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

Abstract

Comparative ecology of Crotalus viridis (western rattlesnake) and Pituophis me- lanoleucus (gopher snake) was studied from 1975-1980 within and near the Snake River Birds of Prey Area in southwestern Idaho. We handled 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) food. Overlap in resource use was relatively high for all dimensions measured. Seasonal activity was similar for both species (Horn's index of overlap, R0, was 0.94). Both species had peak seasonal activities in late May and early June, and neither had a secondary peak in late summer or autumn. Both species had unimodal daily activity patterns in spring and overlap was high (Ro = 0.88). By summer, however, C. viridis had a weakly bimodal daily activity pattern and P. melanoleucus was strongly bimodal. Similarity of overlap for summer daily activity was reduced (Ro = 0.79). Both species were seldom captured after darkness. Body temperatures were similar between the species (Ro = 0.92). Habitat was one of the most highly partitioned resources between these species, and Horn's index for this dimension was R0 = 0.70. Crotalus viridis was found primarily in rocky habitats whereas P. melanoleucus was found throughout all habitats. Levins' standardized measure of niche breadth for habitat was 0.21 for C. viridis and 0.78 for P. melanoleucus, indicating a greater habitat specialization for C. viridis than for P. melanoleucus. Crotalus viridis tended to specialize on one prey species whereas P. melanoleucus showed no distinct prey preferences. Food habits based on type (species) of prey was the other most highly partitioned resource with Horn's index of overlap (Ro = 0.70) equal to that for habitat. There was a significant positive correlation between snake body size and prey size over the entire size ranges of both species of snakes. Once adult body size was attained, however, there was little such correlation. Rattlesnakes showed strong prey selection (P 0.70). Major differences in the life-history traits that we studied were related to reproduction. Crotalus viridis tended to have larger fat reserves and females often gave birth biennially whereas P. melanoleucus tended to have smaller fat reserves and females repro- duced and deposited eggs annually. Differences in resource utilization recorded between these two snake species appear to be shaped by phylogenetically determined life-history and morphological differences.