SEARS, Michael W.; DUNHAM, Arthur E.: The influence of thermal heterogeneity on life history variation among elevationally separated populations of sagebrush lizards

Geographic variation in the thermal environment has been proposed as a mechanism producing life history variation among populations of ectotherms. The longer elevated temperatures can be maintiained for activity, the more time individuals have to forage to support growth and reproduction. In the sagebrush lizard, adult body size is influenced by thermal constraints on activity time; a long activity season is associated with a relatively large maximum adult body size. Contrary to other studied populations, sagebrush lizards along an elevational gradient in SW Utah exhibit an unexpected cline in life history. Lizards at the highest elevation exhibited fastest growth despite having the shortest activity seasons. Heterogeneity of environmental temperatures among habitats may also produce variation in growth capacity among populations by limiting foraging opportunities. Since operative environmental temperatures are distributed temporally and spatially, an individual can be active but not have uninhibited access to its prey. Thus, the influence of environmental heterogeneity on operative temperatures is especially important for small-bodied organisms because large fluctuations in operative environmental temperatures will make some portions of the habitat unavailable. Although lizards at low-elevation can be active at their preferred body temperature for a longer duration, much of their habitat exceeds their critical thermal maximum during mid-day. At higher elevations, the thermal environment is less restrictive at mid-day. Thus, lizards at high high-elevation might have greater success at foraging even when the abundance of prey is the same at all elevations, resulting in higher growth at high elevation.