Thermal performance curves, activity and survival in a free-ranging ectotherm.

Temperature profoundly influences the distribution and diversity of ectotherms, yet in natural settings, interactions between environmental temperatures, behaviour, physiological function and the influence of these factors on individual survival remain poorly understood. In particular, it is unclear as to how trade-offs between these factors are optimised in wild, free-ranging species. We combined temperature-sensitive radio transmitters and accelerometers to measure in situ body temperatures and field-based thermal locomotor performance, estimating thermal optimum and maximum performance. This allowed us to quantify the effectiveness of thermoregulation in the wild and determine whether seasonal trade-offs in thermoregulatory behaviour shape thermal performance and influence survival in the Australian central bearded dragon (Pogona vitticeps). Lizards adjusted their behaviour to maintain optimal body temperatures, achieving greater thermoregulatory precision in spring and summer when environmental costs of thermoregulation were low, but reducing that precision in winter when costs were higher. Activity time and maximum locomotor performance were higher during seasons when thermoregulatory precision was high. Maximum locomotor performance in the field was a strong predictor of survival, regardless of sex, even though survival probabilities were higher in males than females. Higher locomotor performance was associated with increased mortality risk, but survival was not influenced by activity levels or thermoregulatory indices. These findings highlight the complex trade-offs that ectotherms must navigate to balance behavioural thermoregulation and survival. Our data demonstrate the important influence of seasonal and sex-specific variation on behaviour and fitness-related outcomes. Interpreting field-derived thermal performance curves alongside laboratory measures is crucial for distinguishing 'true' physiological capacity from the integrated ecological contexts that shape performance and fitness in nature. Such insights are vital for predicting how ectotherms may respond to future climate warming.