Altitudinal variation of the thermal biology and running performance in the lizard Podarcis tiliguerta

Summary We studied, in the field and laboratory, aspects of the thermal biology in two populations of the lizard Podarcis tiliguerta along a 1450 m altitudinal gradient. Body temperatures (T_b) at high altitudes average lower, are more variable, but are more elevated above environmental temperatures than at sea level. Lizards partially reduced the impact of altitudinal changes in thermal loads through presumable subtle behavioural adjustments. A comparison of the thermal preferences in the laboratory, the maximal operative temperatures predicted from a biophysical model, and the activity T_b's at both sites, indicates that the main response to changing environmental conditions is an active shift in thermoregulatory set points. Integration of field T_b's and laboratory data on temperature specific sprint speeds, predicts that the mountainous lizards experience reduced running abilities that are especially acute in the early morning. Despite this impairment of running performance, the thermal sensitivity of running speed has not evolved to match the T_b's experienced by both populations. This result supports the view that the thermal physiology of this lizard is evolutionarily conservative, but the lack of information on the relation between running performance and fitness components impedes rejection of alternative hypotheses.