Rapid Adaptation over Phenotypic Plasticity: Physiological Responses to Novel Climates in the Introduced Mediterranean House Gecko (Hemidactylus turcicus)

Understanding how organisms respond to novel environmental conditions is critical for predicting the persistence of species under changing global climates. Introduced species serve as ideal models for investigating physiological responses to change because they endure novel climates and must persist in atypical environments. We conducted the first large-scale analysis of thermal tolerance in the Mediterranean House Gecko (Hemidactylus turcicus) across 22 cities, spanning the majority of its introduced range in the United States. We measured critical thermal limits (minima-CTmin and maxima-CTmax) and mouthgaping (thermal panting), an evaporative cooling mechanism, to evaluate the influences of climate, sex, and body mass on these factors. We also compared thermal breadth to seasonality using online climate data. We found regional climate to be the strongest predictor for CTmin, with evidence of rapid adaptation to colder winters on the northern front of their introduced range. CTmax showed little variation across climate and was more constrained overall. Evaporative cooling via mouth gaping was observed in 76.5% of individuals during CTmax trials, with the highest frequency in Arid Desert populations, suggesting variation in reliance on behavioral cooling across climate. However, temperature at which mouth gaping occurred and the difference between CTmax and mouth gaping were not significantly influenced by climate, sex, or body size. Lastly, we found that seasonality influenced thermal breadth. Together, our results suggest that H. turcicus, as a nocturnal species, relies more on physiological adaptation rather than behavioral thermoregulation to cope with thermal extremes. We provide key insights into the ecological and evolutionary processes shaping thermal tolerance in an introduced lizard.