Venom Ontogeny in the Pacific Rattlesnakes Crotalus viridis helleri and C. v. oreganus

Abstract

Ontogenetic variation in venom composition was examined in the Pacific rattlesnakes Crotalus viridis helleri and C. v. oreganus. Venoms were analyzed for protease, phospholipase A2, L-amino acid oxidase, exonuclease and elastinolytic activities, and toxicity toward a native prey (Sceloporus graciosus). Protease activity increased significantly with size; L-amino acid oxidase and exonuclease activities also tended to increase. Phospholipase A2 activity decreased significantly with size, as did venom toxicity. These factors produce a highly toxic venom with low protease activity in juvenile snakes, which facilitates efficient handling of lizards and young rodents. Analysis of gut contents of museum specimens showed that lizards constitute a major fraction of prey taken by juvenile rattlesnakes. Lizards continue to be taken with high frequency until snakes reach approx. 500 mm in total length; above this size, mammals are taken exclusively. As snakes increase in size, they feed on larger mammalian prey, and a functionally different venom is produced. Venom from adult Pacific rattlesnakes is less toxic but has high protease activity, aiding in the digestion of prey in a thermally variable environment.