Ontogenetic Variation in Venom Composition and Diet of Crotalus oreganus concolor: A Case of Venom Paedomorphosis?

Ontogenetic shifts in diet are common for snakes, and such shifts in diet for venomous snakes may be associated with changes in venom composition. The present study investigated whether an ontogenetic shift in diet and venom composition, as observed for Crotalus oreganus helleri and Crotalus oreganus oreganus, occurs in Crotalus oreganus concolor. Like C. o. helleriandC. o. oreganus, and at similar body sizes, C. o. concolor show an ontogenetic shift in diet. Juvenile snakes primarily feed on small lizards, whereas adults typically consume small rodents. However, C. o. concolor do not show the same pattern of venom ontogeny as do C. o. helleri and C. o. oreganus. Because of the presence of a phospholipase A2-based β-neurotoxin (concolor toxin) and several myotoxins, C. o. concolor venom is particularly toxic, but mouse LD50 assays demonstrated no significant difference in toxicity between adult (0.38 μg/g) and juvenile (0.45 μg/g) venoms. Metalloprotease activity (correlated with extensive tissue damage and prey predigestion) was extremely low in both juvenile and adult venoms. Levels of peptide myotoxins and several serine proteases that interfere with hemostasis (specifically thrombin-like and plasmin-like activities) showed a positive correlation with size. Human envenomations recorded during this study showed symptoms consistent with biochemical analyses, with numbness associated with the bite, coagulation abnormalities and essentially no tissue damage. Results suggest that the occurrence of potent neurotoxic component(s) in a venom minimizes predigestive components (metalloproteases). Further, concurrence of these functional components in the venom of an individual may be selected against, and highly toxic venom in both juvenile and adult C. o. concolor may represent a form of venom paedomorphosis.