|Bushmasters are one of the snakes known to feed on venom-resistant opossums|
Opossums and pitvipers are sympatric throughout most of the New World. Opossums are were not known to feed on pit vipers and pit vipers were not known to feed on opossums. In the mid 1940 Vellard discovered that three species South American Didelphis were resistant to the venom of several pitviper species. The resistance is from endogenous toxin-neutralizing serum proteins, and possibly other molecules. Vellard proposed that venom resistance evolved as an adaptation for preying on venomous snakes at the same time he implied the hypothesis might be proven wrong if opossums too small to eat venomous snakes, were also found to be venom resistant. Later researchers reported venom resistance in the North American opossum (Didelphis virginiana), as well as several Neotropical opossums such as, the lutrine opossum ,and the gray four-eyed opossum To date, all of the species found to be venom resistant belong to the Didelphini clade all and are relatively large, about 500 g, and are known to feed on pit vipers. By contrast, experiments have demonstrated the brown four-eyed opossum, a small (200-300 g) species is not resistant to snake venom.
Voss (2013) reviewed the literature records of snakes feeding on opossums and found that snakes feeding on large, venom resistant opossums were sometimes based upon mis-identifications. He did find that (1) some pitvipers are preyed upon by large venom-resistant opossums; (2) many small opossums that are not known to be venom-resistant are preyed upon by pitvipers; and (3) venom-resistant opossums are preyed upon by some large pitvipers. The last situation has not been previously recognized in the literature Voss suggests two possible explanations. First, venom may be able to overcome the resistance if young individuals of venom-resistant species are bitten by large snakes. Secondly some pitvipers may have evolved unusually potent venom as a result of co-evolution with venom-resistant predatory opossums.
Voss, R. S. 2012. Opossums (Mammalia: Didelphidae) in the diets of Neotropical piutvipers (Serpentes: Crotalinae): evidence for alternative coevolutionary outcomes? Toxicon 66:1-6.