Serum-based inhibition of pitviper Venom by Eastern Indigo Snakes (Drymarchon couperi)

When organisms possess chemical defenses, their predators may eventually evolve resistance to their toxins. Eastern indigo snakes ( Drymarchon couperi ; EIS) prey on pitvipers and are suspected to possess physiological resistance to their venom. In this study, we formally investigated this hypothesis using microassays that measured the ability of EIS blood sera to inhibit (A) hemolytic and (B) snake venom metalloproteinase (SVMP) activity of copperhead ( Agkistrodon contortrix ) venom. To serve as controls, we also tested the inhibitory ability of sera from house mice ( Mus musculus ) and checkered gartersnakes ( Thamnophis marcianus ), a snake that does not feed on pitvipers. Sera from both EIS and gartersnakes inhibited over 60% of SVMP activity, while only EIS sera also inhibited venom hemolytic activity (78%). Our results demonstrate that EIS serum is indeed capable of inhibiting two of the primary classes of toxins found in copperhead venom, providing the first empirical evidence suggesting that EIS possess physiological resistance to venom upon injection. Because we documented resistance to hemolytic components of pitviper venom within EIS but not gartersnakes, we speculate this resistance may be driven by selection from feeding on pitvipers while resistance to SVMP may be relatively widespread among snakes.