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It's complicated! Ontogenetic variation in venom composition of rattlesnakes

Hayes, William K.

Fox, Gerad A.

Kelln, Wayne

Department of Earth and Biological Sciences

Loma Linda University

Loma Linda, California USA

Gren, Eric C. K.

University of Montana, Bitterroot College Campus

Bitterroot Community Science Center

Hamilton, Montana USA

Cochran, Chip

Travis, Zachary D.

Department of Earth and Biological Sciences

Loma Linda University

Loma Linda, California USA

Dugan, Eric A.

Dugan Biological Services, LLC

Upland, California USA

Lee, Kyung H.

Neurology Residency

Dartmouth Hitchcock Medical Center

Lebanon, New Hampshire USA

Cardwell, Michael D.

Biology Department

San Diego State University

San Diego, California USA

Goode, Matthew J.

School of Natural Resources

University of Arizona

Tucson, Arizona USA

Morris, Cale

Heritage Academy

Mesa, Arizona USA

Schuett, Gordon W.

Department of Biology ǀ Neuroscience Institute

Georgia State University

Atlanta, Georgia USA

Chiricahua Desert Museum

Rodeo, New Mexico USA

When young, the small size of rattlesnakes severely constrains the prey they can capture and consume. As the snakes grow their larger size and greater venom supply expands their range of prey intake, often from smaller ectothermic prey to larger, endothermic prey. Smaller snakes are also subject to a greater range of predators. We examined ontogenetic changes in venom composition and complexity of seven southwestern North American rattlesnake species to test four hypotheses: that (1) change is greater in species or populations possessing largely proteolytic (type I or type B) rather than largely neurotoxic (type II or type A) venom; (2) change is greater in species that transition from reptile to mammal prey than those which specialize on mammals throughout their life; (3) change occurs continuously as an individual grows, even after attaining reproductive size; and (4) venom complexity is moderate in small individuals, greatest during the period of ontogenetic shift in toxins, and least in adults. We separated venom components of individual venoms using reverse-phase high-pressure liquid chromatography (RP-HPLC), and quantified protein content of individual chromatogram peaks. We identified specific toxins using mass spectrometry. To associate ontogenetic changes in venom with diet, we dissected prey material from the stomachs and/or guts of snakes. Results indicated that ontogenetic change in venom composition exists in all seven species. Moreover, we found support for all but one hypothesis. Hypothesis 2 proved wrong, as the most profound ontogenetic change occurs in C. ruber, which feeds largely on mammals throughout its life. Collectively, our findings demonstrate the remarkably varied patterns of venom ontogeny that exist within a closely related group of vipers, and even within a single species, leading to few, if any, general patterns. They also underscore the likely adaptive importance of venom composition changes coincident with dietary shifts.

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