We monitored ultraviolet-B (UVB) irradiance exposure (measured using the ultraviolet index [UVI]) of the Western Sagebrush Lizard (Sceloporus graciosus gracilis) in Lassen Volcanic National Park, California, and the related Dunes Sagebrush Lizard (S. arenicolus) in eastern New Mexico. For S. graciosus gracilis, on sunny days between 0900 and 1100 h, average lizard UVI tracked available solar UVI. Periodic shade-seeking between 1100 and 1600 h resulted in lizard UVI being less than the available solar UVI. This daily pattern of exposure was evident for the activity period from June until August. This general pattern was also evident for S. arenicolus except when air temperatures exceeded 37°C; at these times, all S. arenicolus subjects retreated underground for much of the afternoon. Individuals of S. graciosus gracilis exhibited large within-day variations in the durations of time that they basked in full sun, sought shade, and retreated underground. Two lizards followed for most of a day showed very different patterns of UVI exposure, yet the calculated daily UVB dose (irradiance × time) was similar for both individuals. Some, but not all, S. graciosus gracilis periodically retreated underground during their daily activity; retreat was more frequent and UVI exposure more intense on a cooler day at the California study site.
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