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Diet and Venom Ontogeny in Insular and High-Altitude Populations of the Southern Pacific Rattlesnake

Gren, Eric

Travis, C. K.

Kelln, Z. D.

Fox, Wade

Gerad A.

Person, Carl E.

Hayes, William K.

Department of Earth and Biological Sciences

Loma Linda University

Loma Linda, CA, 92350 USA

The Southern Pacific Rattlesnake (Crotalus helleri) exhibits pronounced geographic and ontogenetic variation in venom composition. As the snake grows, its venom transitions to become less toxic and more proteolytic as the diet shifts from lizard to rodent prey. Here, we compared diet and venom ontogeny in three populations occurring in different environments and having very distinctive venoms. Specimens from Santa Catalina Island and the Transverse Ranges express proteolytic (digestive) venom typical of the species, whereas those in the San Jacinto Mountains possess neurotoxic venom that has been interpreted as paedomorphic—the retention of a juvenile characteristic (highly toxic venom) into adulthood. We hypothesized that, if diet influences venom composition, (1) rattlesnakes having more neurotoxic venom will consume a higher proportion of lizards, which are more easily digested, and (2) venom of the neurotoxic population will exhibit less ontogenetic change. We analyzed stomach and fecal contents from snakes of all sizes, and used reverse-phase high-pressure liquid chromatography (RP-HPLC) to compare the venoms of juvenile and adults. Snakes in all three populations consumed primarily lizards when young, and transitioned to rodents as adults. But as hypothesized, the San Jacinto Mountains snakes consumed the highest proportion of lizards. Contrary to the second hypothesis, snakes from San Jacinto Mountains showed a similar amount of ontogenetic change in venom as the Transverse Ranges snakes. Although a diet of lizards may select for neurotoxic venom, the difference in diet could result from a large lizard (Sceloporus orcutti) common in the San Jacinto Mountains but absent from the other populations. Rattlesnakes in mountain ranges further south lack Mojave toxin despite the lizard’s presence. The question remains whether venom paedomorphism should be interpreted based on ontogenetic changes in overall venom composition, or ontogenetic changes in venom function (relative toxicity and proteolytic activity).

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