new feather synthesis precedes the loss of old feathers leading to an accumulation of two feathers layers (double coat) before the old plumage is shed. We hypothesize that the combination of the high metabolism required for new feathers synthesis and the potentially high thermal insulation linked to the double coat could lead to a thermal challenge requiring additional peripheral circulation to thermal windows to dissipate extra-heat. To test this hypothesis, we measured the surface temperature of different body regions of captive Gentoo penguins (Pygoscelis papua) throughout the moult under constant environmental conditions. The surface temperature of the main body trunk decreased during the initial stages of the moult, therefore suggesting a higher thermal insulation. On the opposite, the periorbital region, a potential proxy of core temperature in birds, increased during these same early moulting stages. The surface temperature of bill, flipper and foot (thermal windows) tended to initially increase during the moult period, highlighting the likely need for extra heat dissipation in moulting penguins. These results raise questions regarding the thermoregulatory capacities of wild penguins during the challenging period of moulting on land in the current context of global warming.

Amphibians that reproduce in early spring at northern latitudes may encounter environmental ice while migrating to their breeding sites. Due to the nucleation properties of ice, contact with environmental ice may induce rapid freezing of body tissues, which can cause irreversible damage to cells and lead to death. Although some species of salamanders are known to move over ice during early spring migration, freeze-intolerant species are expected to avoid physical contact with ice crystals to minimise the risk of freezing. Here, we documented the thermal biology of the freeze-intolerant blue-spotted salamander (Ambystoma laterale Hallowell, 1856) migrating at sub-zero temperatures in Algonquin Provincial Park, Ontario, Canada. During our surveys, we found sheltered, inactive, and migrating individuals; some in direct contact with ice. Our field measurements of skin temperature using high resolution thermal imaging suggest that A. laterale can sustain activity in a supercooled state (i.e., chilled below the freezing point of body fluids but not frozen). By migrating in a supercooled state, these salamanders may overcome the risk of freezing while simultaneously prolonging their breeding season and potentially avoiding predators.

Thermal tolerances, such as critical temperatures, are important indices for understanding an organisms vulnerability to changing environmental temperature. Differences in thermal tolerance over ontogeny may generate a thermal bottleneck that sets the climate vulnerability for organisms with complex life cycles. However, a species microhabitat preference and life history can hinder our ability to assess climate change vulnerability in large-scale comparative studies as these reflect local adaptation. Here, we used phylogenetically informed, multi-level models with a global dataset of upper critical temperatures from 438 Anuran species that explicitly included microhabitat preferences and ontogenetic stage to examine variation in upper critical temperatures. We found ontogenetic trends in thermal tolerance were similar across microhabitat preferences. We then used microclimate-driven models of the heat and water budget of a thermoregulating frog to show the contribution of behavioural thermoregulation towards mitigating exposure to thermal stress. Our results suggested thermal bottlenecks are not strongly present in Anurans but instead implied strong developmental or genetic conservatism of thermal tolerance within families and ecotypes, and that behavioural thermoregulation has a strong potential for buffering frogs against extreme temperatures.

Temperature affects most aspects of ectotherms life history, including physiology and behavior. Studying thermal sensitivity of jumping performance in frogs can help understanding the influence of temperature on different aspects of frog life. Still, studies on the effects of temperature on amphibians are commonly carried out on terrestrial and tree species, creating a gap for aquatic species. We experimentally tested the thermal sensitivity of jumping performance of the Uruguay Harlequin Frog, Lysapsus limellum, assessing three measures: response time, distance of first jump, and total distance travelled. We hypothesized that individuals submitted to extreme temperatures would increase response time, decrease first jump distance, and increase total jump distance. We used an arena with a gradient of air temperature (Ta) ranging from 20 to 40 {degrees}C. We placed frogs at different Ta and stimulated them to jump. Then, we analysed the influence of Ta on the three estimates of jumping performance, using generalized additive models. We found that temperature affected all three measurements of jumping performance, but some relationships were stronger than others. Extreme temperatures increased response time, reduced first jump distance, and increased total distance. The effect was weaker for response time and first jump distance, but substantially stronger for total distance jumped. Although individuals under extreme temperatures experience a reduced jumping performance, they travelled longer distances to find areas with milder temperatures. Thus, we showed that L. limellum thermoregulates by means of behavior, moving through places at different thermal conditions. Additionally, benefits of displacing to thermally suitable places -in terms of enhanced jumping performance-are bigger than the costs of jumping at reduced locomotor performance, at least under experimental conditions. Our results can help understand how climate change affects the locomotor performance of Neotropical amphibians.

AimWe aimed to understand the sublethal impacts of climate change on native bee ecology. We evaluated how flight performance, a key trait for predator escape, dispersal, and pollination, is impacted by temperature. We aimed to understand how species geographic ranges shape species thermal performance curves (TPCs) and determine species vulnerability to further warming. LocationAustralia Time periodPresent Major taxa studiedStingless bees MethodsWe tested the flight speed and acceleration of two species of stingless bees, Austroplebeia australis and Tetragonula carbonaria at seven test temperatures. We examined how the climate experienced by species throughout their ranges shapes their TPCs. Inferences were made on how sublethal increases in temperature will likely impact key ecological activities associated with flight by estimating the proportion of species geographic ranges that experience temperatures that exceed their thermal optima. ResultsSpecies TPCs reflected the thermal environments they inhabit. A. australis, which experiences greater thermal variation and hotter temperatures throughout their range, had a broader TPC and higher thermal optima. However, A. australis also had faster flight performance than T. carbonaria, rejecting the jack-of-all-trades master-of-none hypothesis. Further increases in temperature will reduce flight performance of T. carbonaria at a faster rate (due to their narrower TPCs) than A. australis, however, a larger proportion of the A. australis range is currently exposed to temperatures above their thermal optima. Main conclusionsClimates will impact species ecology before they reach their lethal limits. Our study supports the idea that species fundamental niche breadths are linked to their geographic ranges, but that trade-offs are not associated with flight performance and ability to perform across a broad range of temperatures. When assessing vulnerability to climate change, it is important to consider how temperature impacts species traits, but also the climates that species experience throughout their unique geographic ranges.

Understanding how endotherms manage energy expenditure at sub-thermoneutral temperatures is crucial for predicting their physiological flexibility and adaptive potential. This study investigated the metabolic responses of captive-bred common waxbills (Estrilda astrild) to decreasing ambient temperatures, with a focus on three questions: whether basal metabolic rate (BMR) predicts standard metabolic rate (SMR), how consistent individual metabolic responses are, and whether waxbills employ cold-induced energy-saving mechanisms. We found that BMR did not reliably predict SMR, underscoring the limits of BMR as a proxy under ecologically relevant conditions. SMR showed moderate repeatability across individuals, but variation was mainly due to baseline metabolic level rather than slope differences in thermal reaction norms. A two-breakpoint model best described the relationship between temperature and metabolic rate, with a marked decline in SMR below ~18.8{degrees}C. This decrease, along with observed reductions in core body temperature in some individuals, reveals an energy-saving strategy involving shallow hypothermia at relatively mild sub-thermoneutral temperatures. These findings suggest that energy-saving mechanisms may be intrinsic to the species rather than purely plastic responses. Overall, our results highlight both the limitations of BMR and the importance of measuring metabolic responses across thermal gradients to detect subtle but ecologically relevant hypometabolic strategies.

Birds maintain some of the highest body temperatures (Tb) among endothermic animals. Often deemed a selective advantage for heat tolerance, high Tb also limits the capacity to increase Tb before reaching lethal levels. Recent thermal modelling suggests that sustained effort in Arctic birds might be restricted at mild air temperatures (Ta) during energetically demanding life history stages, which may force reductions in activity to avoid overheating, with expected negative impacts on reproductive performance. Consequently, understanding how Arctic birds will cope with increasing Ta has become an urgent concern. We examined within-individual changes in Tb in response to an experimental increase in activity in outdoor captive Arctic cold-specialised snow buntings (Plectrophenax nivalis), exposed to naturally varying Ta from -15 to 36 {degrees}C. Calm buntings exhibited a modal Tb range from 39.9 - 42.6 {degrees}C. However, we detected a dramatic increase in Tb within minutes of shifting birds to active flight, with strong evidence for a positive effect of Ta on Tb (slope = 0.04 {degrees}C/{degrees}C). Importantly, by Ta of 9 {degrees}C, flying buntings were already generating Tb [≥] 45{degrees}C, approaching the upper thermal limits of organismal performance (i.e., Tb = 45 - 47 {degrees}C). Under scenarios of elevated Tb, buntings must increase rates of evaporative water loss and/or reduce activity to avoid overheating. With known limited evaporative heat dissipation capacities, we argue buntings operating at peak energy levels will increasingly rely on behavioral thermoregulatory strategies (i.e., reducing activity) to regulate Tb, at the potential detriment to nestling growth and survival.

Many species exhibit color polymorphisms which have distinct physiological and behavioral characteristics. However, the consistency of morph trait covariation patterns across species, time, and ecological contexts remains unclear. This trait covariation is especially relevant in the context of invasion biology and urban adaptation. Specifically, physiological and behavioral traits pertaining to energy maintenance are crucial to fitness, given their immediate ties to individual reproduction, growth, and population establishment. We investigated the physiological and behavioral traits of Podarcis muralis, a versatile color polymorphic species that thrives in urban environments (including invasive populations in Ohio, USA). We measured five physiological and behavioral traits (circulating corticosterone and triglycerides, hematocrit, body condition, and field body temperature) which compose an integrated multivariate phenotype. We then tested variation among stable color polymorphisms in the context of establishment in an urban environment. We found that the traits describing physiological status and strategy shifted across the active season in a morph-dependent manner-the white and yellow morphs exhibited clearly different multivariate physiological phenotypes, characterized primarily by differences in circulating corticosterone. This suggests that morphs have different strategies in physiological regulation, the flexibility of which is crucial to urban adaptation. The white-yellow morph exhibited an intermediate phenotype, suggesting an intermediary energy maintenance strategy. Orange morphs also exhibited distinct phenotypes, but the low prevalence of this morph in our study populations precludes clear interpretation. Our work provides insight into how differences among stable polymorphisms exist across axes of the phenotype and how this variation may aid in establishment within novel environments. O_QDEpigraph: "If you are only moved by color relationships, you are missing the point." - Mark Rothko, Latvian-American abstract painter C_QD

O_LICoping with stressors can require substantial energetic investment, and when resources are limited, such investment can preclude simultaneous expenditure on other biological processes. Among endotherms, energetic demands of thermoregulation can be immense, yet whether a stress response is sufficient to induce changes in thermoregulatory investment appears unexplored. C_LIO_LIWe tested the hypothesis that stress-induced changes in surface temperature, a well-documented phenomenon across vertebrates, stem from a trade-off between thermoregulation and stress responsiveness, whereby individuals seek to reduce energetic expenditure on thermoregulation in challenging environments (the "Thermoprotective Hypothesis"). We predicted that surface temperature and dry heat loss of individuals that are resource-limited would fall under stress exposure at low ambient temperatures and rise under stress exposure at high ambient temperatures when compared with non-resource limited individuals. C_LIO_LITo test our predictions, we exposed Black-capped Chickadees to rotating stressors and control treatments (ndays/treatment = 30; paired treatments) across an ambient temperature gradient whilst remotely monitoring both feeding behaviour and surface temperature. C_LIO_LISupporting the Thermoprotective Hypothesis, our results showed that: 1) social subordinates (n = 12), who fed less than social dominants and alone suffered stress-induced declines in mass, displayed significantly larger changes in surface temperature following stress exposure than social dominants (n = 8), and 2) stress-induced changes in surface temperature significantly increased heat conservation at low ambient temperature, and heat dissipation at high ambient temperature among social subordinates alone. C_LIO_LIThese results suggest that Black-capped Chickadees adjust their thermoregulatory strategies under stress when resources are limited and support the hypothesis that stress-induced changes in temperature are functionally significant. C_LI Data Availability Statement: All data and R code used for the construction of this study are available at: https://datadryad.org/stash/share/QzmRV875_S5IYpMT0jxOYIsoO-ua0T3LX6AA9u-XpdQ.

Polar and sub-polar animals evolved to thrive in cold climates and may thus be particularly vulnerable to rising temperatures associated with climate change. Penguins may be especially vulnerable due to their dual habitat, alternating between foraging in cold waters and breeding/moulting on an increasingly warm land. Here, we characterized heat stress occurrence in breeding king penguins through behavioural observations (e.g. panting occurrence) and body temperature measurements. We observed that behavioural signs of heat stress are frequent in king penguins breeding in the sub-Antarctic region (> 20% of observations at mid-day), and that subcutaneous temperatures increase under high heat load, especially in penguins showing behavioural signs of heat stress. Subcutaneous and core body temperatures were moderately correlated and both increased with heat load. Yet, their responses were not parallel since core body temperature is markedly less sensitive to heat load than subcutaneous temperature. Air temperature alone was a poor predictor of heat stress occurrence, whereas the combination of high solar radiation, low wind speed and high air temperatures provided the strongest predictive power. Finally, reproductive failures were more likely to occur on warmer days, suggesting that heat stress may have significant sublethal effects that could ultimately affect population dynamics. O_FIG O_LINKSMALLFIG WIDTH=150 HEIGHT=200 SRC="FIGDIR/small/611977v2_ufig1.gif" ALT="Figure 1"> View larger version (95K): org.highwire.dtl.DTLVardef@f2c985org.highwire.dtl.DTLVardef@18c4688org.highwire.dtl.DTLVardef@6a99dforg.highwire.dtl.DTLVardef@9c32fa_HPS_FORMAT_FIGEXP M_FIG C_FIG

Ecophysiological studies of invasive species tend to focus on captive individuals and their invasive range. However, the importance of gaining a thorough understanding of their physiology in their native range, where autecological knowledge is limited, has played a crucial role in assessing species ecophysiological responses to the often novel environmental conditions encountered in their invasive range. Here, we investigated the ecophysiological characteristics of a population of the wild-caught common waxbill (Estrilda astrild), a successful global invader, in part of its native range (South Africa). We investigated how this species adjusts its resting metabolic rate over a range of temperatures to identify its thermoneutral zone (TNZ) as an indicator of a species thermal tolerance. The observed TNZ curve predominantly followed the classic Scholander-Irving model, with metabolic rates increasing linearly at temperatures outside the TNZ. However, we found an inflection point at moderately cold temperatures (16{degrees}C) where the common waxbill began to decrease its metabolic rate. This finding highlights the potential use of an energy-saving strategy as an adaptive response to cold, such as facultative hypothermic responses through a reduction in body temperature, which may explain their success as an invasive species. We argue that although metabolic infection points have been repeatedly identified in studies of TNZ, the specific mechanisms behind metabolic down-regulation at low temperatures remain underexplored in the literature. We therefore suggest that future research should focus on investigating body temperature variation, with particular emphasis on its potential contribution to metabolic adaptation in colder environments.

Endothermy, understood as the maintenance of continuous and high body temperatures (TB) due to the combination of metabolic heat production and an insulative cover, is severely challenged in small endotherms of cold environments. As a response, social clustering, and nest use (collectively, "communal nesting") are common strategies for heat conservation in small mammals and birds. To quantify the actual amount of energy that is saved by communal nesting, we studied the social marsupial Dromiciops gliroides (monito del monte), a relict marsupial species from the cold forests of southern South America. It is hypothesized that sociability in this marsupial was driven by the cold, for which we calculated the energetic benefits that communal nesting confers. Using biophysical models and experimental coolings, we simulated heat exchanges experienced by grouped or solitary individuals, and also individuals within nests, collected from the field. Assuming a model of passive cooling, we calculated the net energy cost of euthermic maintenance (Ecost: the total energy needed to maintain euthermia). We adjusted 50 cooling curves, to exponential decay models, and found in all cases that the strategy minimizing heat loss is to be clustered within a nest, for which the Ecost was the minimum. This was significantly lower than the clustered condition, outside the nest, a reduction that represents almost half of energy consumption per day in a resting, thermoneutral condition for this marsupial. Overall, our results suggest that the strategy that significantly maximized heat conservation, compared with alternative strategies, was communal nesting. These findings support the idea that, in this social mammal, sociality is driven by bioenergetic benefits.

Aedes aegypti, a primary vector of dengue, Zika, and chikungunya, displays remarkable adaptability across ecological gradients. Central to this resilience is the egg stage, which must withstand fluctuating moisture and temperature conditions. Environmental transitions, particularly changes in moisture availability, significantly influence egg hatching success in mosquitoes. This study investigates how parental exposure to variable hydration conditions shapes key reproductive traits in Ae. aegypti. Using four environmental regimes, continuous wet, continuous dry, wet-to- dry, and dry-to-wet, we assessed egg output, hatching success, thermotolerance, and egg nutrient composition across three Ae. aegypti populations. Our results show that oviposition timing and egg production are significantly affected by the hydration environment experienced by the parental generation. While the wet, dry, and dry-to-wet groups exhibited a consistent oviposition peak beginning four days post-blood feeding, the wet-to-dry group showed delayed reproductive investment, with peak egg production occurring later. Egg output was highest under continuous wet conditions and significantly reduced in the dry and wet-to-dry treatments across all populations. Interestingly, the wet-to-dry group showed significantly higher egg-thermotolerance than any other group, and this pattern was consistent across all three populations under high- temperature stress conditions (41{degrees}C and 45{degrees}C). Nutritional composition showed an increased glycogen level in eggs when parents were exposed to wet conditions before blood feeding. By integrating physiological and ecological metrics such as hatching rates and thermal stress resilience, we demonstrate how parental environments shape subsequent egg performance, highlighting adaptive responses that enable Ae. aegypti persistence under increasing climate variability.

At high air temperatures (Ta) atmospheric water vapor can markedly influence an animals heat dissipation by reducing evaporative cooling capacities. At low Ta however, the impact of increased water vapor on heat exchange has sparsely been investigated, despite human populations at northern latitudes insisting on feeling colder on days with higher humidity (i.e., humid-cold). Here, we aimed to investigate the humid-cold perception by determining whether Black-capped Chickadees (Poecile atricapillus) exhibited greater energy expenditure through increased heat loss under humid-cold conditions relative to dry-cold. We measured resting metabolic rates (RMR) of captive chickadees (n=10) during exposure to two Ta treatments, below freezing (Ta {approx} -7{degrees}C) and above freezing (Ta {approx} 10{degrees}C), with either dry or saturated air. We found that Ta substantially impacted RMR, with RMR {approx}1.5-fold greater at -7{degrees}C compared with 10{degrees}C. Conversely, humidity did not have a statistically significant impact on RMR at either Ta. Our data thus suggest that humidity does not significantly influence an individuals heat exchange with the environment at cold Ta. This presumably reflects the fact that cold air can hold minimal amounts of water vapor and any possible influence on heat loss is negligible. Instead, we suggest that a greater contributor to the humid-cold myth is the frequency of overcast days during which direct solar radiation is blocked. Summary statementBlack-capped chickadees significantly increased their resting metabolic rates at air temperatures below freezing relative to air temperatures above freezing, whereas humidity level did not significantly affect resting metabolic rates.