ASBURY, D/A*; ADOLPH, S/C; Indiana State University; Harvey Mudd College: Field experiments demonstrate plasticity of microhabitat use in Western Fence Lizards (Sceloporus occidentalis)

Geographic variation in microhabitat use among populations of a single species could reflect genetic divergence in habitat use behavior, behavioral plasticity or a combination thereof. Very few studies of vertebrates have explored the possible genetic basis of geographic variation in habitat use. We documented microhabitat use patterns for three populations of the phrynosomatid lizard Sceloporus occidentalis in southern California. In the wild, S. occidentalis microhabitat use covaried with average environmental temperatures: lizards chose the highest, shadiest perches at the hottest (desert) site, the lowest, sunniest perches at the coolest (montane) site, and intermediate perches at the inland valley site. We then used lizards native to these three populations in a microhabitat choice experiment in field enclosures at the inland valley site, the Bernard Field Station in Claremont, CA. In these experiments, three measures of microhabitat choice (perch height, substrate and basking) were independent of source population. This result suggests that geographic variation of microhabitat use in this species results from behavioral plasticity rather than genetic variation or differences in experience. Both the population differences in the wild and the diel changes observed in the choice experiment showed that microhabitat use is strongly influenced by thermal conditions. In the enclosure experiment, operative temperature measurements indicated that lizards preferred perch sites that had microclimates closest to their preferred body temperature (near 35�C).