When spring becomes summer: global warming modifies seasonal patterns of thermoregulatory behaviour in a heliothermic lizard

In heliothermic lizards, thermoregulatory strategies differ seasonally: in spring, activity is shorter, with frequent shuttling between sun and shade; in summer, activity starts earlier, with more shade-seeking at midday. We hypothesized that elevated spring temperatures caused by global warming might elicit ‘summer-like’ behaviours earlier in the season. To test this hypothesis, we compared data collected for our model species, the lacertid lizard Psammodromus algirus, in May 2022 –a post-warming spring– with identical measurements from May and July 1997 in the same Mediterranean open forest. Preferred temperatures measured in a laboratory thermal gradient (T_sel) were higher in May 2022 than in May 1997, approaching July 1997 values. Field body temperatures in May 2022 were also higher, less variable (higher precision), and closer to T_sel (higher accuracy) than in May 1997; thermoregulatory effectiveness (the extent to which body temperatures are closer to T_sel than are operative temperatures) was likewise greater in May 2022. For all these variables, post-hoc tests revealed significant differences between May 2022 and May 1997, but not between May 2022 and July 1997. Activity began 1 h later in May 1997 compared to the other periods. Microhabitat use in May 2022 (full sun, filtered sun, full shade) closely resembled July 1997, but differed markedly from May 1997, when patch selection contributed least to thermoregulation. These findings suggest that hotter spring conditions under climate warming can enhance thermoregulatory accuracy, precision, and effectiveness to previously unsurpassed levels, provided that shaded microhabitats remain thermally suitable (i.e., below the upper limit of T_sel).