Rattlesnakes of Colorado: Natural History and Venom Biochemistry

Mackessy, Stephen P.

School of Biological Sciences

University of Northern Colorado

Greeley, CO 80639 USA

Though the Rocky Mountains are a canonical feature of the landscape, much of Colorado is characterized by shortgrass prairie and desert habitats, both of which support a greater diversity and abundance of rattlesnakes than the montane regions. Three species occur in the state: Sistrurus tergeminus edwardsii, Crotalus oreganus concolor and C. viridis viridis. In the appropriate habitat for each species, snakes can be exceptionally abundant, and our work over the last twenty years will be briefly summarized. Sistrurus is limited to the southeastern shortgrass steppe of Colorado below 5000 ft., and it appears to be most abundant in more mesic regions within the drier prairie; in one population, over 750 individuals were PIT-tagged over 3 seasons. Crotalus o. concolor is limited to the northwestern part of the state, also generally below 5000 ft. elevation. This species appears to den in small numbers, is surface-active at low (~14°C) temperatures, and appears to grow much more slowly than the other two species. Prairie Rattlesnakes (C. v. viridis) are widely distributed across the state, occurring from the lowest elevations to greater than 9,000 ft. Large dens were common before human persecution, but dens with over 270 individuals are still encountered, and at one study site on the eastern plains, we have PIT-tagged over 2,000 individuals over the last 15 years. Venom composition is variable among the three species: Sistrurus and C. v. viridis both produce type I venoms, with moderate toxicity and metalloproteinase activity, but C. o. concolor produces type II venom, with high toxicity and nearly undetectable metalloproteinase activity. Concolor toxin, the presynaptic neurotoxin that gives this venom its high toxicity, shows near sequence identity (~99%) with crotoxin and Mojave toxin. Continuing work is aimed at evaluating admixture at points of contact between C. viridis and C. oreganus and its effects on venom composition.