Comparative Biology of Sympatric Red Diamond and Southern Pacific Rattlesnakes in Southern California

I compared the biology of two sympatric rattlesnakes within the context of resource use and niche partitioning. Using radio-telemetry and mark-recapture involving passive integrated transponders (PIT) tags, I studied Red Diamond (Crotalus ruber) and Southern Pacific (C. oreganus helleri) Rattlesnakes in Chino Hills State Park, California, from March 2003 through March 2010. The first of three studies compared the two species in terms of home range size, movements, and mating phenology. Crotalus o. helleri occupied larger home ranges than C. ruber, and exhibited 1.9–2.8-fold greater movement distances (depending on year) than C. ruber. Mating phenology also varied, with C. ruber mating only in spring, and C. o. helleri exhibiting a spring and fall bi-modal mating system. The second study examined interspecific differential resource use. I compared both species along all four major niche axes (spatial, temporal, thermal, and diet) to test the general notion that sympatric vipers partition primarily macrohabitat, which contrasts with the general pattern in snakes of partitioning diet. Pianka\u27s and Czekanowski\u27s indices of niche overlap revealed modest overlap in macrohabitat and low overlap in diet; however, diet may be influenced by macrohabitat and body size differences. The two species exhibited broad overlap of microhabitat, thermal, and temporal (seasonal and circadian) resources. Null model tests using Monte Carlo simulations indicated that only diet (prey size) overlap was significantly less than expected. My third study examined and described the diet of C. ruber throughout the species\u27 range (C. o. helleri’s diet had been examined previously) based on trophic data collected from museum specimens, live animals from my Chino Hills study site, roadkilled snakes, opportunistic field observations, and existing literature. The diet of C. ruber consisted primarily of mammals (91.6%), but also included occasional lizards (7.5%) and birds, with no ontogenetic shift in prey type. Males averaged larger than females. Coastal snakes averaged larger and consumed a higher proportion of rodents and prey of larger body size than snakes from desert populations. Feeding occurred occasionally during winter, even at the northern extreme of its range. This dissertation provides the most detailed and complete analysis of resource use and niche separation by sympatric North American viper species. If niche partitioning exists among the rattlesnakes studied, I suggest it occurs subtly along more than one axis and has resulted largely from non-competitive interactions. I was unable to detect significant habitat partitioning using contemporary methodologies. These findings call into question the generality of habitat partitioning by vipers (Luiselli, 2006a, 2006b: Luiselli et al., 2007), and suggest the need for further study