Ontogenetic and geographic venom variation in the Great Basin Rattlesnake, Crotalus oreganus lutosus

Venom composition among the species of the Western Rattlesnake clade is often quite variable, depending on several factors such as geographic location and ontogeny. Venom composition not only affects the ability of a snake to acquire prey efficiently, but it can also significantly impact snakebite symptomology. Currently, there has been limited characterization of the venom from the Great Basin Rattlesnake (Crotalus oreganus lutosus), a lineage that is broadly distributed in the intermontane western United States. In this study we sample 67 individual Great Basin Rattlesnakes collected in Idaho, Utah, California, and Arizona. We find evidence for substantial ontogenetic and geographic variation in venom composition. Of the six toxin families assessed, all showed ontogenetic shifts to varying extents, with some trends differing from those observed in other rattlesnake species, suggesting species-specific ontogenetic patterns. Notably, the P-I snake venom metalloproteinases and disintegrins were absent or significantly reduced in neonates and juveniles yet abundant in adults. Geographic trends were also observed, with L-amino acid oxidase activity being higher in the California population, while thrombin-like serine proteinase activity and phospholipase A₂ activity was significantly different between Idaho and Utah populations - both trends may be related to local prey specificity. The observed variation in venom activities across populations suggests the presence of venom phenotypic metapopulations. This study shows that in this broadly distributed species, both ontogeny and geographic population structure contribute significantly to range-wide venom compositional variation, which has both ecological and clinical relevance.