Higher CTmax Research Articles

Temporal and trophic niche partitioning among arboreal ants in a Neotropical woodland savanna

Abstract Niche partitioning is a key mechanism for explaining species coexistence, including the coexistence of ants in trees of the Brazilian savanna (cerrado). However, we have limited information on the extent to which arboreal ant species exploit different food resources and/or have different daily foraging schedules. We tested these ideas through a baiting experiment, and by measuring the isotopic signature (δ15N) and the critical thermal maximum (CTmax) of the 14 most common species found in a typical cerrado tree species. Although most species foraged on all bait types offered, species‐specific preferences were noted for about one‐third of the species. We also found a wide variation in mean δ15N between species, reflecting interspecific differences in trophic position. Most (71.4%) species foraged predominantly on a given period of the day, ranging from strictly nocturnal species to those that foraged mainly in the afternoon. Species with a higher heat tolerance (higher CTmax) often foraged at warmer periods of the day than those with a lower tolerance. Despite the evidence of trophic and temporal niche partitioning, other mechanisms, such as nesting site specialization and behavioural trade‐offs, are required to explain species coexistence in this arboreal ant assemblage, as several species pairs largely overlapped both in their diet and time of foraging. Importantly, our results provide additional support for the idea that physiological restriction to high temperatures is important for understanding interspecific differences in foraging activity schedules.

High-resolution climate data reveal an increasing risk of warming-driven activity restriction for diurnal and nocturnal lizards.

Widespread species experience a variety of climates across their distribution, which can structure their thermal tolerance, and ultimately, responses to climate change. For ectotherms, activity is highly dependent on temperature, its variability and availability of favourable microclimates. Thermal exposure and tolerance may be structured by the availability and heterogeneity of microclimates for species living along temperature and/or precipitation gradients - but patterns and mechanisms underlying such gradients are poorly understood. We measured critical thermal limits (CTmax and CTmin) for five populations of two sympatric lizard species, a nocturnal gecko (Chondrodactylus bibronii) and a diurnal skink (Trachylepis variegata) and recorded hourly thermal variation for a year in three types of microclimate relevant to the activity of lizards (crevice, full sun and partial shade) for six sites across a precipitation gradient. Using a combination of physiological and modelling approaches, we derived warming tolerance for the present and the end of the century. In the present climate, we found an overall wider thermal tolerance for the nocturnal species relative to the diurnal species, and no variation in CTmax but variable CTmin along the precipitation gradient for both species. However, warming tolerances varied significantly over the course of the day, across months and microhabitats. The diurnal skink was most restricted in its daily activity in the three driest sites with up to six daily hours of restricted activity in the open (i.e. outside refugia) during the summer months, while the impacts for the nocturnal gecko were less severe, due to its higher CTmax and night activity. With climate change, lizards will experience more months where activity is restricted and increased exposure to high temperatures even within the more sheltered microhabitats. Together our results highlight the importance of considering the relevant spatiotemporal scale and habitat for understanding the thermal exposure of diurnal and nocturnal species.

Effects of temperature acclimation on the upper thermal tolerance of two Arctic fishes.

The thermally dynamic nearshore Beaufort Sea, Alaska, is experiencing climate change-driven temperature increases. Measuring thermal tolerance of broad whitefish (Coregonus nasus) and saffron cod (Eleginus gracilis), both important species in the Arctic ecosystem, will enhance understanding of species-specific thermal tolerances. The objectives of this study were to determine the extent that acclimating broad whitefish and saffron cod to 5°C and 15°C changed their critical thermal maximum (CTmax) and HSP70 protein and mRNA expression in brain, muscle and liver tissues. After acclimation to 5°C and 15°C, the species were exposed to a thermal ramping rate of 3.4°C · h-1 before quantifying the CTmax and HSP70 protein and transcript concentrations. Broad whitefish and saffron cod acclimated to 15°C had a significantly higher mean CTmax (27.3°C and 25.9°C, respectively) than 5°C-acclimated fish (23.7°C and 23.2°C, respectively), which is consistent with trends in CTmax between higher and lower acclimation temperatures. There were species-specific differences in thermal tolerance with 15°C-acclimated broad whitefish having higher CTmax and HSP70 protein concentrations in liver and muscle tissues than saffron cod at both acclimation temperatures. Tissue-specific differences were quantified, with brain and muscle tissues having the highest and lowest HSP70 protein concentrations, respectively, for both species and acclimation temperatures. The differences in broad whitefish CTmax between the two acclimation temperatures could be explained with brain and liver tissues from 15°C acclimation having higher HSP70a-201 and HSP70b-201 transcript concentrations than control fish that remained in lab-acclimation conditions of 8°C. The shift in CTmax and HSP70 protein and paralogous transcripts demonstrate the physiological plasticity that both species possess in responding to two different acclimation temperatures. This response is imperative to understand as aquatic temperatures continue to elevate.

Cross-species gut microbiota transplantation predictably affects host heat tolerance.

The gut microbiome is known to influence and have regulatory effects in diverse physiological functions of host animals, but only recently has the relationship between host thermal biology and gut microbiota been explored. Here, we examined how early-life manipulations of the gut microbiota in larval amphibians influenced their critical thermal maximum (CTmax) at different acclimation temperatures. We stripped the resident microbiome from egg masses of wild-caught wood frogs (Lithobates sylvaticus) via an antibiotic wash, and then inoculated the eggs with pond water (control), no inoculation, or the intestinal microbiota of another species that has a wider thermal tolerance - green frogs (Lithobates clamitans). We predicted that this cross-species transplant would increase the CTmax of the recipient wood frog larvae relative to the other treatments. In line with this prediction, green frog microbiome-recipient larvae had the highest CTmax while those with no inoculum had the lowest CTmax. Both the microbiome treatment and acclimation temperature significantly influenced the larval gut microbiota communities and α-diversity indices. Green frog microbiome-inoculated larvae were enriched in Rikenellaceae relative to the other treatments, which produce short-chain fatty acids and could contribute to greater energy availability and enhanced heat tolerance. Larvae that received no inoculation had a higher relative abundance of potentially pathogenic Aeromonas spp., which negatively affects host health and performance. Our results are the first to show that cross-species gut microbiota transplants alter heat tolerance in a predictable manner. This finding has repercussions for the conservation of species that are threatened by climate change and demonstrates a need to further explore the mechanisms by which the gut microbiota modulate host thermal tolerance.

Thermal tolerance in Spodoptera frugiperda: Influence of age, sex, and mating status

Temperature plays a key role in shaping the physiology and behavior of ectotherms where responses can depend on age, sex and ontogeny. We investigated the effects of age, sex and mating status on thermal tolerance of Spodoptera frugiperda 3-, 6- and 9-days old adults heat shocked at adult, pupal and larval stages, at 40 °C for 2 h. Our results showed varied significant critical thermal maxima across mating status, age, sex and life stage in tested insects after heat shock. The virgin females CTmax increased with age while it fluctuated in males and 6 days old virgin females had higher CTmax than males. The 3 day old mated females had the highest heat tolerance while 6 days old virgin males the lowest heat tolerance. There was no significant difference in CTmax recorded between virgin males and females aged 3 and 9 days as well as 3, 6 and 9 day mated males and females. There was also no significant interaction between mating status and sex but there was a significant interaction effect between life stage and age. Mated males had higher CTmax than females across all heat shocked developmental stages. The results show a high level of fitness and survival in S. frugiperda females, which may be advantageous in invasion. This study also showed “carry over” effects in adults heat shocked at juvenile stage, with aged adults showing decline in CTmax. This study suggests that plasticity of thermal tolerance may be an important approach for S. frugiperda invasion.

Developmental diet, life stage and thermal acclimation affect thermal tolerance of the fall armyworm, Spodoptera frugiperda

AbstractInsect thermal tolerance affects survival and distribution of species but can vary within and between individuals due to the environmental conditions they experience. The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), recently invaded Africa and its local hotspots are associated with weather and crop phenology. We investigated the effects of larval diet (maize plants, wheat plants or chickpea‐based artificial diet), life stages and acclimation temperature on thermal tolerance traits of FAW to explore how these variables might contribute to its presence in the field. First and sixth instar larvae and adults reared on each diet at 25°C were acclimated for 24 h at 20, 25 or 30°C. We then recorded the critical thermal minimum (CTmin) and critical thermal maximum (CTmax) of individuals. Sixth instars had the highest CTmin but the effects of acclimation and diet on this trait depended on the life stage being tested. CTmin of first instars increased with acclimation temperature when fed on an artificial diet, but sixth instars and adults were not affected by acclimation or larval diet. CTmax was lowest among adults but acclimation and diet again had effects that differed between life stages. CTmax of first instars and adults increased with acclimation temperature but not in sixth instars. Sixth instars and adults reared on the artificial diet had the highest CTmax but diet had no effect on first instar CTmax. Our results show the complexity of thermal tolerance across FAW life stages and suggest the need to consider local temperature variation and available dietary resources when predicting their potential distribution.

Significant colony differences in thermal tolerances and decoupling of high and low critical temperatures in the army ant Eciton burchellii parvispinum

Abstract Differences in behaviour and physiology among social insect colonies provide opportunities for evolutionary responses to environmental selection. Colony differences in thermal physiology can indicate potential for social insect populations to accommodate temperature extremes and determine how populations will respond to changing climates. We tested whether army ant (Eciton burchellii parvispinum) colonies differed in their distributions of worker tolerances of extreme low temperatures (CTmin) and extreme high temperatures (CTmax). We sampled workers across a range of body sizes in each colony, and we collected data from colonies at both warmer low elevation sites and cooler high elevation sites. Colonies differed significantly in the distributions of both CTmin and CTmax; colony mean CTmin and CTmax were not correlated. The colony differences were not associated with worker body size variation. Some thermal physiology differences were associated with local climates: colonies from warmer lower elevation sites had significantly higher CTmin and higher CTmax, but much of the inter‐colony variation was unexplained. If significant colony variation in thermal physiology is associated with genetic differences, thermal environments could drive evolutionary changes in thermal physiology.

The feces of sea urchins as food improves survival, growth, and resistance of small sea cucumbers Apostichopus japonicus in summer

Mass mortality and low growth highly decrease the production efficiency and sustainable aquaculture development of the sea cucumber Apostichopus japonicus in summer. Sea urchin feces was proposed to address the summer problems. A laboratory study was conducted for ~ 5 weeks to investigate survival, food consumption, growth and resistance ability of A. japonicus cultured with the feces of sea urchins fed kelp (KF feces, group KF), the feces of sea urchins fed prepared feed (FF feces, group FF), and the prepared sea cucumber feed (group S) at high temperature (25 °C). The sea cucumbers of group KF had better survival (100%) than those of the group FF (~ 84%), higher CTmax (35.9 °C) than those of the group S (34.5 °C), and the lowest skin ulceration proportion (0%) when they were exposed to an infectious solution among the three groups. These results suggest that the feces of sea urchins fed kelp is a promising diet for improving the survival and enhancing the resistance in A. japonicus aquaculture in summer. Sea cucumbers fed significantly less FF feces after 24 h of ageing than the fresh FF feces, suggesting this kind of feces became unsuitable for A. japonicus in a short time (within 48 h). However, the 24 h of ageing at 25 °C for the high fiber feces of sea urchins fed kelp had no significant effects on the fecal consumption of sea cucumbers. In the present study, both fecal diets provide better individual growth to sea cucumbers than the prepared feed. Yet, the feces of sea urchins fed kelp provided the highest weight gain rate (WGR) to sea cucumbers. Therefore, the feces of sea urchins fed kelp is a promising food to reduce the mortality, to address the problems of summer, and to achieve higher efficiency in A. japonicus aquaculture in summer.

Relationship between aerobic scope and upper thermal limits of Pacific white shrimp (Litopenaeus vannamei) in low-salinity culture systems

Aquaculture of the Pacific white shrimp Litopenaeus vannamei in low-salinity water is a viable industry and production strategy in the southeastern United States. A major challenge facing this industry is a phenomenon called late-term mortality which is thought to be driven by thermal stress at the end of the growing season when water temperatures can reach or exceed 36 °C in shrimp production ponds. To investigate the physiological mechanisms behind upper lethal limits in shrimp, we evaluated linkages between empirically measured thermal limits and absolute aerobic scope (AAS), or ability to provide energy above that needed for basic maintenance. In this study, we tested whether thermal tolerance decreases with increasing shrimp age/size and whether AAS is a useful concept for understanding the physiological basis of thermal tolerance in shrimp. We exposed two size classes (small: 2.07 ± 0.86 and large: 24.64 ± 2.55 g) of shrimp to increasing temperature at a rate of 1 °C/h from 28 to 42 °C. At each temperature, we used intermittent respirometry to estimate resting metabolic rate and we directly measured lethal thermal tolerance by evaluating critical thermal maximum (CTmax). Additionally, we used the electron transport system assay to estimate maximum metabolic rate (RMR) at temperatures from 9 to 45 °C. Small shrimp had a higher CTmax than large shrimp, with upper lethal limits of 40.6 and 39.0 °C, respectively. For both size-classes, AAS reached its minimum (AASmin) at temperatures near the peak RMR (RMRpeak) and within 2 °C of CTmax. Large shrimp exhibited a lower temperature at AASmin than that of the smaller shrimp. Reductions in AAS appear to be one of the underlying physiological drivers of thermal tolerance in L. vannamei and an indicator of increasing thermal stress. Changes in the temperature at which AAS reaches its minimum may be a useful predictor of shifts in thermal tolerance among shrimp size-classes.

Carbohydrate‐rich diet increases critical thermal maximum in ants

AbstractTo understand species’ responses to climate change, we must better comprehend the factors shaping physiological critical thermal limits. One factor of potential importance is nutrient availability. Carbohydrates are an energy source that can directly affect an organism's physiological state. Ants are among the most omnipresent and ecologically relevant animal groups on Earth, and many ant species consume carbohydrate‐based diets. Additionally, as ectotherms, ants are highly vulnerable to the effects of climate change. Here, we examined the relationship between foraging temperature, carbohydrate availability, and critical thermal maximum (CTmax) in ants (Hymenoptera: Formicidae). First, we conducted a laboratory experiment using 3–4 colonies of two species that forage at high temperatures (Camponotus blandus Smith and Dorymyrmex thoracicus Gallardo) and two species that forage at lower temperatures (Nylanderia fulva Mayr and Dolichoderus quadridenticulatus Roger). Each colony was divided into two experimental subcolonies, which were given diets containing different carbohydrate concentrations (5 vs. 20% sucrose solutions). We then measured CTmax. We also collected ants belonging to these species in the field and measured their CTmax. We found that CTmax was highest for the two species that forage at higher temperatures. For C. blandus and D. thoracicus, workers given 20% sucrose had higher CTmax than workers given 5% sucrose. No diet‐mediated differences in CTmax were seen for N. fulva and D. quadridenticulatus workers. Additionally, the experimental ants in both treatment groups had higher CTmax than their field‐collected conspecifics, except in the case of C. blandus. If carbohydrate‐rich diets can boost heat tolerance in some species, it is possible that changes in resource availability could determine how climate change affects ants, especially species with carbohydrate‐based diets. Furthermore, these impacts could ripple across the entire trophic network.

Phenology and plasticity can prevent adaptive clines in thermal tolerance across temperate mountains: The importance of the elevation-time axis.

Critical thermal limits (CTmax and CTmin) decrease with elevation, with greater change in CTmin, and the risk to suffer heat and cold stress increasing at the gradient ends. A central prediction is that populations will adapt to the prevailing climatic conditions. Yet, reliable support for such expectation is scant because of the complexity of integrating phenotypic, molecular divergence and organism exposure. We examined intraspecific variation of CTmax and CTmin, neutral variation for 11 microsatellite loci, and micro‐ and macro‐temperatures in larvae from 11 populations of the Galician common frog (Rana parvipalmata) across an elevational gradient, to assess (1) the existence of local adaptation through a PST‐FST comparison, (2) the acclimation scope in both thermal limits, and (3) the vulnerability to suffer acute heat and cold thermal stress, measured at both macro‐ and microclimatic scales. Our study revealed significant microgeographic variation in CTmax and CTmin, and unexpected elevation gradients in pond temperatures. However, variation in CTmax and CTmin could not be attributed to selection because critical thermal limits were not correlated to elevation or temperatures. Differences in breeding phenology among populations resulted in exposure to higher and more variable temperatures at mid and high elevations. Accordingly, mid‐ and high‐elevation populations had higher CTmax and CTmin plasticities than lowland populations, but not more extreme CTmax and CTmin. Thus, our results support the prediction that plasticity and phenological shifts may hinder local adaptation, promoting thermal niche conservatism. This may simply be a consequence of a coupled variation of reproductive timing with elevation (the “elevation‐time axis” for temperature variation). Mid and high mountain populations of R. parvipalmata are more vulnerable to heat and cool impacts than lowland populations during the aquatic phase. All of this contradicts some of the existing predictions on adaptive thermal clines and vulnerability to climate change in elevational gradients.

Thermal physiology of Tropical House Geckos (Hemidactylus mabouia) in a cool temperate region of South Africa.

ABSTRACT Temperature profoundly influences the functioning of ectotherms and understanding the thermal biology of invasive species affords valuable insights regarding the suitability of different habitats. Yet, this remains an understudied aspect in the field of invasion biology. Here, we investigated the thermal physiology of the highly invasive Tropical House Gecko (Hemidactylus mabouia) in a coastal region of South Africa. The study site is near to the southernmost extent of the species’ distribution and we expected the physiology of individuals at the study site to differ compared to that of individuals inhabiting tropical regions. Relative to tropical geckos, we observed a decrease of 3.4°C in preferred body temperature (Tpref) to 24.0 °C, which was likely driven by selection for improved functioning within the cooler temperate climate. However, the lower (CTmin) and upper (CTmax) critical thermal limits were higher in geckos at the study site. CTmin was similar between sexes at 11.9 °C, whereas males had a higher CTmax by ≈ 1.9 °C reaching 39.7 °C. A broader tolerance range is typical of a highly variable climate, and we presume that the increase in CTmax allows geckos to better cope with occasional high temperatures. Further, sex-specific differences are uncommon in reptiles, but our study supports claims of improved heat tolerance in male geckos, argued to be related to their aggressive and territorial behaviour. The results also support claims that tropical geckos are more adaptable than previously thought, allowing them to invade new areas. Thereafter, their aggressive behaviour, dietary flexibility and year-round activity likely provides house geckos with a competitive advantage over some native reptiles.

Biogeography of cereal stemborers and their natural enemies: forecasting pest management efficacy under changing climate.

Climate warming presents physiological challenges to insects, manifesting as loss of key life-history fitness traits and survival. For interacting host-parasitoid species, physiological responses to heat stress may vary, thereby potentially uncoupling trophic ecological relationships. Here, we assessed heat tolerance traits and sensitivity to prevailing and future maximum temperatures for the cereal stemborer pests, Chilo partellus, Busseola fusca and Sesamia calamistis and their endo-parasitoids, Cotesia sesamiae and Cotesia flavipes. We further used the machine learning algorithm, Maximum Entropy (MaxEnt), to model current and potential distribution of these species. The mean critical thermal maxima (CTmax ) ranged from 39.5 ± 0.9°C to 44.6 ± 0.6°C and from 46.8 ± 0.7°C to 48.5 ± 0.9°C for parasitoids and stemborers, with C. sesamiae and Ch. partellus exhibiting the lowest and highest CTmax respectively. From the current climate to the 2050s scenario, parasitoids recorded a significant reduction in warming tolerance compared with their hosts. Habitat suitability for all stemborer-parasitoid species was spatially heterogeneous under current and future climatic scenarios. Cotesia sesamiae C. flavipes and B. fusca exhibited significant habitat loss, whereas Ch. partellus and S. calamistis showed a significant habitat gain under future 2050s predictions. Model metrics based on mean area under the curve ranged from 0.72 to 0.84 for all species, indicating a good predictive performance of the models. These results suggest C. sesamiae and C. flavipes may face survival constraints or extirpation compared with their pest hosts when environmental temperature reaches their upper thermal limits earlier, likely reducing pest regulation through density-mediated effects. The results demonstrate potential destabilization of stemborer-parasitoid trophic systems potentially compromising biocontrol efficacy under climate warming. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Acute critical thermal maximum does not predict chronic incremental thermal maximum in Atlantic salmon (Salmo salar)

Atlantic salmon is an important aquaculture species farmed in ocean net-pens and therefore subjected to changing environmental conditions, including rising temperatures. This creates a need for research on the thermal tolerance of this species for the future of sustainable aquaculture. We investigated the thermal tolerance of individually tagged Atlantic salmon post-smolts subjected sequentially to two common high-temperature challenges: critical thermal maximum (CTmax) followed by incremental thermal maximum (ITmax). Our goals were (1) to determine whether CTmax can predict ITmax for individual fish, and (2) to examine connections between various body size (mass, length, condition factor), cardiac (absolute and relative ventricle mass) and blood (hematocrit) metrics and thermal tolerance. We found no relationship between CTmax and ITmax. This is of concern because CTmax, which is a quick and easy test, is often used to predict upper lethal limits in fish despite not using real-world rates of temperature increase and not using death as the experimental endpoint (unlike ITmax). Also, some metrics which correlated in one direction with CTmax had the opposite correlation with ITmax. For instance, smaller fish or fish with smaller ventricles had a higher CTmax but a lower ITmax than larger fish or fish with larger ventricles. Taken together, these results highlight the need to take care when using acute thermal tolerance tests to predict real-world responses to rising temperatures.

Insect thermal limits in warm and perturbed habitats: Dragonflies and damselflies as study cases.

Disturbance (e.g. loss of plant cover) increases ambient temperature which can be lethal for ectotherm insects especially in hot places. We compared the thorax temperatures of 26 odonate species as a function of body size, habitat quality ("conserved" and cooler vs "perturbed" and warmer) and suborder (Anisoptera vs Zygoptera), as well as critical thermal maximum (CTmax) and as a function of habitat quality in Argia pulla (Zygoptera) and Orthemis ferruginea (Anisoptera). We expected thorax temperatures to differ between suborders based on their differences in body size and habitat quality status, and that populations in perturbed sites would have higher critical thermal maxima compared to those in conserved sites. This study was done in a tropical region with high ambient temperatures. Anisopterans had a higher body temperature than zygopterans, with no difference between habitats. Thoracic and air temperature were positively related, yet body temperatures were higher than the ambient temperature. A. pulla had higher CTmax in the perturbed sites, while O. ferruginea showed the opposite trend. Microenvironmental changes increase the ambient temperature, perhaps filtering insect species. The apparent resilience of odonates to disturbance should be examined more closely (using more species), especially in small species like the zygopterans which appear to be more strongly affected by ambient temperature.

No consistent effect of daytime versus night-time measurement of thermal tolerance in nocturnal and diurnal lizards.

While essential in understanding impacts of climate change for organisms, diel variation remains an understudied component of temporal variation in thermal tolerance limits [i.e. the critical thermal minimum (CTmin) and maximum (CTmax)]. For example, a higher Ctmax might be expected for an individual if the measurement is taken during the day (when heat stress is most likely to occur) instead of at night. We measured thermal tolerance (Ctmin and Ctmax) during both the daytime and night-time in 101 nocturnal and diurnal geckos and skinks in Hong Kong and in South Africa, representing six species and covering a range of habitats. We found that period of measurement (day vs. night) only affected Ctmin in South Africa (but not in Hong Kong) and that Ctmax was unaffected. Body size and species were important factors for determining Ctmax in Hong Kong and Ctmin in South Africa, respectively. Overall, however, we did not find consistent diel variation of thermal tolerance and suggest that measurements of critical thermal limits may be influenced by timing of measurement-but that such effects, when present, are likely to be context-dependent.

Impact of age, sex, and size on the thermal tolerance of the adult black soldier fly (Diptera: Stratiomyidae)

Thermal stresses from both environmental conditions and organismal crowding are common in mass production of the black soldier fly, Hermetia illucens L. (Diptera: Stratiomyidae). In this study, upper and lower critical thermal (CT) limits (i.e. knockdown CTmax and CTmin) for the adult black soldier fly were determined. Impacts of size, age, and sex on these critical temperatures were also assessed. The CTmax ranged from 45.0-51.0 °C with larger and older adults having a ~1 °C higher CTmax than smaller and younger adults. However, no differences in the CTmax were found between sexes, regardless of age or size. The CTmin ranged from 8.0 to 13.0 °C with larger and older females having a ~1 °C higher CTmin than males and smaller or younger females. While reporting the upper and lower critical temperatures, this study also revealed the thermal breadth (i.e. the range of body temperatures over which organisms can locomote) for adult black soldier flies across age, sex, and size. Based on these data, and when recognising not all fly populations are the same, mass-rearing facilities should determine the CTmax and CTmin for their fly population in order to optimise mating and fertile egg production, and ultimately maximise profits and sales. One degree of temperature can be the difference between success and failure in industrialised facilities.

Arboreality drives heat tolerance while elevation drives cold tolerance in tropical rainforest ants.

Determining how species thermal limits correlate with climate is important for understanding biogeographic patterns and assessing vulnerability to climate change. Such analyses need to consider thermal gradients at multiple spatial scales. Here we relate thermal traits of rainforest ants to microclimate conditions from ground to canopy (microgeographic scale) along an elevation gradient (mesogeographic scale) and calculate warming tolerance to assess climate change vulnerability in the Australian Wet Tropics Bioregion. We test the thermal adaptation and thermal niche asymmetry hypotheses to explain interspecific patterns of thermal tolerance at these two spatial scales. We tested cold tolerance (CTmin ), heat tolerance (CTmax ), and calculated thermal tolerance range (CTrange ), using ramping assays for 74 colonies of 40 ant species collected from terrestrial and arboreal habitats at lowland and upland elevation sites and recorded microclimatic conditions for one year. Within sites, arboreal ants were exposed to hotter microclimates and on average had a 4.2°C (95% CI: 2.7-5.6°C) higher CTmax and 5.3°C (95% CI: 3.5-7°C) broader CTrange than ground-dwelling ants. This pattern was consistent across the elevation gradient, whether it be the hotter lowlands or the cooler uplands. Across elevation, upland ants could tolerate significantly colder temperatures than lowland ants, whereas the change in CTmax was less pronounced, and CTrange did not change over elevation. Differential exposure to microclimates, due to localized niche preferences, drives divergence in CTmax , while environmental temperatures along the elevation gradient drive divergence in CTmin . Our results suggest that both processes of thermal adaptation and thermal niche asymmetry are at play, depending on the spatial scale of observation, and we discuss potential mechanisms underlying these patterns. Despite the broad thermal tolerance range of arboreal rainforest ants, lowland arboreal ants had the lowest warming tolerance and may be most vulnerable to climate change.

Thermal traits predict the winners and losers under climate change: an example from North American ant communities

AbstractAcross the globe, temperatures are predicted to increase with consequences for many taxonomic groups. Arthropods are particularly at risk as temperature imposes physiological constraints on growth, survival, and reproduction. Given that arthropods may be disproportionately affected in a warmer climate—the question becomes which taxa are vulnerable and can we predict the supposed winners and losers of climate change? To address this question, we resurveyed 33 ant communities, quantifying 20‐yr differences in the incidence of 28 genera. Each North American ant community was surveyed with 30 1‐m2 plots, and the incidence of each genus across the 30 plots was used to estimate change. From the original surveys in 1994–1997 to the resurveys in 2016–2017, temperature increased on average 1°C (range, −0.4°C to 2.5°C) and ~64% of ant genera increased in more than half of the sampled communities. To test Thermal Performance Theory's prediction that genera with higher average thermal limits will tend to accumulate at the expense of those with lower limits, we quantified critical thermal maxima (CTmax: the high temperatures at which they lose muscle control) and minima (CTmin: the low temperatures at which ants first become inactive) for common genera at each site. Consistent with prediction, we found a positive decelerating relationship between CTmax and the proportion of sites in which a genus had increased. CTmin, by contrast, was not a useful predictor of change. There was a strong positive correlation (r = 0.85) between the proportion of sites where a genus was found with higher incidence after 20 yr and the average difference in number of plots occupied per site, suggesting genera with high CTmax values tended to occupy more plots at more sites after 20 yr. Thermal functional traits like CTmax have thus proved useful in predicting patterns of long‐term community change in a dominant, diverse insect taxon.

Progeny of Xenopus laevis from altitudinal extremes display adaptive physiological performance.

Environmental temperature variation generates adaptive phenotypic differentiation in widespread populations. We used a common garden experiment to determine whether offspring with varying parental origins display adaptive phenotypic variation related to different thermal conditions experienced in parental environments. We compared burst swimming performance and critical thermal limits of African clawed frog (Xenopus laevis) tadpoles bred from adults captured at high (∼2000 m above sea level) and low (∼ 5 m above sea level) altitudes. Maternal origin significantly affected swimming performance. Optimal swimming performance temperature (Topt) had a >9°C difference between tadpoles with low altitude maternal origins (pure- and cross-bred, 35.0°C) and high-altitude maternal origins (pure-bred, 25.5°C; cross-bred, 25.9°C). Parental origin significantly affected critical thermal (CT) limits. Pure-bred tadpoles with low-altitude parental origins had higher CTmax (37.8±0.8°C) than pure-bred tadpoles with high-altitude parental origins and all cross-bred tadpoles (37.0±0.8 and 37.1±0.8°C). Pure-bred tadpoles with low-altitude parental origins and all cross-bred tadpoles had higher CTmin (4.2±0.7 and 4.2±0.7°C) than pure-bred tadpoles with high-altitude parental origins (2.5±0.6°C). Our study shows that the varying thermal physiological traits of Xenopus laevis tadpoles are the result of adaptive responses to their parental thermal environments. This study is one of few demonstrating potential intraspecific evolution of critical thermal limits in a vertebrate species. Multi-generation common garden experiments and genetic analyses would be required to further tease apart the relative contribution of plastic and genetic effects to the adaptive phenotypic variation observed in these tadpoles.