VENOM ONTOGENY IN THE PACIFIC RATTLESNAKES CROTALUS-VIRIDIS-HELLERI AND CROTALUS-VIRIDIS-OREGANUS

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 (Sceloporous 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 fration 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 large 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.