Experimental winter warming increases activity with signs of potential DNA damage in common wall lizards.

Winter is warming faster than summer, posing a substantial threat to hibernating ectotherms, whose physiology depends directly on environmental conditions. While the effects of active season warming are increasingly well understood, the consequences of winter warming remain understudied. Research has predominantly focused on single, constant thermal regimes, overlooking the role of thermal variability. Furthermore, the specific warming patterns most disruptive to dormancy, their effects on winter activity and the subsequent physiological consequences are poorly understood. Here, we experimentally manipulated overwintering temperatures in the common wall lizard (Podarcis muralis), from a population introduced in southern UK, to assess the effects of different winter warming regimes on activity, body condition and oxidative stress. Lizards were exposed to three treatments for 3.5 months: a typical cold winter (4±1°C), a constant mild winter (8±1°C) and a fluctuating winter temperature (5 days cold: 4±1°C; 2 days mild: 8±1°C). Constant mild warming significantly increased activity, whereas the fluctuating regime did not, suggesting a temperature-duration threshold for full arousal. Despite increased activity, body condition, total antioxidant capacity and lipid peroxidation remained largely unaffected, indicating limited physiological disruption. However, the mild regime showed a trend toward increased oxidative DNA damage, highlighting a previously unrecognised physiological vulnerability that merits further investigation. Overall, our findings suggest behavioural resilience of common wall lizards to moderate winter warming, though hidden molecular costs could emerge under sustained mild conditions. We encourage integrating behavioural sensitivity and subtle physiological responses into models predicting species resilience to climate change.