Habitat and Prey Selection of Sceloporus Occidentalis and Sceloporus Graciosus

Sympatric populations of the western fence lizard, Sceloporus occidentalis and the sagebrush lizard, Sceloporus graciosus were compared with respect to morphological measurements, and habitat and resource utilization. An allopatric population of S. occidentalis was studied to determine whether any differences among sympatric populations result from species—specific differences or from a shift in characteristics of S. occidentalis because of the presence of a congener. The presence of sympatric S. graciosus does not alter the size of prey selected by S. occidentalis. Sceloporus graciosus selects a significantly smaller mean length of prey (4.6 mm) than S. occidentalis (5.4 mm). Two factors theoretically contribute to the variation in size of prey taken by a population: each individual shows variation in length of prey, and morphological variation in size of adult lizards results in differences among individuals in average length of prey. The first factor explains the variation in size of prey taken by these populations, because there is no correlation between mean prey length and adult body length for either species. Although there is a significant difference between the two species in mean size of prey taken, extensive overlap in prey utilization is revealed directly by the high overlap values between species (66%) and indirectly from the low value of niche separation to niche width ratio. The significantly larger mean prey size of adults to immatures reduces the intraspecific competition between age classes. Adult S. graciosus, with a smaller mean prey length than S. occidentalis, overlaps the most with immatures, particularly S. occidentalis immatures. The significantly larger head size of sympatric compared with allopatric S. occidentalis is not explained by character displacement in trophic apparatus to decrease overlap between species in prey utilization because the same size prey is selected in both areas and by both sexes. The larger head size of S. occidentalis males in sympatry may increase species identification between species in push—up displays. Sympatric S. occidentalis and S. graciosus segregate themselves significantly between habitats, and within a habitat between structural microhabitats. This minimal spatial overlap between sympatric species allows their coexistence despite extensive overlap in prey utilization.