COMPARATIVE ECOLOGY OF 2 SNAKE SPECIES (CROTALUS-VIRIDIS AND PITUOPHIS-MELANOLEUCUS) IN SOUTHWESTERN IDAHO

Comparative ecology of Crotalus viridis (western rattlesnake) and Pitu ophis melanoleucus (gopher snake) was studied from 1975-1980 within an d near the Snake River Birds of Prey Area in southwestern Idaho. We ha ndled a total of 454 C. viridis and 455 P. melanoleucus (including 123 and 24 recaptures, respectively) during the study. Variables studied were (1) seasonal and daily activity patterns, (2) body temperatures, (3) habitat, and (4) fond. Overlap in resource use was relatively high for all dimensions measured. Seasonal activity was similar for both s pecies (Horn's index of overlap, R(0), was 0.94). Both species had pea k seasonal activities in late May and early June, and neither had a se condary peak in late summer or autumn. Both species had unimodal daily activity patterns in spring and overlap was high (R(0) = 0.88). By su mmer, however, C. viridis had a weakly bimodal daily activity pattern and P. melanoleucus was strongly bimodal. Similarity of overlap for su mmer daily activity was reduced (R(0) = 0.79). Both species were seldo m captured after darkness. Body temperatures were similar between the species (R(0) = 0.92). Habitat was one of the most highly partitioned resources between these species, and Horn's index for this dimension w as R(0) = 0.70. Crotalus viridis was found primarily in rocky habitats whereas P. melanoleucus was found throughout all habitats. Levins' st andardized measure of niche breadth for habitat was 0.21 for C. viridi s and 0.78 for P. melanoleucus, indicating a greater habitat specializ ation for C. viridis than for P. melanoleucus. Crotalus viridis tended to specialize on one prey species whereas P. melanoleucus showed no d istinct prey preferences. Food habits based on type (species) of prey was the other most highly partitioned resource with Horn's index of ov erlap (R(0) = 0.70) equal to that for habitat. There was a significant positive correlation between snake body size and prey size over the e ntire size ranges of both species of snakes. Once adult body size was attained, however, there was little such correlation. Rattlesnakes sho wed strong prey selection (P < 0.001) for Townsend ground squirrels (S permophilus towndendii) whereas gopher snakes showed no preference for any of its major prey species (P > 0.70). Major differences in the li fe-history traits that we studied were related to reproduction. Crotal us viridis tended to have larger fat reserves and females often gave b irth biennially whereas P. melanoleucus tended to have smaller fat res erves and females reproduced and deposited eggs annually. Differences in resource utilization recorded between these two snake species appea r to be shaped by phylogenetically determined life-history and morphol ogical differences.