Thermal physiology helps us understand how ectotherms respond to novel environments and how they persist when introduced to new locations. Researchers generally measure thermal physiology traits immediately after animal collection or after a short acclimation period. Because many of these traits are plastic, the conclusions drawn from such research can vary depending on the duration of the acclimation period. In this study, we measured the rate of change and extent to which cold tolerance (critical thermal minimum [CTmin]) of nonnative Italian wall lizards (Podarcis siculus) from Hempstead, New York, changed during a cold acclimation treatment. We also examined how cold acclimation affected heat tolerance (critical thermal maximum [CTmax]), thermal preference (Tpref), evaporative water loss (EWL), resting metabolic rate (RMR), and respiratory exchange ratio (RER). We predicted that CTmin, CTmax, and Tpref would decrease with cold acclimation but that EWL and RMR would increase with cold acclimation. We found that CTmin decreased within 2 wk and that it remained low during the cold acclimation treatment; we suspect that this cold tolerance plasticity reduces risk of exposure to lethal temperatures during winter for lizards that have not yet found suitable refugia. CTmax and Tpref also decreased after cold acclimation, while EWL, RMR, and RER increased after cold acclimation, suggesting trade-offs with cold acclimation in the form of decreased heat tolerance and increased energy demands. Taken together, our findings suggest that cold tolerance plasticity aids the persistence of an established population of invasive lizards. More generally, our findings highlight the importance of accounting for the plasticity of physiological traits when investigating how invasive species respond to novel environments.

Ectotherms survive exposure to subzero temperatures through freeze tolerance or freeze avoidance. Among vertebrate ectotherms, glucose is commonly used as a cryoprotectant in freeze tolerant strategies and as an osmolyte in freeze avoidant strategies, while also functioning as a metabolic substrate. Whereas some lizard species are capable of both freeze tolerance and freeze avoidance, Podarcis siculus is limited to freeze avoidance through supercooling. We hypothesized that, even in a freeze-avoidant species such as P. siculus, plasma glucose would accumulate with cold acclimation and would increase in response to acute exposure to subzero temperatures. To investigate this, we tested whether plasma glucose concentration and osmolality would increase in response to a subzero cold challenge before and after cold acclimation. In addition, we examined the relationship between metabolic rate, cold acclimation, and glucose by measuring metabolic rate during the cold challenge trials. We found that plasma glucose increased during the cold challenge trials, and that the increase was more pronounced after cold acclimation. However, baseline plasma glucose decreased throughout cold acclimation. Interestingly, total plasma osmolality did not change, and the increase in glucose only slightly altered freezing point depression. Metabolic rate during the cold challenge decreased after cold acclimation, and changes in respiratory exchange ratio suggest an increased relative use of carbohydrates. Overall, our findings demonstrate an important role for glucose in the response of P. siculus to an acute cold challenge, thus adding evidence for glucose as an important molecule for overwintering ectotherms that use freeze avoidant strategies.

Caudal autotomy (the voluntary loss of a tail) is an important antipredator defense in many lizard species. The subsequent regrowth of the tail, seen in most lizards with the ability to autotomize, may pose some physiological costs both during the regrowth process and during potential repeated autotomy. In this study, we examined the physiology and behavior of the lacertid lizard Podarcis siculus, as well as the physiology and activity of the detached tails, following autotomy. Evaporative water loss and respiratory exchange ratio increased after autotomy, showing both an increased loss and a redistribution of important resources inside the animal. The lizards exhibited no change in behavioral thermoregulation during regrowth, as thermal preference did not change. Males and females differed in tail energy content, suggesting differing energy storage needs between the sexes. The rate of regeneration and tail movement between the sexes were not different, but tail movement did vary between previously regenerated tails and original tails, indicating a possible detriment to future autotomy events. Overall, autotomy appears to influence more than just a lizard's ability to escape, as our study provides evidence of potential physiological tradeoffs associated with this antipredator behavior.

Heat tolerance plasticity is predicted to be an important buffer against global warming. Nonetheless, basal heat tolerance often correlates negatively with tolerance plasticity ('trade-off hypothesis'), a constraint that could limit plasticity benefits. We tested the trade-off hypothesis at the individual level with respect to heat hardening in two lizard species, Anolis carolinensis and Anolis sagrei. Heat hardening is a rapid increase in heat tolerance after heat shock that is rarely measured in reptiles but is generally considered to be a first line of physiological defense against heat. We also employed a biophysical model of operative habitat temperatures to estimate the performance consequences of hardening under ecologically relevant conditions. Anolis carolinensis hardened by 2 h post-heat shock and maintained hardening for several hours. However, A. sagrei did not harden. Biophysical models showed that hardening in A. carolinensis reduces their overheating risk in the field. Therefore, while not all lizards heat harden, hardening has benefits for species that can. We initially found a negative relationship between basal tolerance and hardening within both species, consistent with the trade-off hypothesis. However, permutation analyses showed that the apparent trade-offs could not be differentiated from statistical artifact. We found the same result when we re-analyzed published data supporting the trade-off hypothesis in another lizard species. Our results show that false positives may be common when testing the trade-off hypothesis. Statistical approaches that account for this are critical to ensure that the hypothesis, which has broad implications for thermal adaptation and responses to warming, is assessed appropriately.

Temperature has a substantial effect on both the physiology and behavior of ectothermic animals such as lizards. Physiology and behavior can also be influenced by ontogenetic and sex differences, but these effects are largely understudied in lizards. We examined ontogenetic and sex-based differences in thermal tolerances, preferred temperature, and temperature-dependent evaporative water loss rates in Italian Wall Lizards, Podarcis siculus, collected from an introduced population near Los Angeles, California, USA that were acclimated to laboratory conditions. Podarcis siculus has been introduced to multiple localities in the USA and the Mediterranean region and has demonstrated remarkable ability to adapt to novel climatic conditions. In the California population, adults of both sexes had a higher critical thermal maximum (CTmax) than juveniles, and adult females had a lower critical thermal minimum (CTmin) than juveniles and adult males. Thus, adult females had a significantly wider thermal breadth (CTmax - CTmin) compared to adult males and juveniles. Mass-specific evaporative water loss was higher in juveniles compared to adult males at intermediate temperatures. There was no significant difference among groups for preferred temperature. This implies that thermal tolerance, a physiological characteristic, varies with age and sex for this population, whereas thermal preference, a behavioral characteristic, does not. Interestingly, CTmin for all age and sex classes was above temperatures likely experienced by some nonnative populations in winter, suggesting individuals need to find urban thermal retreats. These results add to the growing literature demonstrating that thermal tolerances and breadths can vary between sexes and across age classes in squamate species.