We studied the impact of restrictions by the thermal environments on body temperatures (Tbs) and microhabitat use of the lacertid lizards Podarcis melisellensis and Podarcis muralis in a Mediterranean area (Croatia) during autumn. The thermal conditions at available microhabitats were assessed with copper models that measured the operative temperatures (Te) at different sites. We estimated the thermal suitability of microhabitats by quantifying the extent of similarity between the Tes and the range of selected temperatures (i.e., the Tbs that lizards maintain in zero-cost conditions in a laboratory thermogradient; Tsel). Both species maintained, throughout most of day, Tbs that were on average 2–4 C below both Tsel and the Tbs recorded in the field during summer. Nevertheless, the Tbs measured during autumn were in the upper range of available Tes and were much closer to the Tsel than were the Tes. In addition, lizards were most often observed in the warmest microhabitats and were often seen basking. These results indicate that lizards were actively thermoregulating. The Te measurements show that lizards encounter suitable thermal conditions (i.e., where Tes is within Tsel) in only a restricted subset of the available microhabitats and during only a relatively short period of the day. Thus, lizards are constrained to maintain relatively low Tbs during autumn by the generally low available Tes. Although lizards were clearly thermoregulating, they appeared to accept lower Tbs during activity in autumn than in summer. A possible reason for this seasonal shift in activity Tbs is that achievement of higher Tbs during autumn is only feasible by confining activities to some specific microhabitats, which may severely constrain other behaviors, such as foraging.
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Vol. 2001 • No. 1