Trade‐Offs in Activity Time and Physiological Performance for Thermoregulating Desert Lizards, Sceloporus Merriami

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Abstract
Average body temperatures (Tb) of summer—active Sceloporus merriami were lower in the morning (33.3°C) than in the evening 37.0°) at an arid locality in Big Bend National Park, Texas. During midafternoon, lizards were inactive and retreated to deep crevices and caves. According to a multivariate analysis, overall rates of lizard activities (movements, feeding, and social displays) were similar between morning and evening activity, indicating that lizards were behaving similarly during the different activity periods. However, at 37°, estimated rates of metabolism and water loss were greater and sprint speed was lower relative to these rates at 33°, suggesting that substantial resource and performance costs may have been imposed by the observed higher evening activity Tb. To investigate this unusual pattern of body temperature variation, I compared the morning and evening distributions of available operative environmental temperatures (Te) as estimated by an array of randomly deployed and oriented copper models of S. merriami placed on rock and ground surfaces within lizard home ranges. In the morning, lizards had access to a wide range of Te(30°—60°), and a large fraction of the Te distribution was below 36° at the time of peak morning activity. During the morning, a hypothetical increase in Tb to within the interval 36°—38° would have allowed lizards to occupy 90% depending on the exact time), and microclimates below 36° were unavailable until just before dark. The available Te distributions in the evening forced lizards to accept a higher activity Tb, and concomitantly higher metabolic and water loss rates, and lower locomotor performance, or remain inactive in crevices and caves. Thus in the evening, and not in the morning, the gain in usable home range due to accepting a higher Tb was great, and the lizards exploited this fraction at an increased Tb only in the evening. These data suggest that the distribution of available Te can directly influence the cost/benefit economics of lizard body temperature selection.