Thermally Imposed Time Constraints on the Activity of the Desert Lizard Sceloporus Merriami

Abstract

We investigated the thermal biology of the lizard Sceloporus merriami in a hot, arid locality in west Texas. The effects of the thermal environment on lizard activity were hypothesized to be extensive because S. merriami body temperatures average lower than the body temperatures of any other North American desert iguanid (32.2°C). To test this hypothesis, we compared lizard thermal microclimate use and rates of lizard movements, feeding, and social displays with estimates of the relative availability of thermal microclimates near 32.2°. These comparisons were made throughout the lizards' morning activity period. The thermal environment had a major impact on S. merriami and restricted individual activities (movement rates, feeding strikes, and social displays) to a 2—h period beginning around local sunrise (1000) and to a brief period in late afternoon. In the morning, rates of activities were maximal within 1 h after local sunrise. This activity maximum coincided with the time that the average available environmental temperature approximated the mean body temperature for S. merriami. Also at this time, lizards occupied thermal microclimates randomly with respect to microclimate availability on their home ranges, whereas when activity rates were lower, lizard microclimate use was a nonrandom. Thus, when thermally favorable microclimates were most available, lizard activity was least constrained. S. merriami timed the initiation, peak, and cessation of overall and microclimate—specific activity to coincide with the overall and microclimate—specific availability of favorable operative temperatures.