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

Most research in physiological ecology has focused on the effects of mean changes in temperature under the classic "hot vs cold" acclimation treatment; however, current evidence suggests that an increment in both the mean and variance of temperature could act synergistically to amplify the negative effects of global temperature increase and how it would affect fitness and performance-related traits in ectothermic organisms. We assessed the effects of acclimation to daily variance of temperature on thermal performance curves of swimming speed in helmeted water toad tadpoles (Calyptocephalella gayi). Acclimation treatments were 20°C ± 0.1 SD (constant) and 20°C ± 1.5 SD (fluctuating). We draw two key findings: first, tadpoles exposed to daily temperature fluctuation had reduced maximal performance (Zmax), and flattened thermal performance curves, thus supporting the "vertical shift or faster-slower" hypothesis, and suggesting that overall swimming performance would be lower through an examination of temperatures under more realistic and ecologically-relevant fluctuating regimens; second, there was significant interindividual variation in performance traits by means of significant repeatability estimates. Our present results suggest that the widespread use of constant acclimation temperatures in laboratory experiments to estimate thermal performance curves (TPCs) may lead to an overestimation of actual organismal performance. We encourage the use of temperature fluctuation acclimation treatments to better understand the variability of physiological traits, which predict ecological and evolutionary responses to global change.

Thermal ecological physiology of native and invasive frog species: do invaders perform better?

Biological invasions are recognized as an important biotic component of global change that threatens the composition, structure and functioning of ecosystems, resulting in loss of biodiversity and displacement of native species. Although ecological characteristics facilitating the establishment and spread of non-native species are widely recognized, little is known about organismal attributes underlying invasion success. In this study, we tested the effect of thermal acclimation on thermal tolerance and locomotor performance in the invasive Xenopus laevis and the Chilean native Calyptocephalella gayi. In particular, the maximal righting performance (µMAX), optimal temperature (TO), lower (CTmin) and upper critical thermal limits (CTmax), thermal breadth (Tbr) and the area under the performance curve (AUC) were studied after 6 weeks acclimation to 10 and 20°C. We observed higher values of µmax and AUC in X. laevis in comparison to C. gayi. On the contrary, the invasive species showed lower values of CTmin in comparison to the native one. In contrast, CTmax, TO and Tbr showed no inter-specific differences. Moreover, we found that both species have the ability to acclimate their locomotor performance and lower thermal tolerance limit at low temperatures. Our results demonstrate that X. laevis is a better performer than C. gayi. Although there were differences in CTmin, the invasive and native frogs did not differ in their thermal tolerance. Interestingly, in both species the lower and upper critical thermal limits are beyond the minimal and maximal temperatures encountered in nature during the coldest and hottest month, respectively. Overall, our findings suggest that both X. laevis and C. gayi would be resilient to climate warming expectations in Chile.

Impact of global warming at the range margins: phenotypic plasticity and behavioral thermoregulation will buffer an endemic amphibian.

When dispersal is not an option to evade warming temperatures, compensation through behavior, plasticity, or evolutionary adaptation is essential to prevent extinction. In this work, we evaluated whether there is physiological plasticity in the thermal performance curve (TPC) of maximum jumping speed in individuals acclimated to current and projected temperatures and whether there is an opportunity for behavioral thermoregulation in the desert landscape where inhabits the northernmost population of the endemic frog Pleurodema thaul. Our results indicate that individuals acclimated to 20°C and 25°C increased the breath of their TPCs by shifting their upper limits with respect to when they were acclimated at 10°C. In addition, even when dispersal is not possible for this population, the landscape is heterogeneous enough to offer opportunities for behavioral thermoregulation. In particular, under current climatic conditions, behavioral thermoregulation is not compulsory as available operative temperatures are encompassed within the population TPC limits. However, for severe projected temperatures under climate change, behavioral thermoregulation will be required in the sunny patches. In overall, our results suggest that this population of Pleurodema thaul will be able to endure the worst projected scenario of climate warming as it has not only the physiological capacities but also the environmental opportunities to regulate its body temperature behaviorally.

Energy metabolism and the postprandial response of the Chilean tarantulas, Euathlus truculentus (Araneae: Theraphosidae).

One of the most ubiquitous consequences of feeding in animals is specific dynamic action (SDA), a drastic increment in metabolic rate after a meal, which lasts from a few hours to several days. According to a recent exhaustive review by Secor (2009), studies in SDA are abundant, encompassing all kinds of vertebrates and invertebrates. However, important exceptions are arachnids, as few studies have characterized SDA in this group. Here, we measured the standard metabolic rate (SMR) of the Chilean tarantulas Euathlus truculentus (body mass=7.32±0.7 g; N=32; T(A)=25°C), its inter-individual variation (i.e., repeatability) and its SDA. We measured SMR three or four times in each individual, and we also conducted predation experiments where a prey was consumed by each spider, during a respirometry trial. The SMR of E. truculentus was 0.00049±0.000079 mlCO(2) g(-1) min(-1) which corresponds to 1524 µW (assuming a protein-based diet), 108.4% of the predicted value for arachnids. According to the standard nomenclature for SDA studies, the scope of the SDA for a meal size of 1.26±0.04 g (18% of the spider size) was 6.55±1.1 times the baseline, the time to peak was 45 min, and the magnitude of the SDA was 0.28±0.03 kj, which is 85% of the expected value for invertebrates. Our SMR data are in concordance with previous findings suggesting remarkably low energy metabolism in arachnids, compared with other arthropods. On the other hand, the exceedingly high scope of the postprandial response contrasts with the comparatively low SDA. This fact suggests that spiders spend most of the energy for digestion in a short period after prey capture, which could be a consequence of their external digestion.