Lizard thermoregulation revisited after two decades of global warming

Although the effects of global warming on thermoregulation are usually explored using predictions of climate envelop modelling, such effects should best be analysed empirically, studying the same population with the same methods after a long enough period of temperature rise. We used a 30-year long database about body temperatures (T(b)s) of field-active Psammodromus algirus lizards inhabiting a well-conserved temperate open forest, and we focused on the summers of 1997 and 2017 to compare T(b)s, environmental operative temperatures (T(e)s), their proximity to the selected thermal range (T-sel), and the selection of sunlit and shaded patches all along the day. From these data, we estimated the precision (standard deviation of T(b)s), accuracy (average distance between T(b)s and T-sel) and effectiveness (extent to which T(b)s are closer to T-sel than T(e)s) of thermoregulation. Of the highest 5% of all T(b)s in the database, 95% were recorded in 2017, when the adjustment to T-sel was much better for T(b)s selected in a laboratory thermogradient than for field T(b)s (percentages of T(b)s above T-sel of 2% and 52% respectively). In 2017, especially after 12:00 h, the selection of shaded patches (87% of lizards in full shade vs. <1% in full sun) was more intense than in 1997, contributed more to overall thermoregulation, and produced a larger difference between T(e)s and T(b)s. In spite of this, T(b)s were lower-and closer to T-sel-in 1997 (when most shaded patches offered favourable thermal opportunities, with T(e)s within or below T-sel) than in 2017 (when only 33% of full shade T(e)s, and 8% of all T(e)s, were within or below T-sel). As a consequence, estimates of the accuracy and effectiveness of thermoregulation decreased over the 20-year period examined. We conclude that given the low availability of T(e)s within or below T-sel, lizards cannot longer prevent the rise of their T(b)s above T-sel, at least in hot summer days. Thus, the effects of global warming are already hindering the ability of lizards to buffer environmental change by behavioural means, even in temperate forests with a fine-grained mosaic of sun and shade patches. Read the free Plain Language Summary for this article on the Journal blog.