Resistance to rattlesnake venom

[NicoleRussell]

Previous studies have shown that some mammals are able to neutralize
venom from snake predators. California ground squirrels
(Spermophilus beecheyi) show variation among populations in their
ability to bind venom and minimize damage from northern Pacific
rattlesnakes (Crotalus oreganus), but the venom toxins targeted by
resistance have not been investigated. Four California ground
squirrel populations, selected for differences in local density or
type of rattlesnake predators, were assayed for their ability to
neutralize digestive and hemostatic effects of venom from three
rattlesnake species. In Douglas ground squirrels (S. b. douglasii),
we found that animals from a location where snakes are common showed
greater inhibition of venom metalloprotease and hemolytic activity
than animals from a location where snakes are rare. Effects on
general proteolysis were not different. Douglas ground squirrels
also reduced the metalloprotease activity of venom from sympatric
northern Pacific rattlesnakes (Crotalus oreganus oreganus) more than
the activity of venom from allopatric western diamondback
rattlesnakes (C. atrox), but enhanced fibrinolysis of sympatric
venom almost 1.8 times above baseline levels. Two Beechey ground
squirrel (S. b. beecheyi) populations had similar inhibition of
venoms from northern and southern Pacific rattlesnakes (C. o.
helleri), despite differences between the populations in the locally
prevalent predator. However, the venom toxins inhibited by Beechey
squirrels did vary among venom from Pacific rattlesnake subspecies,
and between these venoms and venom from allopatric western
diamondback rattlesnakes. Blood plasma from Beechey squirrels showed
highest inhibition of metalloprotease activity of northern Pacific
rattlesnake venom, general proteolytic activity and hemolysis of
southern Pacific rattlesnake venom, and hemolysis by allopatric
western diamondback venom. These results reveal previously cryptic
variation in venom activity against resistant prey that suggests
reciprocal adaptation at the molecular level.