Lower thermal tolerance in nocturnal than in diurnal ants: a challenge for nocturnal ectotherms facing global warming

1. The thermal adaptation hypothesis proposes that because thermoregulation involves a high metabolic cost, thermal limits of organisms must be locally adapted to temperatures experienced in their environments. There is evidence that tolerance to high temperatures decreases in insects inhabiting colder habitats and microclimates. However, it is not clear if thermal limits of ectotherms with contrasting temporal regimes, such as diurnal and nocturnal insects, are also adapted to temperatures associated with their circadian activities. 2. This study explores differences in heat tolerance among diurnal and nocturnal ant species in four ecosystems in Mexico: tropical montane, tropical rainforest, subtropical dry forests, and high‐elevation semi‐desert. 3. The critical thermal maximum (CT_max), i.e. the temperature at which ants lost motor control, was estimated for diurnal and nocturnal species. CT_max for 19 diurnal and 12 nocturnal ant species distributed among 45 populations was also estimated. 4. Semi‐desert and subtropical dry forest ants displayed higher tolerances to high temperatures than did ants in tropical rainforest. The lowest tolerance to high temperatures was recorded in tropical montane forest ants. In general, among all habitats, the CT_max of nocturnal ants was lower than that of diurnal ants. 5. An increase in nocturnal temperatures, combined with lower tolerance to high temperatures, may represent a substantial challenge for nocturnal ectotherms in a warming world.     

Desiccation resistance in tropical insects: causes and mechanisms underlying variability in a Panama ant community

Desiccation resistance, the ability of an organism to reduce water loss, is an essential trait in arid habitats. Drought frequency in tropical regions is predicted to increase with climate change, and small ectotherms are often under a strong desiccation risk. We tested hypotheses regarding the underexplored desiccation potential of tropical insects. We measured desiccation resistance in 82 ant species from a Panama rainforest by recording the time ants can survive desiccation stress. Species' desiccation resistance ranged from 0.7 h to 97.9 h. We tested the desiccation adaptation hypothesis, which predicts higher desiccation resistance in habitats with higher vapor pressure deficit (VPD) – the drying power of the air. In a Panama rainforest, canopy microclimates averaged a VPD of 0.43 kPa, compared to a VPD of 0.05 kPa in the understory. Canopy ants averaged desiccation resistances 2.8 times higher than the understory ants. We tested a number of mechanisms to account for desiccation resistance. Smaller insects should desiccate faster given their higher surface area to volume ratio. Desiccation resistance increased with ant mass, and canopy ants averaged 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Water content was on average 2.5% higher in canopy ants, but total water content was not a good predictor of ant desiccation resistance or critical thermal maximum (CT_max), a measure of an ant's thermal tolerance. In canopy ants, desiccation resistance and CT_max were inversely related, suggesting a tradeoff, while the two were positively correlated in understory ants. This is the first community level test of desiccation adaptation hypothesis in tropical insects. Tropical forests do contain desiccation‐resistant species, and while we cannot predict those simply based on their body size, high levels of desiccation resistance are always associated with the tropical canopy.     

Rain shadow effects predict population differences in thermal tolerance of leaf-cutting ant workers (Atta cephalotes)

Tests of hypotheses for the evolution of thermal physiology often rely on mean temperatures, but mounting evidence suggests geographic variation in temperature extremes is also an important predictor of species’ thermal tolerances. Although the tropics are less thermally variable than higher latitude regions, rain shadows on the leeward sides of mountains can experience greater diel and seasonal variation in temperature than windward sites. Rain shadows provide opportunities to test predictions about the relationships of extreme temperatures with thermal physiology while controlling for latitude. We tested the hypothesis that populations of leaf-cutting ants (Atta cephalotes) in leeward, montane, and windward sites in Costa Rica would differ in upper thermal tolerances (CT_max) of workers. As predicted from rain shadow effects via extreme high temperatures, the leeward rain shadow site yielded the highest mean CT_max (rain shadow site 42.1 ± 0.3°C, Montane site 38.2 ± 0.5°C, and windward site 38.2 ± 0.3°C). This suggests that high-temperature extremes in tropical rain shadow forests can select for higher thermal tolerances. CT_max increased with worker body size within sites, but CT_max increased with body size more gradually at the two lowland sites, as predicted if local high temperatures selected more strongly on the most thermally vulnerable society members (small workers). This suggests that warmer lowland climates selected for colonies with less variation in heat tolerance than cooler high elevation climates.     

Out on a limb: Thermal microenvironments in the tropical forest canopy and their relevance to ants

Small, cursorial ectotherms like ants often are immersed in the superheated air layers that develop millimeters above exposed, insolated surfaces (i.e., the thermal boundary layer). We quantified the thermal microenvironments around tree branches in the tropical rainforest canopy, and explored the effects of substrate color on the internal body temperature and species composition of arboreal ants. Branch temperatures during the day (09:00–16:00) were hottest (often > 50 °C) and most variable on the upper surface, while the lowest and least variable temperatures occurred on the underside. Temperatures on black substrates declined with increasing distance above the surface in both the field and the laboratory. By contrast, a micro-scale temperature inversion occurred above white substrates. Wind events (ca. 2 m s⁻¹) eliminated these patterns. Internal temperatures of bodies of Cephalotes atratus workers experimentally heated in the laboratory were 6 °C warmer on white vs. black substrates, and 6 °C cooler than ambient in windy conditions. The composition of ant species foraging at baits differed between black-painted and unpainted tree branches, with a tendency for smaller ants to avoid the significantly hotter black surfaces. Collectively, these outcomes show that ants traversing canopy branches experience very heterogeneous thermal microenvironments that are partly influenced in predictable ways by branch surface coloration and breezy conditions.     

The role of nest surface temperatures and the brain in influencing ant metabolic rates

Thermal limits of insects can be influenced by recent thermal history: here we used thermolimit respirometry to determine metabolic rate responses and thermal limits of the dominant meat ant, Iridomyrmex purpureus. Firstly, we tested the hypothesis that nest surface temperatures have a pervasive influence on thermal limits. Metabolic rates and activity of freshly field collected individuals were measured continuously while ramping temperatures from 44 °C to 62 °C at 0.25 °C/minute. At all the stages of thermolimit respirometry, metabolic rates were independent of nest surface temperatures, and CT_max did not differ between ants collected from nest with different surface temperatures. Secondly, we tested the effect of brain control on upper thermal limits of meat ants via ant decapitation experiments (‘headedness’). Decapitated ants exhibited similar upper critical temperature (CT_max) results to living ants (Decapitated 50.3±1.2 °C: Living 50.1±1.8 °C). Throughout the temperature ramping process, ‘headedness’ had a significant effect on metabolic rate in total (Decapitated V̇CO₂ 140±30 µl CO₂ mg⁻¹ min⁻¹: Living V̇CO₂ 250±50 CO₂ mg⁻¹ min⁻¹), as well as at temperatures below and above CT_max. At high temperatures (>44 °C) pre- CT_max the relationships between I. purpureus CT_max values and mass specific metabolic rates for living ants exhibited a negative slope whilst decapitated ants exhibited a positive slope. The decapitated ants also had a significantly higher Q₁₀:25–35 °C when compared to living ants (1.91±0.43 vs. 1.29±0.35). Our findings suggest that physiological responses of ants may be able to cope with increasing surface temperatures, as shown by metabolic rates across the thermolimit continuum, making them physiologically resilient to a rapidly changing climate. We also demonstrate that the brain plays a role in respiration, but critical thermal limits are independent of respiration levels.     

City limits: Heat tolerance is influenced by body size and hydration state in an urban ant community

Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. We used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CT_max) in urban ants—specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CT_max. We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CT_max for 11 species common to cities in California's Central Valley that exhibit nearly 300‐fold variation in body size. There was a moderate phylogenetic signal influencing CT_max, and inter (but not intra) specific variation in body size influenced CT_max where larger species had higher CT_max. The sensitivity of ants’ CT_max to water availability exhibited species‐specific thresholds where short‐term water limitation (8 hr) reduced CT_max and body water content in some species while longer‐term water limitation (32 hr) was required to reduce these traits in other species. However, CT_max was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CT_max and estimates of thermal safety margin in native species were more sensitive to water availability relative to non‐native species. In sum, we provide evidence of links between heat tolerance and water availability, which will become critically important in an increasingly warm, dry, and urbanized world that others have shown may be selecting for smaller (not larger) body size.     

Variation in thermal tolerance of North American ants

Changing climates are predicted to alter the distribution of thermal niches. Small ectotherms such as ants may be particularly vulnerable to heat injury and death. We quantified the critical thermal maxima of 92 ant colonies representing 14 common temperate ant species. The mean CT_max for all measured ants was 47.8 °C (±0.27; range=40.2–51.2 °C), and within-colony variation was lower than among-colony variation. Critical thermal maxima differed among species and were negatively correlated with body size. Results of this study illustrate the importance of accounting for mass, among and within colony variation, and interspecific differences in diel activity patterns, which are often neglected in studies of ant thermal physiology.     

Thermal constraints on foraging of tropical canopy ants

Small cursorial ectotherms risk overheating when foraging in the tropical forest canopy, where the surfaces of unshaded tree branches commonly exceed 50 °C. We quantified the heating and subsequent cooling rates of 11 common canopy ant species from Panama and tested the hypothesis that ant workers stop foraging at temperatures consistent with the prevention of overheating. We created hot experimental “sunflecks” on existing foraging trails of four ant species from different clades and spanning a broad range of body size, heating rate, and critical thermal maxima (CT_max). Different ant species exhibited very different heating rates in the lab, and these differences did not follow trends predicted by body size alone. Experiments with ant models showed that heating rates are strongly affected by color in addition to body size. Foraging workers of all species showed strong responses to heating and consistently abandoned focal sites between 36 and 44 °C. Atta colombica and Azteca trigona workers resumed foraging shortly after heat was removed, but Cephalotes atratus and Dolichoderus bispinosus workers continued to avoid the heated patch even after >5 min of cooling. Large foraging ants (C. atratus) responded slowly to developing thermal extremes, whereas small ants (A. trigona) evacuated sunflecks relatively quickly, and at lower estimated body temperatures than when revisiting previously heated patches. The results of this study provide the first field-based insight into how foraging ants respond behaviorally to the heterogeneous thermal landscape of the tropical forest canopy.     

Thermal biology of the alien ground beetle <Emphasis Type="Italic">Merizodus soledadinus</Emphasis> introduced to the Kerguelen Islands

Thermal tolerance is one of the major determinants of successful establishment and spread of invasive aliens. Merizodus soledadinus (Coleoptera, Carabidae) was accidentally introduced to Kerguelen from the Falkland Islands in 1913. On Kerguelen, the climate is cooler than the Falklands Islands but has been getting warmer since the 1990s, in synchrony with the rapid expansion of M. soledadinus. We aimed to investigate the thermal sensitivity in adults of M. soledadinus and hypothesised that climate warming has assisted the colonisation process of M. soledadinus. We examined (1) survival of constant low temperatures and at fluctuating thermal regimes, (2) the critical thermal limits (CT_min and CT_max) of acclimated individuals (4, 8 and 16°C), (3) the metabolic rates of acclimated adults at temperatures from 0 to 16°C. The FTRs moderately increased the duration of survival compared to constant cold exposure. M. soledadinus exhibited an activity window ranged from −5.5 ± 0.3 to 38 ± 0.5°C. The Q ₁₀ after acclimation to temperatures ranging from 0 to 16°C was 2.49. Our work shows that this species is only moderately cold tolerant with little thermal plasticity. The CT_min of M. soledadinus are close to the low temperatures experienced in winter on Kerguelen Islands, but the CT_max are well above summer conditions, suggesting that this species has abundant scope to deal with current climate change.     

Intraspecific geographic variation in thermal limits and acclimatory capacity in a wide distributed endemic frog

Intraspecific variation in physiological traits and the standard metabolic rate (SMR) is common in widely distributed ectotherms since populations at contrasting latitudes experiences different thermal conditions. The climatic variability hypothesis (CVH) states that populations at higher latitudes presents higher acclimation capacity than those at lower latitudes, given the wider range of climatic variability they experience. The endemic four-eyed frog, Pleurodema thaul is widely distributed in Chile. We examined the variation in maximum and minimum critical temperatures (CT_max and CT_min), preferred temperature (T_Pref), SMR and their acclimatory capacity in two populations from the northern and center of its distribution. All the traits are higher in the warmer population. The capacity for acclimation varies between traits and, with the exception of CT_max and T_Pref, it is similar between populations. This pattern could be explained by the higher daily thermal variability in desert environments, that increases plasticity to the levels found in the high latitude population. However, we found low acclimatory capacity in all physiological traits, of only about 3% for CT_min, 10% for CT_max and T_Pref, and 1% for SMR. Thus, despite the fact that Pleurodema thaul possess some ability to adjust thermal tolerances in response to changing thermal conditions, this acclimatory capacity seems to be unable to prevent substantial buffering when body temperatures rise. The low acclimatory capacity found for P. thaul suggests that this species use behavioral rather than physiological adjustments to compensate for environmental variation, by exploiting available micro-environments with more stable thermal conditions.     

Thermoregulatory behaviour explains countergradient variation in the upper thermal limit of a rainforest skink

The vulnerability of a terrestrial ectotherm to high environmental temperatures depends on the animal's thermal physiology and thermoregulatory behaviour. These variables – environment, physiology, and behaviour – interact with each other, complicating assessment of species vulnerability to global warming. We previously uncovered a counterintuitive pattern in rainforest sunskinks Lampropholis coggeri: a negative relationship between their critical thermal maximum (CT_max) and the temperature of their environment. Could this result be explained by a three‐way interaction between environment, physiology, and behaviour? Here we find that sunskink thermal preference is correlated positively with CT_max, but, importantly, skinks from hotter environments prefer lower temperatures than conspecifics from cooler environments. In an acclimation experiment, we find that CT_max is plastic and shifts in alignment with acclimation temperature. We also found heritable variation in this trait in a common garden study, but this variation was small relative to the plastic shifts observed in CT_max. Thus, our previous observation of a negative correlation between field CT_max and temperature is explained, at least in part, by the lizard's thermoregulatory behaviour: lizards from hot environments preferentially choose cool microenvironments, and their physiology acclimates to these cooler experienced temperatures. Our results suggest that behavioural adjustments to the environment can produce countergradient variation in physiological traits. More broadly, our work underscores the importance of interactions between environment, behaviour, and physiology in ectotherms. Understanding these interactions will be crucial in assessing vulnerability to climate change.     

Divergence of thermal physiological traits in terrestrial breeding frogs along a tropical elevational gradient

Critical thermal limits are thought to be correlated with the elevational distribution of species living in tropical montane regions, but with upper limits being relatively invariant compared to lower limits. To test this hypothesis, we examined the variation of thermal physiological traits in a group of terrestrial breeding frogs (Craugastoridae) distributed along a tropical elevational gradient. We measured the critical thermal maximum (CT_max; n = 22 species) and critical thermal minimum (CT_min; n = 14 species) of frogs captured between the Amazon floodplain (250 m asl) and the high Andes (3,800 m asl). After inferring a multilocus species tree, we conducted a phylogenetically informed test of whether body size, body mass, and elevation contributed to the observed variation in CT_max and CT_min along the gradient. We also tested whether CT_max and CT_min exhibit different rates of change given that critical thermal limits (and their plasticity) may have evolved differently in response to different temperature constraints along the gradient. Variation of critical thermal traits was significantly correlated with species’ elevational midpoint, their maximum and minimum elevations, as well as the maximum air temperature and the maximum operative temperature as measured across this gradient. Both thermal limits showed substantial variation, but CT_min exhibited relatively faster rates of change than CT_max, as observed in other taxa. Nonetheless, our findings call for caution in assuming inflexibility of upper thermal limits and underscore the value of collecting additional empirical data on species’ thermal physiology across elevational gradients.     

How repeatable is CTmax within individual brook trout over short- and long-time intervals?

As stream temperatures increase due to factors such as heated runoff from impervious surfaces, deforestation, and climate change, fish species adapted to cold water streams are forced to move to more suitable habitat, acclimate or adapt to increased thermal regimes, or die. To estimate the potential for adaptation, a (within individual) repeatable metric of thermal tolerance is imperative. Critical thermal maximum (CT_max) is a dynamic test that is widely used to measure thermal tolerance across many taxa and has been used in fishes for decades, but its repeatability in most species is unknown. CT_max tests increase water temperature steadily over time until loss of equilibrium (LOE) is achieved. To determine if CT_max is a consistent metric within individual fish, we measured CT_max on the same lab-held individually-marked adult brook trout Salvelinus fontinalis at three different times (August & September 2016, September 2017). We found that CT_max is a repeatable trait (Repeatability ± S.E.: 0.48 ± 0.14). CT_max of individuals males was consistent over time, but the CT_max of females increased slightly over time. This result indicates that CT_max is a robust, repeatable estimate of thermal tolerance in a cold-water adapted fish.     

Can temperate insects take the heat? A case study of the physiological and behavioural responses in a common ant, Iridomyrmex purpureus (Formicidae), with potential climate change

Insects in temperate regions are predicted to be at low risk of climate change relative to tropical species. However, these assumptions have generally been poorly examined in all regions, and such forecasting fails to account for microclimatic variation and behavioural optimisation. Here, we test how a population of the dominant ant species, Iridomyrmex purpureus, from temperate Australia responds to thermal stress. We show that ants regularly forage for short periods (minutes) at soil temperatures well above their upper thermal limits (upper lethal temperature = 45.8 ± 1.3 °C; CT_max = 46.1 °C) determined over slightly longer periods (hours) and do not show any signs of a classic thermal performance curve in voluntary locomotion across soil surface temperatures of 18.6–57°C (equating to a body temperature of 24.5–43.1 °C). Although ants were present all year round, and dynamically altered several aspects of their thermal biology to cope with low temperatures and seasonal variation, temperature-dependence of running speed remained invariant and ants were unable to elevate high temperature tolerance using plastic responses. Measurements of microclimate temperature were higher than ant body temperatures during the hottest part of the day, but exhibited a stronger relationship with each other than air temperatures from the closest weather station. Generally close associations of ant activity and performance with microclimatic conditions, possibly to maximise foraging times, suggest I. purpureus displays highly opportunistic thermal responses and readily adjusts behaviour to cope with high trail temperatures. Increasing frequency or duration of high temperatures is therefore likely to result in an immediate reduction in foraging efficiency. In summary, these results suggest that (1) soil-dwelling temperate insect populations may be at higher risks of thermal stress with increased frequency or duration of high temperatures resulting from climate change than previously thought, however, behavioural cues may be able to compensate to some extent; and (2) indices of climate change-related thermal stress, warming tolerance and thermal safety margin, are strongly influenced by the scale of climate metrics employed.     

Heat tolerance in Drosophila subobscura along a latitudinal gradient: Contrasting patterns between plastic and genetic responses

Susceptibility to global warming relies on how thermal tolerances respond to increasing temperatures through plasticity or evolution. Climatic adaptation can be assessed by examining the geographic variation in thermal‐related traits. We studied latitudinal patterns in heat tolerance in Drosophila subobscura reared at two temperatures. We used four static stressful temperatures to estimate the thermal death time (TDT) curves, and two ramping assays with fast and slow heating rates. Thermal death time curves allow estimation of the critical thermal maximum (CT_max), by extrapolating to the temperature that would knock down the flies almost “instantaneously,” and the thermal sensitivity to increasing stressful temperatures. We found a positive latitudinal cline for CT_max, but no clinal pattern for knockdown temperatures estimated from the ramping assays. Although high‐latitude populations were more tolerant to an acute heat stress, they were also more sensitive to prolonged exposure to less stressful temperatures, supporting a trade‐off between acute and chronic heat tolerances. Conversely, developmental plasticity did not affect CT_max but increased the tolerance to chronic heat exposition. The patterns observed from the TDT curves help to understand why the relationship between heat tolerance and latitude depends on the methodology used and, therefore, these curves provide a more complete and reliable measurement of heat tolerance.     

Thermal niche variation among individuals of the poison frog,Oophaga pumilio,in forest and converted habitats

The conversion of natural habitats to human land uses often increases local temperatures, creating novel thermal environments for species. The variable responses of ectotherms to habitat conversion, where some species decline while others persist, can partly be explained by variation among species in their thermal niches. However, few studies have examined thermal niche variation within species and across forest‐land use ecotones, information that could provide clues about the capacity of species to adapt to changing temperatures. Here, we quantify individual‐level variation in thermal traits of the tropical poison frog, Oophaga pumilio, in thermally contrasting habitats. Specifically, we examined local environmental temperatures, field body temperatures (T_b), preferred body temperatures (T_pref), critical thermal maxima (CT_max), and thermal safety margins (TSM) of individuals from warm, converted habitats and cool forests. We found that frogs from converted habitats exhibited greater mean T_b and T_pref than those from forests. In contrast, CT_max and TSM did not differ significantly between habitats. However, CT_max did increase moderately with increasing T_b, suggesting that changes in CT_max may be driven by microscale temperature exposure within habitats rather than by mean habitat conditions. Although O. pumilio exhibited moderate divergence in T_pref, CT_max appears to be less labile between habitats, possibly due to the ability of frogs in converted habitats to maintain their T_b below air temperatures that reach or exceed CT_max. Selective pressures on thermal tolerances may increase, however, with the loss of buffering microhabitats and increased frequency of extreme temperatures expected under future habitat degradation and climate warming. Abstract in Spanish is available with online material.     

Constant and fluctuating temperature acclimations have similar effects on phenotypic plasticity in springtails

Much interest exists in the extent to which constant versus fluctuating temperatures affect thermal performance traits and their phenotypic plasticity. Theory suggests that effects should vary with temperature, being especially pronounced at more extreme low (because of thermal respite) and high (because of Jensen's inequality) temperatures. Here we tested this idea by examining the effects of constant temperatures (10 to 30 °C in 5 °C increments) and fluctuating temperatures (means equal to the constant temperatures, but with fluctuations of ±5 °C) temperatures on the adult (F2) phenotypic plasticity of three thermal performance traits – critical thermal minimum (CT_min), critical thermal maximum (CT_max), and upper lethal temperature (ULT₅₀) in ten species of springtails (Collembola) from three families (Isotomidae 7 spp.; Entomobryidae 2 spp.; Onychiuridae 1 sp.). The lowest mean CT_min value recorded here was -3.56 ± 1.0 °C for Paristoma notabilis and the highest mean CT_max was 43.1 ± 0.8 °C for Hemisotoma thermophila. The Acclimation Response Ratio for CT_min was on average 0.12 °C/°C (range: 0.04 to 0.21 °C/°C), but was much lower for CT_max (mean: 0.017 °C/°C, range: -0.015 to 0.047 °C/°C) and lower also for ULT₅₀ (mean: 0.05 °C/°C, range: -0.007 to 0.14 °C/°C). Fluctuating versus constant temperatures typically had little effect on adult phenotypic plasticity, with effect sizes either no different from zero, or inconsistent in the direction of difference. Previous work assessing adult phenotypic plasticity of these thermal performance traits across a range of constant temperatures can thus be applied to a broader range of circumstances in springtails.     

Water deprivation drives intraspecific variability in lizard heat tolerance

Quantifying intraspecific variation in heat tolerance is critical to understand how species respond to climate change. In a previous study, we recorded variability in critical thermal maxima (CT_max) by 3 °C among populations of small Iberian lizard species, which could substantially influence predictions of climate-driven activity restriction. Here, we undertake experiments to examine whether we could reproduce similar levels of heat-tolerance variability in response to water deficit. We hypothesized that deprivation of drinking water should increase variability in CT_max between populations more than deprivation of food under the theoretical expectation that the variation of the more limiting resource must trigger stronger variation in physiological performance. We measured CT_max after manipulating availability of live prey and drinking water in two populations of an arid and a mesic lizard species from the Iberian Peninsula. We quantified a mean CT_max across all studied lizards of 44.2 °C ± 0.2 SE for the arid species and 41.7 °C ± 0.3 SE for the mesic species. Using multimodel inference, we found that water deprivation (combined with food supply) caused population differences in CT_max by 3 to 4 °C which were two to three times wider than population differences due to food deprivation (combined with water supply) or to food and water provision. To highlight the need for more thermo-hydroregulatory research, we examined bias in research effort towards thermal versus hydric environmental effects on heat tolerance through a systematic literature review. We show that environmental temperature has been used five times more frequently than precipitation in ecological studies of heat tolerance of terrestrial species. Studies linking thermal tolerance of ectotherms to the interplay of air temperature and water availability are needed in the face of projected increases in aridity and drought in the 21st century, because the balance of body temperature and water resources are functionally interlinked.     

Upper thermal tolerance plasticity in tropical amphibian species from contrasting habitats: Implications for warming impact prediction

Tropical ectothermic species are currently depicted as more vulnerable to increasing temperatures because of the proximity between their upper thermal limits and environmental temperatures. Yet, the acclimatory capacity of thermal limits has rarely been measured in tropical species, even though they are generally predicted to be smaller than in temperate species. We compared critical thermal maximum (CT_max) and warming tolerance (WT: the difference between CT_max and maximum temperature, T_max), as well as CT_max acclimatory capacity of toad species from the Atlantic forest (AF) and the Brazilian Caatinga (CAA), a semi-arid habitat with high temperatures. Acclimation temperatures represented the mean temperatures of AF and CAA habitats, making estimates of CT_max and WT more ecologically realistic. CAA species mean CT_max was higher compared to AF species in both acclimation treatments. Clutches within species, as well as between AF and CAA species, differed in CT_max plasticity and we discuss the potential biological meaning of these findings. We did not find a trade-off between absolute CT_max and CT_max plasticity, indicating that species can have both high CT_max and high CT_max plasticity. Although CT_max was highly correlated to T_max, CT_max plasticity was not related to T_max or T_max coefficients of variation. CAA species mean WT was lower than for AF species, but still very high for all species, diverging from other studies with tropical species. This might be partially related to over-estimation of vulnerability due to under-appreciation of realistic acclimation treatments in CT_max estimation. Thus, some tropical species might not be as vulnerable to warming as previously predicted if CT_max is considered as a shifting population parameter.     

Standardized ethograms and a device for assessing amphibian thermal responses in a warming world

Most predictions of how populations and species of ectotherms will respond to global warming are based on estimates of the temperature at which organisms lose motor control (i.e., CT_max - the Critical Thermal Maximum). Here, we describe a non-lethal protocol and ethograms to estimate the relative tolerance of amphibians to increasing temperatures. These methods—suitable for field or laboratory conditions—are replicable, inexpensive and applicable to both post-metamorphic stages and organisms with direct development. We illustrate the use of this standardized protocol for four amphibians from a tropical cloud forest in Veracruz, Mexico with contrasting life histories: a lungless salamander (Aquiloeurycea cafetalera: Plethodontidae), a leaf-litter frog (Craugastor rhodopis: Craugastoridae), a semiaquatic frog (Lithobates berlandieri: Ranidae), and a tree frog (Rheohyla miotympanum: Hylidae). We identified four behavioral responses preceding CT_max for all amphibians included in this study: 1) Optimal Activity Range, 2) Supra-optimal Activity Range, 3) Heat Stress Range, and 4) Involuntary Movements Range. Additionally, we identified a fifth parameter associated with resilience to heat shock: 5) Recovery Stage after reaching CT_max. We conclude that the behavioral responses preceding the Critical Thermal Maximum are as informative as CT_max. Using behavioral responses to estimate thermal tolerance has the additional advantage of reducing the risk of injury or death of amphibians during physiological experiments.