When things get hot: Thermoregulation behavior in the lizard Sceloporus aeneus at different thermal conditions

Rising environmental temperatures have become a global threat for ectotherms, with the increasing risk of overheating promoting population declines. Flexible thermoregulatory behavior might be a plausible mechanism to mitigate the effects of extreme temperatures. We experimentally evaluated thermoregulatory behavior in the bunchgrass lizard, Sceloporus aeneus, at three different environmental temperatures (25, 35 and 45 °C) both with and without a thermal refuge. We recorded themoregulatory behaviors (body posture and movement between hot and cold patches) and compared individual lizards across all experimental temperature and shelter combinations. Behavioral thermoregulation in S. aeneus was characterized by the expression of five body postures, whose frequencies varied based on environmental temperature and microthermal conditions. Behavioral responses allowed lizards to maintain a mean body temperature <40 °C, the critical thermal maximum for temperate species, even at extreme environmental temperatures (45 °C). Although S. aeneus express an array of behavioral postures that provide an effective mechanism to cope with elevating temperatures, the presence of a thermal refuge was important to better achieve this. Together, our study offers a novel method to evaluate microhabitat preference that encompasses both behavioral observations and time-space analysis based on the ambient thermal distribution, a consideration that can aid in the formulation of more accurate predictions on ectotherm vulnerability related to increasing global environmental temperatures.     

The effects of thermal biology and refuge availability on the restricted distribution of an alpine lizard

Aim In an effort to disentangle the ecological processes that confine ectotherms to alpine environments, we studied the thermoregulatory and microhabitat selection behaviours of the rock lizard Iberolacerta cyreni, which is endemic to some mountains of central Spain, and of the wall lizard Podarcis muralis, which is a potential competitor of rock lizards. Location We chose three areas in the Sierra de Guadarrama (central Spain) that differed in their thermal quality [mean deviation of environmental operative temperatures from the lizards’ preferred thermal range (PTR)] and refuge availability: a pine forest (1770 m a.s.l.) in which P. muralis was the only species found, and two mixed shrub and rock sites (1770 and 1900 m a.s.l.) where both species were present. Methods In the field we collected data on refuge availability, sun exposure, body temperature (T_b) and operative temperature (T_e). Thus, we estimated the thermal habitat quality of the areas sampled and the thermoregulation accuracy and effectiveness of both species. Results The pine forest had the lowest thermal quality and refuge availability. The lower‐elevation shrub site offered the best thermal quality, but refuges were much scarcer than at the higher‐elevation site. Both species thermoregulated accurately, because mean deviations of body temperature (T_b) from PTR were considerably smaller than those of T_e. Podarcis muralis had higher T_b values than did I. cyreni, which had similar T_b values at both shrub sites, whereas P. muralis had lower T_b values at higher elevation. Overall, the thermoregulatory effectiveness (extent to which T_b values are closer to the PTR than are T_e values) of both species was similar, but whereas I. cyreni thermoregulated more efficiently at higher elevation, the opposite was true for P. muralis. At the lower‐elevation shrub site, I. cyreni remained closer to refuges than did P. muralis. Main conclusions Our results suggest that the pine forest belt might prevent the expansion of rock lizards towards lower elevations as a result of its low thermal quality and scarcity of refuges, that the thermoregulatory effectiveness of rock lizards in alpine environments depends more on refuge availability than on thermal habitat quality, and that competition with wall lizards is unlikely to explain either the distribution or the thermoregulatory effectiveness of rock lizards.     

A heterogeneous thermal environment enables remarkable behavioral thermoregulation in Uta stansburiana

Ectotherms can attain preferred body temperatures by selecting specific temperature microhabitats within a varied thermal environment. The side‐blotched lizard, Uta stansburiana may employ microhabitat selection to thermoregulate behaviorally. It is unknown to what degree habitat structural complexity provides thermal microhabitats for thermoregulation. Thermal microhabitat structure, lizard temperature, and substrate preference were simultaneously evaluated using thermal imaging. A broad range of microhabitat temperatures was available (mean range of 11°C within 1–2 m²) while mean lizard temperature was between 36°C and 38°C. Lizards selected sites that differed significantly from the mean environmental temperature, indicating behavioral thermoregulation, and maintained a temperature significantly above that of their perch (mean difference of 2.6°C). Uta's thermoregulatory potential within a complex thermal microhabitat structure suggests that a warming trend may prove advantageous, rather than detrimental for this population.     

Some (worms) like it hot: fish parasites grow faster in warmer water,and alter host thermal preferences

Elevated environmental temperatures associated with anthropogenic warming have the potential to impact host‐parasite interactions, with consequences for population health and ecosystem functioning. One way that elevated temperatures might influence parasite prevalence and intensity is by increasing life cycle completion rates. Here, we investigate how elevated temperatures impact a critical phase of the life cycle of the bird tapeworm Schistocephalus solidus – the growth of plerocercoid larvae in host fish (three‐spined sticklebacks Gasterosteus aculeatus). By 8 weeks post‐infection, plerocercoids recovered from experimentally infected sticklebacks held at 20 °C weighed on average 104.9 mg, with all exceeding 50 mg, the mass considered consistently infective to definitive hosts. In contrast, plerocercoids from sticklebacks held at 15 °C weighed on average 26.5 mg, with none exceeding 50 mg. As small increases in plerocercoid mass affect adult fecundity disproportionately in this species, enhanced plerocercoid growth at higher temperatures predicts dramatically increased output of infective parasite stages. Subsequent screening of thermal preferences of sticklebacks from a population with endemic S. solidus infection demonstrated that fish harbouring infective plerocercoids show significant preferences for warmer temperatures. Our results therefore indicate that parasite transmission might be affected in at least two ways under anthropogenic warming; by enhancing rates of parasite growth and development, and by increasing the likelihood of hosts being able to seek out proliferating warmer microhabitats. Furthermore, our results suggest the potential for positive feedback between parasite growth and host thermal preferences, which could dramatically increase the effects of even small temperature increases. We discuss the possible mechanisms underpinning our results, their likely ecological consequences and highlight key areas for further research.     

The highest kingdom of Anolis: Thermal biology of the Andean lizard Anolis heterodermus (Squamata: Dactyloidae) over an elevational gradient in the Eastern Cordillera of Colombia

Vertebrate ectotherms may deal with changes of environmental temperatures by behavioral and/or physiological mechanisms. Reptiles inhabiting tropical highlands face extreme fluctuating daily temperatures, and extreme values and intervals of fluctuations vary with altitude. Anolis heterodermus occurs between 1800 m to 3750 m elevation in the tropical Andes, and is the Anolis species found at the highest altitude known. We evaluated which strategies populations from elevations of 2200 m, 2650 m and 3400 m use to cope with environmental temperatures. We measured body, preferred, critical maximum and minimum temperatures, and sprint speed at different body temperatures of individuals, as well as operative temperatures. Anolis heterodermus exhibits behavioral adjustments in response to changes in environmental temperatures across altitudes. Likewise, physiological traits exhibit intrapopulation variations, but they are similar among populations, tended to the “static” side of the evolution of thermal traits spectrum. The thermoregulatory behavioral strategy in this species is extremely plastic, and lizards adjust even to fluctuating environmental conditions from day to day. Unlike other Anolis species, at low thermal quality of the habitat, lizards are thermoconformers, particularly at the highest altitudes, where cloudy days can intensify this strategy even more. Our study reveals that the pattern of strategies for dealing with thermal ambient variations and their relation to extinction risks in the tropics that are caused by global warming is perhaps more complex for lizards than previously thought.     

Too hot to die? The effects of vegetation shading on past,present, and future activity budgets of two diurnal skinks from arid Australia

Behavioral thermoregulation is an important mechanism allowing ectotherms to respond to thermal variations. Its efficiency might become imperative for securing activity budgets under future climate change. For diurnal lizards, thermal microhabitat variability appears to be of high importance, especially in hot deserts where vegetation is highly scattered and sensitive to climatic fluctuations. We investigated the effects of a shading gradient from vegetation on body temperatures and activity timing for two diurnal, terrestrial desert lizards, Ctenotus regius, and Morethia boulengeri, and analyzed their changes under past, present, and future climatic conditions. Both species’ body temperatures and activity timing strongly depended on the shading gradient provided by vegetation heterogeneity. At high temperatures, shaded locations provided cooling temperatures and increased diurnal activity. Conversely, bushes also buffered cold temperature by saving heat. According to future climate change scenarios, cooler microhabitats might become beneficial to warm‐adapted species, such as C. regius, by increasing the duration of daily activity. Contrarily, warmer microhabitats might become unsuitable for less warm‐adapted species such as M. boulengeri for which midsummers might result in a complete restriction of activity irrespective of vegetation. However, total annual activity would still increase provided that individuals would be able to shift their seasonal timing towards spring and autumn. Overall, we highlight the critical importance of thermoregulatory behavior to buffer temperatures and its dependence on vegetation heterogeneity. Whereas studies often neglect ecological processes when anticipating species’ responses to future climate change the strongest impact of a changing climate on terrestrial ectotherms in hot deserts is likely to be the loss of shaded microhabitats rather than the rise in temperature itself. We argue that conservation strategies aiming at addressing future climate changes should focus more on the cascading effects of vegetation rather than on shifts of species distributions predicted solely by climatic envelopes.     

Host‐specific thermal profiles affect fitness of a widespread pathogen

Host behavior can interact with environmental context to influence outcomes of pathogen exposure and the impact of disease on species and populations. Determining whether the thermal behaviors of individual species influence susceptibility to disease can help enhance our ability to explain and predict how and when disease outbreaks are likely to occur. The widespread disease chytridiomycosis (caused by the fungal pathogen Batrachochytrium dendrobatidis, Bd) often has species‐specific impacts on amphibian communities; some host species are asymptomatic, whereas others experience mass mortalities and population extirpation. We determined whether the average natural thermal regimes experienced by sympatric frog species in nature, in and of themselves, can account for differences in vulnerability to disease. We did this by growing Bd under temperatures mimicking those experienced by frogs in the wild. At low and high elevations, the rainforest frogs Litoria nannotis, L. rheocola, and L. serrata maintained mean thermal regimes within the optimal range for pathogen growth (15–25°C). Thermal regimes for L. serrata, which has recovered from Bd‐related declines, resulted in slower pathogen growth than the cooler and less variable thermal regimes for the other two species, which have experienced more long‐lasting declines. For L. rheocola and L. serrata, pathogen growth was faster in thermal regimes corresponding to high elevations than in those corresponding to low elevations, where temperatures were warmer. For L. nannotis, which prefers moist and thermally stable microenvironments, pathogen growth was fastest for low‐elevation thermal regimes. All of the thermal regimes we tested resulted in pathogen growth rates equivalent to, or significantly faster than, rates expected from constant‐temperature experiments. The effects of host body temperature on Bd can explain many of the broad ecological patterns of population declines in our focal species, via direct effects on pathogen fitness. Understanding the functional response of pathogens to conditions experienced by the host is important for determining the ecological drivers of disease outbreaks.     

Plasticity of haemoglobin concentration and thermoregulation in a mountain lizard

The plastic capability of species to cope with the new conditions created by climate change is poorly understood. This is particularly relevant for organisms restricted to high elevations because they are adapted to cold temperatures and low oxygen availability. Therefore, evaluating trait plasticity of mountain specialists is fundamental to understand their vulnerability to environmental change. We transplanted mountain lizards, Iberolacerta cyreni, 800 m downhill to evaluate the plastic response in body condition, thermoregulation traits, haemoglobin level, and haemoparasite load. Initial measurements of body mass, total haemoglobin concentration ([Hb]), hematic parasite intensities, dorsal luminance, and thermoregulatory behaviour were resampled after two and four weeks of acclimation. We also tested whether an anti-parasitic drug reduced haemoparasite intensity. After only two weeks of acclimation to a lower elevation, lizards decreased 42% in [Hb], had 17% less parasite intensities, increased body condition by 25%, and raised by ~3% their mean preferred temperatures and their voluntary thermal maximum. The anti-parasitic treatment had no significant effect on the intensity of hematic parasites, but our results suggest that negative effects of haemoparasites on [Hb] are relaxed at lower elevation. The rapid plastic changes observed in thermal preferences, body condition, [Hb], and parasite intensity of I. cyreni demonstrate a potential plastic response of a mountain specialist. This may be adaptive under the climatic extremes typical of mountain habitats. However, there is uncertainty in whether the observed plasticity can also help overcome long term environmental changes.     

Growth and thermal tolerance of a Tibet fish Schizopygopsis younghusbandi juveniles acclimated to three temperature levels

Schizopygopsis younghusbandi is an endemic fish of Tibet characterized by slow growth. Artificial stock enhancement was applied to rebuild the natural population of S. younghusbandi in recent years. However, the optimal growth temperature and thermal tolerance of S. younghusbandi has not been studied, which restricts the production of S. younghusbandi fingerling for stock enhancement. The purpose of this paper is to determine the growth, critical thermal maximum (CTMax), lethal thermal maximum (LTMax) and acclimation response ratio (ARR) of S. younghusbandi juveniles (body weight 5.7 ± 1.2 g) at three acclimation temperature levels (10, 15, 20°C). The results showed that acclimation temperature significantly affected the growth, CTMax, LTMax and ARR of the experimental fish. Largest final weight (7.5 ± 2.3 g) was recorded in 15°C group. At a heating rate of 1°C/30 min, CTMax ranged from 30.98 to 32.01°C and LTMax ranged from 31.76 to 32.31°C in the three acclimation temperatures. Schizopygopsis younghusbandi had lower ARR value (0.097) than most other fish species. Low ARR value indicates that S. younghusbandi may have narrower thermal tolerance range and weaker acclimation ability to global warming. For successful aquaculture of S. younghusbandi juveniles, temperature should be maintained around 15°C.     

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.     

The capacity to cope with climate warming declines from temperate to tropical latitudes in two widely distributed Eucalyptus species

As rapid climate warming creates a mismatch between forest trees and their home environment, the ability of trees to cope with warming depends on their capacity to physiologically adjust to higher temperatures. In widespread species, individual trees in cooler home climates are hypothesized to more successfully acclimate to warming than their counterparts in warmer climates that may approach thermal limits. We tested this prediction with a climate‐shift experiment in widely distributed Eucalyptus tereticornis and E. grandis using provenances originating along a ~2500 km latitudinal transect (15.5–38.0°S) in eastern Australia. We grew 21 provenances in conditions approximating summer temperatures at seed origin and warmed temperatures (+3.5 °C) using a series of climate‐controlled glasshouse bays. The effects of +3.5 °C warming strongly depended on home climate. Cool‐origin provenances responded to warming through an increase in photosynthetic capacity and total leaf area, leading to enhanced growth of 20–60%. Warm‐origin provenances, however, responded to warming through a reduction in photosynthetic capacity and total leaf area, leading to reduced growth of approximately 10%. These results suggest that there is predictable intraspecific variation in the capacity of trees to respond to warming; cool‐origin taxa are likely to benefit from warming, while warm‐origin taxa may be negatively affected.     

Miami heat: Urban heat islands influence the thermal suitability of habitats for ectotherms

The urban heat island effect, where urban areas exhibit higher temperatures than less‐developed suburban and natural habitats, occurs in cities across the globe and is well understood from a physical perspective and at broad spatial scales. However, very little is known about how thermal variation caused by urbanization influences the ability of organisms to live in cities. Ectotherms are sensitive to environmental changes that affect thermal conditions, and therefore, increased urban temperatures may pose significant challenges to thermoregulation and alter temperature‐dependent activity. To evaluate whether these changes to the thermal environment affect the persistence and dispersal of ectothermic species in urban areas, we studied two species of Anolis lizards (Anolis cristatellus and Anolis sagrei) introduced to Miami‐Dade County, FL, USA, where they occur in both urban and natural habitats. We calculated canopy openness and measured operative temperature (T_e), which estimates the distribution of body temperatures in a non‐thermoregulating population, in four urban and four natural sites. We also captured lizards throughout the day and recorded their internal body temperature (T_b). We found that urban areas had more open canopies and higher T_e compared to natural habitats. Laboratory trials showed that A. cristatellus preferred lower temperatures than A. sagrei. Urban sites currently occupied by each species appear to lower thermoregulatory costs for both species, but only A. sagreihad field T_b that were more often within their preferred temperature range in urban habitats compared to natural areas. Furthermore, based on available T_e within each species' preferred temperature range, urban sites with only A. sagrei appear less suitable for A. cristatellus, whereas natural sites with only A. cristatellus are less suitable for A. sagrei. These results highlight how the thermal properties of urban areas contribute to patterns of persistence and dispersal, particularly relevant for studying species invasions worldwide.     

Bet hedging in a warming ocean: predictability of maternal environment shapes offspring size variation in marine sticklebacks

Bet hedging at reproduction is expected to evolve when mothers are exposed to unpredictable cues for future environmental conditions, whereas transgenerational plasticity (TGP) should be favoured when cues reliably predict the environment offspring will experience. Since climate predictions forecast an increase in both temperature and climate variability, both TGP and bet hedging are likely to become important strategies to mediate climate change effects. Here, the potential to produce variably sized offspring in both warming and unpredictable environments was tested by investigating whether stickleback (Gasterosteus aculeatus) mothers adjusted mean offspring size and within‐clutch variation in offspring size in response to experimental manipulation of maternal thermal environment and predictability (alternating between ambient and elevated water temperatures). Reproductive output traits of F1 females were influenced by both temperature and environmental predictability. Mothers that developed at ambient temperature (17 °C) produced larger, but fewer eggs than mothers that developed at elevated temperature (21 °C), implying selection for different‐sized offspring in different environments. Mothers in unpredictable environments had smaller mean egg sizes and tended to have greater within‐female egg size variability, especially at 21 °C, suggesting that mothers may have dynamically modified the variance in offspring size to spread the risk of incorrectly predicting future environmental conditions. Both TGP and diversification influenced F2 offspring body size. F2 offspring reared at 21 °C had larger mean body sizes if their mother developed at 21 °C, but this TGP benefit was not present for offspring of 17 °C mothers reared at 17 °C, indicating that maternal TGP will be highly relevant for ocean warming scenarios in this system. Offspring of variable environment mothers were smaller but more variable in size than offspring from constant environment mothers, particularly at 21 °C. In summary, stickleback mothers may have used both TGP and diversified bet‐hedging strategies to cope with the dual stress of ocean warming and environmental uncertainty.     

Communal nesting under climate change: fitness consequences of higher incubation temperatures for a nocturnal lizard

Communal nesting lizards may be vulnerable to climate warming, particularly if air temperatures regulate nest temperatures. In southeastern Australia, velvet geckos Oedura lesueurii lay eggs communally inside rock crevices. We investigated whether increases in air temperatures could elevate nest temperatures, and if so, how this could influence hatching phenotypes, survival, and population dynamics. In natural nests, maximum daily air temperature influenced mean and maximum daily nest temperatures, implying that nest temperatures will increase under climate warming. To determine whether hotter nests influence hatchling phenotypes, we incubated eggs under two fluctuating temperature regimes to mimic current ‘cold’ nests (mean = 23.2 °C, range 10–33 °C) and future ‘hot’ nests (27.0 °C, 14–37 °C). ‘Hot’ incubation temperatures produced smaller hatchlings than did cold temperature incubation. We released individually marked hatchlings into the wild in 2014 and 2015, and monitored their survival over 10 months. In 2014 and 2015, hot‐incubated hatchlings had higher annual mortality (99%, 97%) than cold‐incubated (11%, 58%) or wild‐born hatchlings (78%, 22%). To determine future trajectories of velvet gecko populations under climate warming, we ran population viability analyses in Vortex and varied annual rates of hatchling mortality within the range 78– 96%. Hatchling mortality strongly influenced the probability of extinction and the mean time to extinction. When hatchling mortality was >86%, populations had a higher probability of extinction (PE: range 0.52– 1.0) with mean times to extinction of 18–44 years. Whether future changes in hatchling survival translate into reduced population viability will depend on the ability of females to modify their nest‐site choices. Over the period 1992–2015, females used the same communal nests annually, suggesting that there may be little plasticity in maternal nest‐site selection. The impacts of climate change may therefore be especially severe on communal nesting species, particularly if such species occupy thermally challenging environments.     

Photosynthetic capacity and leaf nitrogen decline along a controlled climate gradient in provenances of two widely distributed Eucalyptus species

Climate is an important factor limiting tree distributions and adaptation to different thermal environments may influence how tree populations respond to climate warming. Given the current rate of warming, it has been hypothesized that tree populations in warmer, more thermally stable climates may have limited capacity to respond physiologically to warming compared to populations from cooler, more seasonal climates. We determined in a controlled environment how several provenances of widely distributed Eucalyptus tereticornis and E. grandis adjusted their photosynthetic capacity to +3.5°C warming along their native distribution range (~16–38°S) and whether climate of seed origin of the provenances influenced their response to different growth temperatures. We also tested how temperature optima (T_opt) of photosynthesis and J_max responded to higher growth temperatures. Our results showed increased photosynthesis rates at a standardized temperature with warming in temperate provenances, while rates in tropical provenances were reduced by about 40% compared to their temperate counterparts. Temperature optima of photosynthesis increased as provenances were exposed to warmer growth temperatures. Both species had ~30% reduced photosynthetic capacity in tropical and subtropical provenances related to reduced leaf nitrogen and leaf Rubisco content compared to temperate provenances. Tropical provenances operated closer to their thermal optimum and came within 3% of the T_opt of J_max during the daily temperature maxima. Hence, further warming may negatively affect C uptake and tree growth in warmer climates, whereas eucalypts in cooler climates may benefit from moderate warming.     

Differences in the thermal physiology of adult Yarrow's spiny lizards (Sceloporus jarrovii) in relation to sex and body size

Sexual size dimorphism (SSD) is often assumed to reflect the phenotypic consequences of differential selection operating on each sex. Species that exhibit SSD may also show intersexual differences in other traits, including field‐active body temperatures, preferred temperatures, and locomotor performance. For these traits, differences may be correlated with differences in body size or reflect sex‐specific trait optima. Male and female Yarrow's spiny lizards, Sceloporus jarrovii, in a population in southeastern Arizona exhibit a difference in body temperature that is unrelated to variation in body size. The observed sexual variation in body temperature may reflect divergence in thermal physiology between the sexes. To test this hypothesis, we measured the preferred body temperatures of male and female lizards when recently fed and fasted. We also estimated the thermal sensitivity of stamina at seven body temperatures. Variation in these traits provided an opportunity to determine whether body size or sex‐specific variation unrelated to size shaped their thermal physiology. Female lizards, but not males, preferred a lower body temperature when fasted, and this pattern was unrelated to body size. Larger individuals exhibited greater stamina, but we detected no significant effect of sex on the shape or height of the thermal performance curves. The thermal preference of males and females in a thermal gradient exceeded the optimal temperature for performance in both sexes. Our findings suggest that differences in thermal physiology are both sex‐ and size‐based and that peak performance at low body temperatures may be adaptive given the reproductive cycles of this viviparous species. We consider the implications of our findings for the persistence of S. jarrovii and other montane ectotherms in the face of climate warming.     

Calling behaviour under climate change: geographical and seasonal variation of calling temperatures in ectotherms

Calling behaviour is strongly temperature‐dependent and critical for sexual selection and reproduction in a variety of ectothermic taxa, including anuran amphibians, which are the most globally threatened vertebrates. However, few studies have explored how species respond to distinct thermal environments at time of displaying calling behaviour, and thus it is still unknown whether ongoing climate change might compromise the performance of calling activity in ectotherms. Here, we used new audio‐trapping techniques (automated sound recording and detection systems) between 2006 and 2009 to examine annual calling temperatures of five temperate anurans and their patterns of geographical and seasonal variation at the thermal extremes of species ranges, providing insights into the thermal breadths of calling activity of species, and the mechanisms that enable ectotherms to adjust to changing thermal environments. All species showed wide thermal breadths during calling behaviour (above 15 °C) and increases in calling temperatures in extremely warm populations and seasons. Thereby, calling temperatures differed both geographically and seasonally, both in terrestrial and aquatic species, and were 8–22 °C below the specific upper critical thermal limits (CT_max) and strongly associated with the potential temperatures of each thermal environment (operative temperatures during the potential period of breeding). This suggests that calling behaviour in ectotherms may take place at population‐specific thermal ranges, diverging when species are subjected to distinct thermal environments, and might imply plasticity of thermal adjustment mechanisms (seasonal and developmental acclimation) that supply species with means of coping with climate change. Furthermore, the thermal thresholds of calling at the onset of the breeding season were dissimilar between conspecific populations, suggesting that other factors besides temperature are needed to trigger the onset of reproduction. Our findings imply that global warming would not directly inhibit calling behaviour in the study species, although might affect other temperature‐dependent features of their acoustic communication system.     

Fluctuating environments impact thermal tolerance in an invasive insect species Bactrocera dorsalis (Diptera: Tephritidae)

The incidence and severity of environmental stressors associated with global climate change are increasing and insects frequently face variability in temperature and moisture regimes at variable spatio-temporal scales. Coincidental with this, is increased thermal and hydric stress on insects as warming increases vapour pressure deficit (VPD), the drying power of the air. While the effects of mean temperatures on fitness are widely documented, fluctuations in both temperature and relative humidity (RH) are largely unexplored. Here, we investigated the effects of dynamic temperature and RH fluctuations (around the mean [28°C; 65% RH]) on low and high thermal tolerance of laboratory-reared adult invasive Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), measured as critical thermal minima (CT_min), critical thermal maxima (CT_max), chill coma recovery time (CCRT) and heat knockdown time (HKDT). Our results show that increased environmental amplitude significantly influenced low and high temperature responses and varied across traits tested. The highest amplitude (δ12°C; 28% RH) compromised CT_min, CCRT and HKDT traits while enhancing CT_max. Similarly, acclimation to δ3°C; 7% RH compromised both low (CT_min and CCRT) and high (CT_max and HKDT) fitness traits. Variations in fitness reported here indicate significant roles of combined thermal and moisture fluctuations on B. dorsalis fitness suggesting caveats that are worthy considering when predicting species responses to climate change. These results are significant for B. dorsalis population phenology, management, quantifying vulnerability to climate variability and may help modelling future biogeographical patterns.     

Living at the edge: lower success of eggs and hatchlings at lower elevation may shape range limits in an alpine lizard

Studies on range limits clarify the factors involved in the extent of species occurrence and shed light on the limits to adaptation. We studied the effects of elevational variation on the thermal dependence of fitness‐related traits (incubation time, hatching rate, and survivorship, size, and condition of hatchlings) to assess the role of incubation requirements in distribution range limits of the alpine endemic Iberolacerta cyreni. We captured gravid females from two core (summit) and two marginal (low‐elevation edge) populations, we incubated their eggs at three temperatures (22, 26, and 30 °C), and we monitored phenotypic effects. Viability of eggs and hatchlings decreased, independently of elevation, as incubation temperature increased. Hatching success and embryo survivorship were lower for clutches from low‐elevation areas than for those from mountain summits, showing that lizards face difficulties thriving at the low‐elevation edge of their range. Such difficulties were partly counterbalanced by faster postnatal growth at lower elevations, leading to increased adult size and higher fecundity. High incubation temperature had detrimental effects also at low‐elevation areas, and no elevational variation in the thermal dependence of hatchling traits was detected. We suggest that temperature effects on egg development and the lack of selective pressures strong enough to foster local adaptation at marginal areas, combined with extended egg retention, may contribute to shape the range limits of these alpine oviparous reptiles.     

Field and laboratory studies reveal interacting effects of stream oxygenation and warming on aquatic ectotherms

Aquatic ecological responses to climatic warming are complicated by interactions between thermal effects and other environmental stressors such as organic pollution and hypoxia. Laboratory experiments have demonstrated how oxygen limitation can set heat tolerance for some aquatic ectotherms, but only at unrealistic lethal temperatures and without field data to assess whether oxygen shortages might also underlie sublethal warming effects. Here, we test whether oxygen availability affects both lethal and nonlethal impacts of warming on two widespread Eurasian mayflies, Ephemera danica, Müller 1764 and Serratella ignita (Poda 1761). Mayfly nymphs are often a dominant component of the invertebrate assemblage in streams, and play a vital role in aquatic and riparian food webs. In the laboratory, lethal impacts of warming were assessed under three oxygen conditions. In the field, effects of oxygen availability on nonlethal impacts of warming were assessed from mayfly occurrence in 42 293 UK stream samples where water temperature and biochemical oxygen demand were measured. Oxygen limitation affected both lethal and sublethal impacts of warming in each species. Hypoxia lowered lethal limits by 5.5 °C (±2.13) and 8.2 °C (±0.62) for E. danica and S. ignita respectively. Field data confirmed the importance of oxygen limitation in warmer waters; poor oxygenation drastically reduced site occupancy, and reductions were especially pronounced under warm water conditions. Consequently, poor oxygenation lowered optimal stream temperatures for both species. The broad concordance shown here between laboratory results and extensive field data suggests that oxygen limitation not only impairs survival at thermal extremes but also restricts species abundance in the field at temperatures well below upper lethal limits. Stream oxygenation could thus control the vulnerability of aquatic ectotherms to global warming. Improving water oxygenation and reducing pollution can provide key facets of climate change adaptation for running waters.