Animal poisons and especially venoms are complex mixtures of toxins ( Casewell et al., 2013 Fox and Serrano, 2008 Fry et al., 2009, 2012). Physiological resistance to toxins may evolve in predators that eat chemically-defended prey ( Brodie, 1990 Rowe and Rowe, 2008) and selection for greater resistance is predicted to be stronger in predators that exhibit greater diet specialization ( Arbuckle et al., 2017). 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. 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. Sera from both EIS and gartersnakes inhibited over 60% of SVMP activity, while only EIS sera also inhibited venom hemolytic activity (78%). 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. 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. Eastern indigo snakes ( Drymarchon couperi EIS) prey on pitvipers and are suspected to possess physiological resistance to their venom. When organisms possess chemical defenses, their predators may eventually evolve resistance to their toxins.
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