Thermal performance curves under daily thermal fluctuation: A study in helmeted water toad tadpoles

Intra and interspecific variation in thermal performance and critical limits in anurans from southern Chile

The relationship between temperature and performance can be illustrated through a thermal performance curve (TPC), which has proven useful in describing various aspects of ectotherms' thermal ecology and evolution. The parameters of the TPC can vary geographically due to large-scale variations in environmental conditions. However, only some studies have attempted to quantify how thermal performance varies over relatively small spatial scales, even in the same location or consistently among individuals within a species. Here, we quantified individual and species variation in thermal sensitivity of locomotor performance in five amphibia Eupsophus species found in the temperate rainforests of southern Chile and compared their estimates against co-occurring species that exhibit a substantially more extensive distributional range. We measured critical thermal limits and jumping performance under five different temperatures. Our results suggest that thermal responses are relatively conserved along the phylogeny, as the locomotor performance and thermal windows for activity remained narrow in Eupsophus species when compared against results observed for Batrachyla taeniata and Rhinella spinulosa. Additionally, we found significant individual differences in locomotor performance within most species, with individual consistency in performance observed across varied temperatures. Further analyses explored the influence of body size on locomotor performance and critical thermal limits within and between species. Our results suggest a trade-off scenario between thermal tolerance breadth and locomotor performance, where species exhibiting broader thermal ranges might have compromised performance. Interestingly, these traits seem partly mediated by body size variations, raising questions about potential ecological implications.

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.

The effect of thermal microenvironment in upper thermal tolerance plasticity in tropical tadpoles. Implications for vulnerability to climate warming

Current climate change is generating accelerated increase in extreme heat events and organismal plastic adjustments in upper thermal tolerances, (critical thermal maximum -CT_max ) are recognized as the quicker mitigating mechanisms. However, current research casts doubt on the actual mitigating role of thermal acclimation to face heat impacts, due to its low magnitude and weak environmental signal. Here, we examined these drawbacks by first estimating maximum extent of thermal acclimation by examining known sources of variation affecting CT_max expression, such as daily thermal fluctuation and heating rates. Second, we examined whether the magnitude and pattern of CT_max plasticity is dependent of the thermal environment by comparing the acclimation responses of six species of tropical amphibian tadpoles inhabiting thermally contrasting open and shade habitats and, finally, estimating their warming tolerances (WT = CT_max  - maximum temperatures) as estimator of heating risk. We found that plastic CT_max responses are improved in tadpoles exposed to fluctuating daily regimens. Slow heating rates implying longer duration assays determined a contrasting pattern in CT_max plastic expression, depending on species environment. Shade habitat species suffer a decline in CT_max whereas open habitat tadpoles greatly increase it, suggesting an adaptive differential ability of hot exposed species to quick hardening adjustments. Open habitat tadpoles although overall acclimate more than shade habitat species, cannot capitalize this beneficial increase in CT_max, because the maximum ambient temperatures are very close to their critical limits, and this increase may not be large enough to reduce acute heat stress under the ongoing global warming.

Frequency of Temperature Fluctuations Subtly Impacts the Life Histories of a Tropical Snail

AbstractMost organisms are faced with daily cyclic changes in a suite of environmental conditions, including temperature. In shallow marine waters, populations of the same species may experience either intertidal or subtidal conditions, such that some individuals experience extreme daily fluctuations as the tide ebbs and flows, while others only a few meters away experience less pronounced or less frequent fluctuations or almost constant thermal conditions. This study used a fully factorial combination of three thermal treatments and two diet treatments to test the hypotheses that (1) individuals experiencing fluctuating temperatures perform more poorly than those experiencing the same mean temperature under constant conditions and that (2) the negative impact of fluctuating temperatures is greater under food-limiting conditions. Five life-history components of the slipper snail Crepidula cf. marginalis were used as response variables. We found that temperature fluctuations impacted size at hatching and time to hatching, as well as growth rate, to some extent. Diet treatments impacted growth rates, clutch sizes, time to first brood, and time to hatching. There were no statistically significant interactions between the two factors. These results show that fluctuations between two temperatures that are typically experienced by these animals in the field can significantly affect fitness-related characters and, therefore, suggest the tidal height at which larvae settle can significantly impact individual fitness. This is one of the first studies to demonstrate that differences in the frequency of fluctuations, in the absence of differences in the magnitude or the mean, can have significant impacts on invertebrate life histories.

The Potential for Physiological Performance Curves to Shape Environmental Effects on Social Behavior

As individual animals are exposed to varying environmental conditions, phenotypic plasticity will occur in a vast array of physiological traits. For example, shifts in factors such as temperature and oxygen availability can affect the energy demand, cardiovascular system, and neuromuscular function of animals that in turn impact individual behavior. Here, we argue that nonlinear changes in the physiological traits and performance of animals across environmental gradients—known as physiological performance curves—may have wide-ranging effects on the behavior of individual social group members and the functioning of animal social groups as a whole. Previous work has demonstrated how variation between individuals can have profound implications for socially living animals, as well as how environmental conditions affect social behavior. However, the importance of variation between individuals in how they respond to changing environmental conditions has so far been largely overlooked in the context of animal social behavior. First, we consider the broad effects that individual variation in performance curves may have on the behavior of socially living animals, including: (1) changes in the rank order of performance capacity among group mates across environments; (2) environment-dependent changes in the amount of among- and within-individual variation, and (3) differences among group members in terms of the environmental optima, the critical environmental limits, and the peak capacity and breadth of performance. We then consider the ecological implications of these effects for a range of socially mediated phenomena, including within-group conflict, within- and among group assortment, collective movement, social foraging, predator-prey interactions and disease and parasite transfer. We end by outlining the type of empirical work required to test the implications for physiological performance curves in social behavior.

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.

Running performance with emphasis on low temperatures in a Patagonian lizard, Liolaemus lineomaculatus

Lizard activity and endurance of cold climate is regulated by several factors such as evolutionary potential, acclimatization capacity, physiological tolerance, and locomotion among thermally advantageous microenvironments. Liolaemus lineomaculatus, a lizard inhabiting a wide range of cold environments in Patagonia, provides an excellent model to test interpopulation variability in thermal performance curves (TPCs) and usage of microhabitats. We obtained critical thermal minima and maxima, and performed running trials at eight temperatures using lizards from both a temperate-site (high-altitude) population at 42° S and a cold-site population at 50° S. The availability of environmental temperatures for running performance in open ground and in potential lizard refuges were recorded, and showed that lizards in the temperate site had a greater availability of thermal environments offering temperatures conducive to locomotion. Generalized additive mixed models showed that the two populations displayed TPCs of different shapes in 0.15 m runs at temperatures near their optimal temperature, indicating a difference in thermal sensitivity at high temperatures. However, the rest of the locomotor parameters remained similar between Liolaemus lineomaculatus from thermal and ecological extremes of their geographic distribution and this may partly explain their ability to endure a cold climate.

Transgenerational and within-generation plasticity shape thermal performance curves

Thermal performance curves (TPCs) compute the effects of temperature on the performance of ectotherms and are frequently used to predict the effect of environmental conditions and currently, climate change, on organismal vulnerability and sensitivity. Using Drosophila melanogaster as an animal model, we examined how different thermal environments affected the shape of the performance curve and their parameters. We measured the climbing speed as a measure of locomotor performance in adult flies and tested the ontogenetic and transgenerational effects of thermal environment on TPC shape. Parents and offspring were reared at 28 ± 0ºC (28C), 28 ± 4ºC (28V), and 30 ± 0ºC (30C). We found that both, environmental thermal variability (28V) and high temperature (30C) experienced during early ontogeny shaped the fruit fly TPC sensitivity. Flies reared at variable thermal environments shifted the TPC to the right and increased heat tolerance. Flies held at high and constant temperature exhibited lower maximum performance than flies reared at the variable thermal environment. Furthermore, these effects were extended to the next generation. The parental thermal environment had a significative effect on TPC and its parameters. Indeed, flies reared at 28V whose parents were held at a high and constant temperature (30C) had a lower heat tolerance than F1 of flies reared at 28C or 28V. Also, offspring of flies reared at variable thermal environment (28V) reached the maximum performance at a higher temperature than offspring of flies reared at 28C or 30C. Consequently, since TPC parameters are not fixed, we suggest cautiousness when using TPCs to predict the impact of climate change on natural populations.

Natural selection on plasticity of thermal traits in a highly seasonal environment

For ectothermic species with broad geographical distributions, latitudinal/altitudinal variation in environmental temperatures (averages and extremes) is expected to shape the evolution of physiological tolerances and the acclimation capacity (i.e., degree of phenotypic plasticity) of natural populations. This can create geographical gradients of selection in which environments with greater thermal variability (e.g., seasonality) tend to favor individuals that maximize performance across a broader range of temperatures compared to more stable environments. Although thermal acclimation capacity plays a fundamental role in this context, it is unknown whether natural selection targets this trait in natural populations. Additionally, understanding whether and how selection acts on thermal physiological plasticity is also highly relevant to climate change and biological conservation. Here, we addressed such an important gap in our knowledge in the northernmost population of the four-eyed frog, Pleurodema thaul. We measured plastic responses of critical thermal limits for activity, behavioral thermal preference, and thermal sensitivity of metabolism to acclimation at 10 and 20°C. We monitored survival during three separate recapture efforts and used mark-recapture integrated into an information-theoretic approach to evaluate the relationship between survivals as a function of the plasticity of thermal traits. Overall, we found no evidence that thermal acclimation in this population is being targeted by directional selection, although there might be signals of selection on individual traits. According to the most supported models, survival increased in individuals with higher tolerance to cold when cold-acclimated, probably because daily low extremes are frequent during the cooler periods of the year. Furthermore, survival increased with body size. However, in both cases, the directional selection estimates were nonsignificant, and the constraints of our experimental design prevented us from evaluating more complex models (i.e., nonlinear selection).

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.