Seasonal variation in the thermal biology of a terrestrial toad, Rhinella icterica (Bufonidae), from the Brazilian Atlantic Forest

Canopy coverage, light, and moisture affect thermoregulatory trade-offs in an amphibian breeding habitat

When amphibians thermoregulate, they face a fundamental trade-off between the ability to maintain activity and an increased rate of dehydration at higher temperatures. Canopy coverage affects both the thermal and hydric conditions of the environment and can therefore influence amphibian thermoregulation. Frogs require proper conditions to thermoregulate to successfully grow, survive, and reproduce. But while we know how canopy and environmental variables typically affect operative temperature, less is known about effects on amphibian water loss rates. In this study, we measure the effect of canopy coverage on the conditions available for thermoregulation at a breeding pond of the California red-legged frog, Rana draytonii. We use agar frog models to estimate the thermal and hydric capacities frogs would experience in locations with different canopy coverage and microhabitats. At each site, we deployed models under four microhabitat treatments: wet/sun, wet/shade, dry/sun, and dry/shade. We modeled how environmental variables affected operative temperature and evaporative water loss from agar frogs. We found positive effects of air temperature, the sun treatment, and reduced canopy cover on operative temperature, and negative direct or indirect effects of these variables on evaporative water loss, consistent with the hypothesized trade-off between thermoregulatory behavior to increase temperature and the increased desiccation risk due to higher water loss. Additionally, our results indicate that the availability of wet microhabitats can allow frogs to reduce water loss, potentially mitigating the risk of desiccation when thermoregulating to achieve higher operative temperatures. Our findings suggest, that with access to proper microhabitats, amphibians can mitigate the fundamental trade-off and receive benefits of thermoregulating at high temperatures.

Acclimation of thermal physiology to new basking regimes in a widespread Australian skink

Changes in thermal environments are a challenge for many ectotherms, as they would have to acclimate their physiology to new thermal environments to maintain high-levels of performance. Time spent basking is key for many ectothermic animals to keep their body temperature within optimal thermal ranges. However, little is known about the impact of changes in basking time on the thermal physiology of ectothermic animals. We investigated how different basking regimes (low intensity vs high intensity) affected key thermal physiological traits of a widespread Australian skink (Lampropholis delicata). We quantified thermal performance curves and thermal preferences of skinks subjected to low and high intensity basking regimes over a 12-week period. We found that skinks acclimated their thermal performance breadth in both basking regimes, with the skinks from the low-intensity basking regime showing narrower performance breadths. Although maximum velocity and optimum temperatures increased after the acclimation period, these traits did not differ between basking regimes. Similarly, no variation was detected for thermal preference. These results provide insight into mechanisms that allow these skinks to successfully overcome environmental constraints in the field. Acclimation of thermal performance curves seems to be key for widespread species to colonise new environments, and can buffer ectothermic animals in novel climatic scenarios.

Immunoendocrinology and Ecoimmunology in Brazilian Anurans

This paper reviews several aspects of immunoendocrinology and ecoimmunology in Brazilian species of anurans under investigation for more than a decade, including (1) patterns of annual covariation of circulating steroids, calling behavior and innate immunity, (2) endocrine and immune correlates of calling performance, (3) behavioral and physiological correlates of parasite load, (4) thermal sensitivity of immune function, and (5) endocrine and immunomodulation by experimental exposure to acute and chronic stressors, as well as to endocrine manipulations and simulated infections. Integrated results have shown an immunoprotective role of increased steroid plasma levels during reproductive activity in calling males. Moreover, a higher helminth parasite load is associated with changes in several behavioral and physiological traits under field conditions. We also found anuran innate immunity is generally characterized by eurythermy, with maximal performance observed in temperatures close to normal and fever thermal preferendum. Moreover, the aerobic scope of innate immune response is decreased at fever thermal preferendum. Experimental exposure to stressors results in increased corticosterone plasma levels and immune redistribution, with an impact on immune function depending on the duration of the stress exposure. Interestingly, the fate of immunomodulation by chronic stressors also depends in part on individual body condition. Acute treatment with corticosterone generally enhances immune function, while prolonged exposure results in immunosuppression. Still, the results of hormonal treatment are complex and depend on the dose, duration of treatment, and the immune variable considered. Finally, simulated infection results in complex modulation of the expression of cytokines, increased immune function, activation of the Hypothalamus-Pituitary-Interrenal axis, and decreased activity of the Hypothalamus-Pituitary-Gonadal axis, as well as reduced melatonin plasma levels, suggesting that anurans have a functional Immune-Pineal axis, homologous to that previously described for mammals. These integrated and complementary approaches have contributed to a better understanding of physiological mechanisms and processes, as well as ecological and evolutionary implications of anuran immunoendocrinology.

A comprehensive database of amphibian heat tolerance

Rising temperatures represent a significant threat to the survival of ectothermic animals. As such, upper thermal limits represent an important trait to assess the vulnerability of ectotherms to changing temperatures. For instance, one may use upper thermal limits to estimate current and future thermal safety margins (i.e., the proximity of upper thermal limits to experienced temperatures), use this trait together with other physiological traits in species distribution models, or investigate the plasticity and evolvability of these limits for buffering the impacts of changing temperatures. While datasets on thermal tolerance limits have been previously compiled, they sometimes report single estimates for a given species, do not present measures of data dispersion, and are biased towards certain parts of the globe. To overcome these limitations, we systematically searched the literature in seven languages to produce the most comprehensive dataset to date on amphibian upper thermal limits, spanning 3,095 estimates across 616 species. This resource will represent a useful tool to evaluate the vulnerability of amphibians, and ectotherms more generally, to changing temperatures.

Empirical estimation of skin resistance to water loss in amphibians: agar evaluation as a non-resistance model to evaporation

Total resistance (RT) to evaporative water loss (EWL) in amphibians is given by the sum of the boundary layer (rb) and the skin resistance (rs). Thus, rs can be determined if the rb component is defined (rs=RT - rb). The use of agar models has become the standard technique to estimate rb under the assumption that agar surface imposes no barrier to evaporation (rs=0). We evaluated this assumption by determining EWL rates and rb values from exposed surfaces of free water, a physiological solution mimicking the osmotic properties of a generalized amphibian, and agar gels prepared at various concentrations either using water or physiological solution as diluent. Water evaporation was affected by both, the presence of solutes and agar concentration. Models prepared with agar at 5% concentration in water provided the most practical and appropriate proxy for the estimation of rb.

Influence of High Temperatures and Heat Wave on Thermal Biology, Locomotor Performance, and Antioxidant System of High-Altitude Frog Nanorana pleskei Endemic to Qinghai-Tibet Plateau

Investigating how highland amphibians respond to changes in ambient temperature may be of great significance for their fate prediction and effective conservation in the background of global warming. Here, using field individuals as the control group, we investigated the influence of high temperatures (20.5 and 25.5°C) and heat wave (15–26.6°C) on the thermal preference, critical thermal limits, locomotor performance, oxidative stress, and antioxidant enzyme activities in high-altitude frog Nanorana pleskei (3,490 m) endemic to the Qinghai-Tibet Plateau (QTP). After 2 weeks of acclimation to high temperatures and heat wave, the thermal preference (Tpref), critical thermal maximum (CTmax), and range of tolerable temperature significantly increased, while the critical thermal minimum (CTmin) was significantly decreased. The total time of jump to exhaustion significantly decreased, and burst swimming speed significantly increased in frogs acclimated in the high temperature and heat wave groups compared with the field group. In the high temperature group, the level of H2O2 and lipid peroxide (malondialdehyde, MDA), as well as the activities of glutathione peroxidase (GPX), glutathione reductase (GR), catalase (CAT), superoxide dismutase (SOD), and total antioxidant capacity (T-AOC) significantly increased in the liver or muscle. However, in the heat wave group, the MDA content significantly decreased in the liver, and antioxidants activities decreased in the liver and muscle except for CAT activities that were significantly increased in the liver. These results indicated that N. pleskei could respond to the oxidative stress caused by high temperatures by enhancing the activity of antioxidant enzymes. The heat wave did not appear to cause oxidative damage in N. pleskei, which may be attributed to the fact that they have successfully adapted to the dramatic temperature fluctuations on the QTP.

Environmental heterogeneity shapes physiological traits in tropical direct-developing frogs

Tropical ectotherm species tend to have narrower physiological limits than species from temperate areas. As a consequence, tropical species are considered highly vulnerable to climate change since minor temperature increases can push them beyond their physiological thermal tolerance. Differences in physiological tolerances can also be seen at finer evolutionary scales, such as among populations of ectotherm species along elevation gradients, highlighting the physiological sensitivity of such organisms.Here, we analyze the influence of elevation and bioclimatic domains, defined by temperature and precipitation, on thermal sensitivities of a terrestrial direct-developing frog (Craugastor loki) in a tropical gradient. We address the following questions: (a) Does preferred temperature vary with elevation and among bioclimatic domains? (b) Do thermal tolerance limits, that is, critical thermal maximum and critical thermal minimum vary with elevation and bioclimatic domains? and (c) Are populations from high elevations more vulnerable to climate warming?We found that along an elevation gradient body temperature decreases as environmental temperature increases. The preferred temperature tends to moderately increase with elevation within the sampled bioclimatic domains. Our results indicate that the ideal thermal landscape for this species is located at midelevations, where the thermal accuracy (db ) and thermal quality of the environment (de ) are suitable. The critical thermal maximum is variable across elevations and among the bioclimatic domains, decreasing as elevation increases. Conversely, the critical thermal minimum is not as variable as the critical thermal maximum.Populations from the lowlands may be more vulnerable to future increases in temperature. We highlight that the critical thermal maximum is related to high temperatures exhibited across the elevation gradient and within each bioclimatic domain; therefore, it is a response to high environmental temperatures.

Community-wide seasonal shifts in thermal tolerances of mosquitoes

The broadening in species' thermal tolerance limits and breadth from tropical to temperate latitudes is proposed to reflect spatial gradients in temperature seasonality, but the importance of seasonal shifts in thermal tolerances within and across locations is much less appreciated. We performed thermal assays to examine the maximum and minimum critical temperatures (CT_max and CT_min , respectively) of a mosquito community across their active seasons. Mosquito CT_min tracked seasonal shifts in temperature, whereas CT_max tracked a countergradient pattern with lowest heat tolerances in summer. Mosquito thermal breadth decreased from spring to summer and then increased from summer to autumn. We show a temporal dichotomy in thermal tolerances with thermal breadths of temperate organisms in summer reflecting those of the tropics ("tropicalization") that is sandwiched between a spring and autumn "temperatization." Therefore, our tolerance patterns at a single temperate latitude recapitulate classical patterns across latitude. These findings highlight the need to understand the temporal and spatial components of thermotolerance variation better, including plasticity and rapid seasonal selection, and the potential for this variation to affect species responses to climate change. With summers becoming longer and increasing winter nighttime temperatures, we expect increasing tropicalization of species thermal tolerances in both space and time.

Water loss and temperature interact to compound amphibian vulnerability to climate change

Ectotherm thermal physiology is frequently used to predict species responses to changing climates, but for amphibians, water loss may be of equal or greater importance. Using physical models, we estimated the frequency of exceeding the thermal optimum (T_opt ) or critical evaporative water loss (EWL_crit ) limits, with and without shade- or water-seeking behaviours. Under current climatic conditions (2002-2012), we predict that harmful thermal (>T_opt ) and hydric (>EWL_crit ) conditions limit the activity of amphibians during ~70% of snow-free days in sunny habitats. By the 2080s, we estimate that sunny and dry habitats will exceed one or both of these physiological limits during 95% of snow-free days. Counterintuitively, we find that while wet environments eliminate the risk of critical EWL, they do not reduce the risk of exceeding T_opt (+2% higher). Similarly, while shaded dry environments lower the risk of exceeding T_opt , critical EWL limits are still exceeded during 63% of snow-free days. Thus, no single environment that we evaluated can simultaneously reduce both physiological risks. When we forecast both temperature and EWL into the 2080s, both physiological thresholds are exceeded in all habitats during 48% of snow-free days, suggesting that there may be limited opportunity for behaviour to ameliorate climate change. We conclude that temperature and water loss act synergistically, compounding the ecophysiological risk posed by climate change, as the combined effects are more severe than those predicted individually. Our results suggest that predictions of physiological risk posed by climate change that do not account for water loss in amphibians may be severely underestimated and that there may be limited scope for facultative behaviours to mediate rapidly changing environments.

Feeding alters the preferred body temperature of Cururu toads, Rhinella diptycha (Anura, Bufonidae)

Ectothermic organisms depend primarily on external heat sources and behavioural adjustments to regulate body temperature. Under controlled conditions, in a thermal gradient, body temperature often clusters around a more or less defined range of preferred body temperatures (T_pref). However, T_pref may be modified in response to environmental parameters and/or physiological state. For example, meal ingestion is sometimes followed by a post-prandial thermophilic response leading to a transient increment in T_pref. Although thought to optimize digestive processes, its occurrence, magnitude, and possible determinants remains scarcely documented for anuran amphibians. Herein, we investigated whether the Cururu toad, Rhinella diptycha, exhibits a post-prandial thermophilic response by monitoring the body temperature of fasting and fed toads while they were maintained in a thermal gradient. We found that the toads' T_pref increased by about 13% from day 2 to 4 after feeding, in comparison with the T_pref recorded under fasting. Also, fed animals exhibited a broader range for T_pref at days 2 and 3 post-prandial, which reflects a greater level of locomotor activity compared to fasting individuals. We conclude that R. diptycha is capable to exhibit a post-prandial thermophilic response under the controlled conditions of a thermal gradient. Although this thermoregulatory adjustment is thought to optimize meal digestion yielding important energetic and ecological benefits, its occurrence in anuran amphibians in nature remains uncertain.

Toads prey upon scorpions and are resistant to their venom: A biological and ecological approach to scorpionism

In recent years, SE Brazil, the most populous region in the country with an estimated population of 88 million, has been experiencing an alarming increase in scorpions accidents (scorpionism), mainly caused by the yellow scorpion (Tityus serrulatus), or "escorpião amarelo" in Portuguese. This species is considered particularly dangerous to humans and can reproduce by parthenogenesis favouring rapid dispersal and colonization of new environments. Since the 1940s, owing to the growing danger represented by scorpionism, public control policies have been developed, including active search for scorpions, together with the use of toxic substances applied in places most likely to serve as their refuges. Even so, the number of accidents is increasing year by year, presently at an alarming rate. It seems evident that the increase in accidents is directly (or primarily) related to the lack of predators that in healthy environmental conditions would naturally control scorpion populations. However, due to environmental changes, leading to a lack of predators, scorpions have been gradually invading the urban environment. Arachnids and insects in general, as well as some other invertebrates, are preyed upon by anuran amphibians (toads, frogs and tree frogs). Toads (family Bufonidae) are nocturnal, large, and highly voracious animals, capable of actively exploring extensive areas and consuming large numbers of insects and arachnids daily. One of the most common toad species in southeastern Brazil is Rhinella icterica. Both R. icterica and T. serrulatus inhabit the same nocturnal environment. The predatory action of toads, specifically on scorpions, is practically unknown from behavioural and toxinological points of view. Thus, we studied the predatory behaviour of this toad against the yellow scorpion and evaluated the resistance of the amphibian to scorpion venom. Our results show that R. icterica is a voracious predator of T. serrulatus and is extremely resistant to its venom. Human/toad relationship throughout western history has always been very conflicted and possibly one of the factors that most has contributed to human ignorance of the role of these amphibians in maintaining ecological balance. Presently, the control of scorpionism is being performed through active search and/or the use of chemical agents, although showing little efficacy in reducing human accidents. In the medium or long term, more effective actions taking into account the biology of scorpions and their predators have never been taken to reduce these accidents.

The body temperature of active desert anurans from hyper-arid environment of South America: The reliability of WorldClim for predicted body temperatures in anurans

The temperature of anuran activity from arid regions have been poorly studied. In recent years, the emphasis was placed on predicting the impacts of global warming on the fauna of ectotherms in general. Many times future impacts are predicted based on global thermal information (macroscale) but not on an individual scale. We used literature data about body temperature in anurans that inhabit the desert region of the Northwest of Argentina to analyze the role of the elevation, eco-region and substrate temperature on the individual scale. Also, we evaluated the predictive capacity that global environmental variables obtained from WorldClim for this region, and compared them with those of local scale. Our results showed that the activity body temperature of the toads in the arid region of western Argentina strongly influenced by elevation and the substrate temperature on the individual scale. We observed that data of the global scale (WorldClim) can predict the body temperature of the studied amphibians, but with a deviation, not less than 8 °C. According to our results, it is evident that to making reliable predictions of how global warming impacts on amphibians in the region, the environmental temperature data need to be recorded at the local scale (operative temperatures).

Ecophysiology of Amphibians: Information for Best Mechanistic Models

Several amphibian lineages epitomize the faunal biodiversity crises, with numerous reports of population declines and extinctions worldwide. Predicting how such lineages will cope with environmental changes is an urgent challenge for biologists. A promising framework for this involves mechanistic modeling, which integrates organismal ecophysiological features and ecological models as a means to establish causal and consequential relationships of species with their physical environment. Solid frameworks built for other tetrapods (e.g., lizards) have proved successful in this context, but its extension to amphibians requires care. First, the natural history of amphibians is distinct within tetrapods, for it includes a biphasic life cycle that undergoes major habitat transitions and changes in sensitivity to environmental factors. Second, the accumulated data on amphibian ecophysiology is not nearly as expressive, is heavily biased towards adult lifeforms of few non-tropical lineages, and overlook the importance of hydrothermal relationships. Thus, we argue that critical usage and improvement in the available data is essential for enhancing the power of mechanistic modeling from the physiological ecology of amphibians. We highlight the complexity of ecophysiological variables and the need for understanding the natural history of the group under study and indicate directions deemed crucial to attaining steady progress in this field.